TOXICOLOGICAL PROFILE FOR

HEXAMETHYLENE DIISOCYANATE

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Public Health Service

Agency for Toxic Substances and Disease Registry

August 1998
HEXAMETHYLENE DIISOCYANATE ii

DISCLAIMER

The use of company or product name(s) is for identification only and does not imply endorsement by the

Agency for Toxic Substances and Disease Registry.
HEXAMETHYLENE DIISOCYANATE iii

UPDATE STATEMENT

Toxicological profiles are revised and republished as necessary, but no less than once every three years. For

information regarding the update status of previously released profiles, contact ATSDR at:

Agency for Toxic Substances and Disease Registry

Division of Toxicology/Toxicology Information Branch

1600 Clifton Road NE, E-29

Atlanta, Georgia 30333















HEXAMETHYLENE DIISOCYANATE 1

1. PUBLIC HEALTH STATEMENT

This public health statement tells you about hexamethylene diisocyanate (HDI) and the effects of

exposure.

The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in

the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for

long-term federal clean-up activities. Hexamethylene diisocyanate has not been found in any of

the 1,445 current or former NPL sites. However, the total number of NPL sites evaluated is not

known. As more sites are evaluated, the number of sites at which HDI is found may increase.

This is important because exposure to this substance may harm you and because these sites may

be sources of exposure.

When a substance is released from a large area, such as an industrial plant, or from a container,

such as a drum or bottle, it enters the environment. This release does not always lead to

exposure. You can be exposed to a substance only when you come in contact with it by

breathing, eating, touching, or drinking.

If you are exposed to hexamethylene diisocyanate, many factors determine whether you’ll be

harmed. These factors include the dose (how much), the duration (how long), and how you come

in contact with it. You must also consider the other chemicals you’re exposed to and your age,

sex, diet, family traits, lifestyle, and state of health.

1.1 WHAT IS HEXAMETHYLENE DIISOCYANATE?

HDI is the common name for hexamethylene diisocyanate. It is also known as 1,6-hexamethylene

diisocyanate, 1,6-diisocyanatohexane, Mondur HX, and Desmodur H. It is a pale

yellow liquid with a strong odor. HDI is found in hardening agents for automobile paints.
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1. PUBLIC HEALTH STATEMENT

1.2 WHAT HAPPENS TO HDI WHEN IT ENTERS THE ENVIRONMENT?

HDI is most often found in air near locations where spray paints that contain it as a hardening

agent are used, HDI in the air can enter the soil and water. HDI can also enter the soil if

products containing it are dumped directly onto the soil. HDI can enter the water supply by

washing out of soil that contains it or if products with HDI are dumped directly into water. Once

it is in soil or water, HDI breaks down very quickly, so evaporation into the air is not expected.

HDI also breaks down very quickly in the air, so it probably will not build up in the environment.

1.3 HOW MIGHT I BE EXPOSED TO HDI?

The most common products that contain HDI are called hardening agents and are used to spraypaint

cars. The most common way a person can be exposed to HDI is by breathing air that

contains it as a vapor or mist, like that made when spray-painting a car. Most of the people who

are exposed to HDI work in the automotive painting industry or in areas where this is done. If

you do this kind of work, you can be exposed to more HDI if you do not wear the right protective

safety equipment such as a respirator or mask. If your safety equipment does not fit right or does

not work properly when you are using products that contain HDI, you may be exposed to larger

amounts. You can probably absorb some HDI through your skin. You could also accidentally

swallow HDI if it is on your hands and you do not wash them before eating, drinking, or smoking.

Unless you have been employed in the automobile refinishing or other business where painters

manually mix two-component polyurethane paint systems, it is unlikely that you will be exposed

to large amounts of HDI.
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1. PUBLIC HEALTH STATEMENT

1.4 HOW CAN HDI ENTER AND LEAVE MY BODY?

The most common way HDI enters your body is by breathing air that has it in it. You can

probably absorb some HDI through your skin, and you can also accidentally swallow HDI if it is

on your hands and you do not wash them before eating, drinking, or smoking. Once inside your

body, HDI breaks down very quickly and is quickly excreted in the urine. Some HDI can attach

itself to protein in your blood, but we do not know how long it takes for this form of HDI to

break down and be excreted.

1.5 HOW CAN HDI AFFECT MY HEALTH?

To protect the public from the harmful effects of toxic chemicals and to find ways to treat people

who have been harmed, scientists use many tests.

One way to see if a chemical will hurt people is to learn how the chemical is absorbed, used, and

released by the body; for some chemicals, animal testing may be necessary. Animal testing may

also be used to identify health effects such as cancer or birth defects. Without laboratory

animals, scientists would lose a basic method to get information needed to make wise decisions

to protect public health. Scientists have the responsibility to treat research animals with care and

compassion. Laws today protect the welfare of research animals, and scientists must comply

with strict animal care guidelines.

How HDI affects your health depends on how much is in the air you breathe. Tests using

laboratory animals showed that breathing in high concentrations of HDI can irritate the nose,

eyes, and throat. High concentrations have also caused pneumonia, difficulty in breathing, and

death in some animals. Swallowing high concentrations of HDI also killed laboratory animals.

When placed on the skin of these animals, HDI caused redness, irritation, and irreversible skin

damage. People would probably be affected in many of the same ways if they were exposed to

large amounts of HDI in air.
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1. PUBLIC HEALTH STATEMENT

Many people who breathe in vapors from products with small amounts of HDI for many months

or many years may develop an allergic, asthma-like reaction. Symptoms usually develop very

slowly over a long time (months or years), but they can also develop within a couple of weeks

after first breathing in HDI. At low concentrations, sensitized workers develop a burning

sensation and a feeling of tightness in the chest, a cough (with and without phlegm), fever, and

chills. They have a hard time breathing during their work day when using a product containing

HDI. These signs usually are not seen on weekends, during vacations, or any time the person is

not using a product that contains HDI. These reactions usually begin again soon after the person

returns to work and begins to use the product with HDI.

Some studies in laboratory animals showed that, when breathed in over a long time, HDI did not

produce cancer. No studies that show that HDI can cause cancer in people have been found.

1.6 IS THERE A MEDICAL TEST TO DETERMINE WHETHER I HAVE BEEN

 EXPOSED TO HDI?

Before you ask for special medical tests for HDI, you should talk with your doctor and tell him

you work in a place that uses products that contain HDI. There are no good medical tests for

finding out if you have been exposed to HDI. Some tests are available that measure the

antibodies against HDI your body makes after you have been exposed to it. However, these

blood tests are not very good because they can react with other substances that look like HDI in

your blood. The test can show that you have been exposed to HDI when really you have not been

exposed to it (false positives). Also, some people do not develop antibodies to HDI after they

have been exposed. Another test looks for the breakdown products of HDI in the urine. This test

is only good if you were exposed to HDI within the last 12-15 hours. It is not a good test to find

out if you have been exposed to low amounts of HDI over many months or years. -

Unless you have been employed in the automobile refinishing or other business where painters

manually mix two-component polyurethane paint systems, it is unlikely that you have been

exposed to significant amounts of HDI. Your doctor can give you more information on medical

tests that are available for determining if you have been exposed to HDI.
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1. PUBLIC HEALTH STATEMENT

1.7 WHAT RECOMMENDATIONS HAS THE FEDERAL GOVERNMENT MADE TO

 PROTECT HUMAN HEALTH?

The federal government develops regulations and recommendations to protect public health.

Regulations can be enforced by law. Federal agencies that develop regulations for toxic

substances include the Environmental Protection Agency (EPA), the Occupational Safety

and Health Administration (OSHA), and the Food and Drug Administration (FDA).

Recommendations provide valuable guidelines to protect public health but cannot be enforced by

law. Federal organizations that develop recommendations for toxic substances include the

Agency for Toxic Substances and Disease Registry (ATSDR) and the National Institute for

Occupational Safety and Health (NIOSH).

Regulations and recommendations can be expressed in not-to-exceed levels in air, water, soil, or

food that are usually based on levels that affect animals, then they are adjusted to help protect

people. Sometimes these not-to-exceed levels differ among federal organizations because of

different exposure times (an 8-hour workday or a 24-hour day), the use of different animal

studies, or other factors.

Recommendations and regulations are also periodically updated as more information becomes

available. For the most current information, check with the federal agency or organization that

provides it. Some regulations and recommendations for HDI include the following.

The EPA proposes to list HDI as a hazardous substance that will be required to be reported

under the Emergency Planning and Community Right-to-Know Act for 1986 and the Pollution

Prevention Act of 1990. Currently, some owners and operators using HDI are required to report

every year how much HDI they release into the environment.

The NIOSH-recommended limit for occupational exposure is 0.035 milligrams of HDI per cubic

meter of air (0.035 mg/m3

), which is the same as 5 parts of HDI per billion parts of air (ppb).

There is no established OSHA permissible exposure limit (PEL) for HDI at this time; however, a

similar substance, toluene diisocyanate (TDI), has a PEL of 5 parts per billion (ppb).
HEXAMETHYLENE DIISOCYANATE 6

1. PUBLIC HEALTH STATEMENT

1.8 WHERE CAN I GET MORE INFORMATION?

If you have any more questions or concerns, please contact your community or state health or

environmental quality department or

Agency for Toxic Substances and Disease Registry

Division of Toxicology

1600 Clifton Road NE, Mailstop E-29

Atlanta, GA 30333

Internet address: http://atsdrl.atsdr.cdc.gov:8080

* Information line and technical assistance

Phone: (800) 447- 1544

Fax: (404) 639-6315 or 6324

ATSDR can also tell you the location of occupational and environmental health clinics. These

clinics specialize in recognizing, evaluating, and treating illnesses resulting from exposure to

hazardous substances.

* To order toxicological profiles, contact

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Phone: (800) 553-6847 or (703) 487-4650
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2. HEALTH EFFECTS

2.1 INTRODUCTION

The primary purpose of this chapter is to provide public health officials, physicians, toxicologists, and

other interested individuals and groups with an overall perspective of the toxicology of hexamethylene

diisocyanate. It contains descriptions and evaluations of toxicological studies and epidemiological

investigations and provides conclusions, where possible, on the relevance of toxicity and toxicokinetic

data to public health.

A glossary and list of acronyms, abbreviations, and symbols can be found at the end of this profile.

2.2 DISCUSSION OF HEALTH EFFECTS BY ROUTE OF EXPOSURE

To help public health professionals and others address the needs of persons living or working near

hazardous waste sites, the information in this section is organized first by route of exposure-inhalation,

oral, and dermal; and then by health effect--death, systemic, immunological, neurological, reproductive,

developmental, genotoxic, and carcinogenic effects. These data are discussed in terms of three exposure

periods-acute (14 days or less), intermediate (15-364 days), and chronic (365 days or more).

Levels of significant exposure for each route and duration are presented in tables and illustrated in

figures. The points in the figures showing no-observed-adverse-effect levels (NOAELs) or lowestobservedadverse-effect levels (LOAELs) reflect the actual doses (levels of exposure) used in the studies.

LOAELS have been classified into “less serious” or “serious” effects. “Serious” effects are those that

evoke failure in a biological system and can lead to morbidity or mortality (e.g., acute respiratory distress

or death). “Less serious” effects are those that are not expected to cause significant dysfunction or death,

or those whose significance to the organism is not entirely clear. ATSDR acknowledges that a

considerable amount of judgment may be required in establishing whether an end point should be

classified as a NOAEL, “less serious” LOAEL, or “serious” LOAEL, and that in some cases, there will

be insufficient data to decide whether the effect is indicative of significant dysfunction. However, the

Agency has established guidelines and policies that are used to classify these end points. ATSDR

believes that there is sufficient merit in this approach to warrant an attempt at distinguishing between

“less serious” and “serious” effects. The distinction between “less serious” effects and “serious” effects
HEXAMETHYLENE DIISOCYANATE 8

2. HEALTH EFFECTS

is considered to be important because it helps the users of the profiles to identify levels of exposure at

which major health effects start to appear. LOAELs or NOAELs should also help in determining

whether or not the effects vary with dose and/or duration, and place into perspective the possible

significance of these effects to human health.

The significance of the exposure levels shown in the Levels of Significant Exposure (LSE) tables and

figures may differ depending on the user’s perspective. Public health officials and others concerned with

appropriate actions to take at hazardous waste sites may want information on levels of exposure

associated with more subtle effects in humans or animals (LOAEL) or exposure levels below which no

adverse effects (NOAELs) have been observed. Estimates of levels posing minimal risk to humans

(Minimal Risk Levels or MRLs) may be of interest to health professionals and citizens alike.

Estimates of exposure levels posing minimal risk to humans (Minimal Risk Levels or MRLs) have been

made for hexamethylene diisocyanate. An MRL is defined as an estimate of daily human exposure to a

substance that is likely to be without an appreciable risk of adverse effects (noncarcinogenic) over a

specified duration of exposure. MRLs are derived when reliable and sufficient data exist to identify the

target organ(s) of effect or the most sensitive health effect(s) for a specific duration within a given route

of exposure. MRLs are based on noncancerous health effects only and do not consider carcinogenic

effects. MRLs can be derived for acute, intermediate, and chronic-duration exposures for inhalation and

oral routes. Appropriate methodology does not exist to develop MRLs for dermal exposure.

Although methods have been established to derive these levels (Barnes and Dourson 1988; EPA 1990),

uncertainties are associated with these techniques. Furthermore, ATSDR acknowledges additional

uncertainties inherent in the application of the procedures to derive less than lifetime MRLs. As an

example, acute inhalation MRLs may not be protective for health effects that are delayed in development

or are acquired following repeated acute insults, such as hypersensitivity reactions, asthma, or chronic

bronchitis. As these kinds of health effects data become available and methods to assess levels of

significant human exposure improve, these MRLs will be revised.

A User’s Guide has been provided at the end of this profile (see Appendix B). This guide should aid in

the interpretation of the tables and figures for Levels of Significant Exposure and the MRLs.
HEXAMETHYLENE DIISOCYANATE 9

2. HEALTH EFFECTS

2.2.1 Inhalation Exposure

Inhalation is the most common route of exposure for hexamethylene diisocyanate (HDI). Over 99% of the

HDI manufactured in or imported into the United States is used to make HDI prepolymers, also

lmown as polyisocyanates. These prepolymers, in turn, are used by paint formulators as hardeners in

two-component polyurethane paint systems, used primarily for painting automobiles. The remaining

fraction of HDI production (<1%) is sold as solid rocket fuel binders and as paint thickeners (CMA

1997). At the time of manufacture, biuret prepolymer contains about 0.7% monomeric HDI. During

storage, the monomeric content can increase to as much as 1.6% due to in situ breakdown of the biuret

(Hulse 1984). The monomeric content of HDI trimer is 0.2% at the time of manufacture and remains

stable at this level during storage. Human inhalation exposures reported in studies discussed in this

chapter are typically in the range of 0.001-0.02 ppm; in many cases, a dose could not be determined.

Because the vast majority of HDI is used to make prepolymers used in paint systems, most of the reports

concerning the respiratory toxicity of HDI focus on that source of exposure. Approximately 50% of HDI

prepolymers are biurets, which contain 0.7-1.6% monomer. The other 50% of HDI prepolymers are

trimers, which contain 0.2% monomer. Because paint formulators typically add solvents to the

prepolymer, the percentage of monomer in the paint hardener is usually less than these percentages. In

large painting operations, the paint hardener is mixed with the paint in closed systems, so that workers

are exposed only to the mixture, further diluting the percentage of monomer. HDI monomer content in

the mixed paints is 0.006-0.5%. As discussed below, workers in the studies discussing the respiratory

effects of HDI would have been exposed to a combination of HDI monomeric and polymeric forms,

making it difficult to determine whether the observed effects were due to the monomer, polymer, or both.

Monomeric HDI vaporizes quite easily, leading to inhalation and dermal exposures of workers who come

in contact with the air containing the HDI vapors. Monomeric HDI, like other diisocyanates, can produce

both a local irritation to the nasal and respiratory tract and an asthma-like condition in sensitized people

at air vapor concentrations (range, from approximately 0.0002 to 0.02 ppm) (Malo et al. 1983; Tornling

et al. 1990). Monomeric HDI also produces clinical signs of respiratory toxicity that are similar to the

other diisocyanates (e.g., toluene diisocyanate). At concentrations greater than 0.0006 ppm, burning and

irritation of the nose, throat and mucous membranes of the lungs; cough; laryngitis; bronchitis; tightness

of the chest; hoarseness; pulmonary edema; emphysema; car pulmonale; and an asthma-like syndrome

have also been reported (Grammar et al. 1988; Malo et al. 1983; Von Burg 1993). Other clinical signs
HEXAMETHYLENE DIISOCYANATE 10

2. HEALTH EFFECTS

may include more vague symptoms, such as headache, fatigue, and an asthma-like condition (Von Burg

1993). Overall, information on the total health effects of HDI on humans and animals is limited.

As stated earlier, over 99% of the monomeric HDI manufactured in the United States is converted into

polymeric forms (biuret and trimer), which are then sold to paint formulators for use in the hardening

component of two-component polyurethane paint systems. HDI biuret and trirner can induce respiratory

and immunological reactions similar to HDI monomer in both humans (Alexandersson et al. 1987; Belin

et al. 1981; Cockcroft and Mink 1979; Grammar et al. 1988; Usui et al. 1992; Vandenplas et al. 1993)

and animals (Ferguson et al. 1987; Weyel et al. 1982). Unlike monomeric HDI, polymeric forms

typically have a very low vapor pressure, making it very unlikely to vaporize at room or paint shop

ambient temperatures. Exposures to polymeric forms, primarily via the inhalation and dermal routes, and

secondarily by the oral route, occur when the paint/hardener combination is ejected from the spray nozzle

onto a metal surface. During the spraying process, small droplets of the monomeric/polymeric mixture

suspended in the surrounding air is inadvertently breathed in by or lands on the skin of an exposed

worker. The exposures discussed in many of the reports mentioned earlier that describe the inhalation

toxicology of monomeric HDI in humans were probably combination exposures of the monomeric form

and polymeric forms of HDI, making it difficult to determine whether the respiratory and immunologic

effects observed in humans and laboratory animals are induced by either one or both forms of HDI.

2.2.1.1 Death

No studies were located regarding death in humans after inhalation exposure to HDI.

Several reports of death after inhalation exposures of acute-duration in laboratory animals were located.

In one study, the acute inhalation toxicity of the HDI and various HDI pre-polymer was tested on male

and female Wistar rats. The rats (n=l0 males and 10 females per group) were exposed to 105, 143,259,

341,383,443,575,589, or 719 mg HDI/m3

 (15.3,20.7,37.6,49.4,55.5,64.2,83.4, 85.4, or 104.3 ppm)

in inhalation chambers for 4 hours and observed for 4 weeks after exposure. Deaths approximately

followed a dose-response pattern in both sexes. Death was not observed in any of the rats in the 105 or

143 mg HDI/m3

 (15.3, 20.7 ppm) groups. Deaths occurred in 4 of 10 males and 1 of 10 females exposed

to 259 mg HDI/m3

 (37.6 ppm); 9 of 10 males and 5 of 10 females exposed to 341 mg HDI/m3 (49.4 ppm);

7 of 10 males and 6 of 10 females exposed to 383 mg HDI/m3

 (55.5 ppm); 8 of 10 males and 8 of

10 females exposed to 443 mg HDI/m3

 (64.2 ppm); 8 of 10 males and 9 of 10 females exposed to 575 mg
HEXAMETHYLENE DIISOCYANATE 11

2. HEALTH EFFECTS

HDI/m3

 (83.4 ppm); 9 of 10 males and 9 of 10 females exposed to 589 mg HDI/m3 (85.4 ppm); and 10 of

10 males and 10 of 10 females exposed to 719 mg HDI/m3

 (104.3 ppm). Deaths occurred between 1 and

20 days after exposure. From this data, the concentration of HDI that resulted in death to 50% of the

exposed population, (LC50) was calculated to be 3 10 mg/m3

 (45 ppm) (Kimmerle 1976).

Groups of 4 male albino ChR-CD rats were exposed to various concentrations of HDI for 4 or 8 hours.

When rats were exposed to 370 ppm, they died after 2-3 hours of exposure. Prior to death, rats showed

signs of irritation, gasping, and convulsions. Tracheitis, pleural effusion, and small areas of pulmonary

hemorrhage were observed at necropsy but were not considered extensive enough to cause death. Rats

survived exposures to 5-72 ppm HDI (Haskell Laboratory 1961). In a similar study, groups of 4 male

albino ChR-CD rats were exposed to 30 ppm HDI for 4 hours daily for 10 days over a 2-week period.

Two of 4 animals (50%) of the HDI-exposed rats died (one during the 8th exposure and the other 6 days

after the last exposure). Bronchitis with purulent obstruction of some bronchial branches was observed

in the rat that died during exposure. Bronchopneumonia was observed in the rat that died after exposure

(Haskell Laboratory 1961).

In another study, male albino Sprague-Dawley rats were exposed to HDI air concentrations of 3,4,6,11,

22,44, or 88 ppm for 6 hours. At 44 ppm, 1 of 6 rats failed to survive the exposure, while 1 additional

rat died within 7 days after exposure to 44 ppm of HDI. All of the rats at the 88 ppm dose died during

exposure. No other deaths were reported at either 7 or 15 days after exposure in any of the other

treatment groups. In the rat that died immediately after exposure to 44 ppm of HDI, lung changes were

limited to moderate congestion; the rats that died at 88 ppm exposure to HDI had moderate-to-severe

pulmonary edema and congestion, which may be indicative of acute irritation and/or heart failure (Dow

Chemical Co. 1964)

Male English smooth-haired guinea pigs were exposed to 0.5 ppm HDI for 6 hours, 1.8 ppm for 2 hours, or

4 ppm for 3 hours. At the 4 ppm dose level, 50% of the animals died within 1 hour during exposure (Karol et

al. 1984).

Fewer studies were located on death in laboratory animals exposed for intermediate and chronicdurations.

One study by Mobay Corporation (1984) determined the toxicity of HDI via inhalation

exposures in Sprague-Dawley rats over a 3-week period. Male and female rats were exposed (head-only)

to HDI vapors at average concentrations of 0.005,0.0175,0.15, or 0.3 ppm for 5 hours a day, 5 days a
HEXAMETHYLENE DIISOCYANATE 12

2. HEALTH EFFECTS

week for 3 weeks. No mortality was observed in any of the treatment groups at any time during or after

exposures. In another unrelated study of longer duration, Fischer 344 rats of both sexes were exposed to HDI

(whole body exposure) over a period of 90 days. Rats were exposed to 0, 0.011,0.041, or 0.143 ppm HDI in

air for 6 hours per day for 66-69 days over a period of approximately 13 weeks. No deaths occurred in any

of the treatment groups during or after exposures (Mobay Corporation 1988).

One study was identified that described the death rates of rats exposed to HDI for a chronic duration. Groups

of 60 male and 60 female Fischer 344 rats were exposed (whole body) to 0,0.005,0.025, or 0.175 ppm HDI

for 2 years. None of the three inhaled concentrations of HDI was shown to have an effect on mortality in

exposed rats compared to control animals (Mobay Corporation 1989).

The LOAEL values resulting in mortality in all species are recorded in Table 2-l and plotted in Figure 2-l.

2.2.1.2 Systemic Effects

Studies regarding the systemic effects that have been observed in humans and animals after inhalation

exposure to HDI are discussed below. The highest NOAEL values and all LOAEL values from each reliable

study for each systemic effect in each species and duration category are recorded in 2-l and plotted in

Figure 2- 1.

Respiratory Effects. Respiratory effects due to inhalation of HDI are the subject of most of the

literature on HDI toxicity, with most reports on humans based on individual case studies (Belin et al.

1981; Cockcroft and Mink 1979; Patterson et al. 1990; Vandenplas et al. 1993). One report described the

case of a 56-year-old man who worked as a foreman in a garage where automobile painting was done and

consequently was exposed intermittently to paints containing HDI for 5-6 years. He reported having

developed respiratory and systemic reactions after exposure to paints (which contained 7% polymeric

HDI) used in the garage. Episodes of shortness of breath, wheezing, malaise, and chills were reported,

with symptoms occurring in the late afternoons of working days and lasting for several hours thereafter.

In an attempt to confum that HDI was the compound responsible, the man was removed from the garage

environment for several weeks and lung parameters were measured, including forced expiratory

volume-l second (FEV1), forced vital capacity (FVC), vital capacity (VC), forced residual capacity

(FRC), total lung capacity (TLC), among others. Body temperature and blood samples were also











HEXAMETHYLENE DIISOCYANATE 24

2. HEALTH EFFECTS

collected. In an inhalation test exposure to one of the spray paints in which the HDI concentration was

measured to be 0.02 ppm (polymeric forms of HDI were not measured), no abnormalities in clinical signs

or lung parameters were noted during the first hour after exposure. At one hour, a burning sensation

began to occur in his chest, followed by a cough, and a drop in FVC (but no change in FEV1/FVC ratio).

A productive cough was later noted with progression to chills, headache, and malaise at the third hour

after exposure. The man was prostrate at 6 hours after exposure. Other clinical signs included bibasal

inspiratory crackles and an increase in body temperature, an elevated white blood cell (WBC) count, and

a normal chest radiograph. Lung functions at 6 hours after exposure showed decreased TLC and VC,

while FRC was higher. Six hours after exposure, clinical signs improved. One day after exposure, lung

volumes and FEV1 also improved. This report suggested that the HD1 and/or the HDI prepolymer is

capable of inducing both an alveolar reaction (characterized by fever, inspiratory crackles, elevated WBC

count and a drop in PaO2) and a bronchial reaction (characterized by drops in FEV1/FVC ratio and an

increase in FRC). The reaction was classified as a late obstructive and restrictive breathing defect after

exposure to HDI (Malo et al. 1983).

Another case involving an auto spray painter who was assumed to have been exposed to HDI was

reported. He worked most of his life as a spray painter (primarily of automobiles). The worker spent

about 25-33% of his time spraying paint on vehicles in a poorly ventilated shop, but he did wear a

respirator with an outside air supply. At one point during his work he began to notice shortness of

breath, chest tightness and wheezes, and dry cough, but no nasal problems. Symptoms occurred shortly

after he began spraying paint and would generally worsen through the night. Attacks could persist for a

week before resolving. The worker identified three paints that he had used that seemed to contribute the

most to his symptomatology, all three of which had a hardener consisting of dimeric HDI. The worker

was challenged by breathing 0.0034 ppm HDI for 15 minutes, 0.0167 ppm for 15 minutes, and 0.007 ppm

for 60 minutes and respiratory parameters (FEV, and methacholine challenge) were measured. The study

failed to induce a bronchoconstriction response in the worker after exposure to HDI. Challenge with

methacholine induced a 20% decline in FEV, after 340 units; however, a subsequent challenge with

methacholine following all of the HDI exposures gave a 20% decline in FEV1 with 360 units. These .

results indicated that the worker had a mild broncho-hyperresponsiveness which did not change

significantly after exposures to HDI. Either the worker is not allergic to HDI and/or the HDI dimer, or

the worker is allergic to just the HDI dimer, which went undetected in this study because the patient was

not tested with the dimer (Tulane Medican 1982a). The authors also indicate that the worker was

involved in the preparatory bodywork prior to spray painting for which he used epoxy resins which he
HEXAMETHYLENE DIISOCYANATE 25

2. HEALTH EFFECTS

both applied and sanded. It has been lmown for a considerable time that many amine curing agents for epoxy

resins are skin sensitizers; some, such as diethylene triamine, have been shown to be sensitizers to both skin

and respiratory tract (ACGIH 1994). This study illustrates the difficulty of relating a specific effect in

humans to the complex exposure situations often encountered in the workplace.

Short-term pulmonary function studies were performed to determine the daily and weekly changes in

pulmonary function of a group of isocyanate/solvent-exposed workers in a urethane molding department

(n=17) when compared with non-exposed workers (n=20). The average age was 30 ±7 years for the exposed

workers and 35 ±10 years for controls; 47% of the exposed workers were smokers compared to 15% of the

controls. Mean personal air samples indicated exposure to 1.55 ±I.63 ppb (approximately 0.002 ppm) HDI

in exposed workers compared to 0.67 ±0.25 ppb (approximately 0.0007 ppm) in controls. Mold operators

were exposed to both isocyanates and volatile organic compounds (VOC) as the urethane paint was sprayed.

While spraying the paint, mold operators wore half-face respirators with cartridges and pre-filters for

protection against organic vapors. Gloves, hearing protection, and safety glasses were also worn. Pre- and

post-shift pulmonary function tests, including forced vital capacity (PVC), forced expiratory volume in

1 second (FEV1), and FEV1% FVC, were performed on 5-7 people from the exposed group and 5-7 people

from the control group on the Monday and Friday of each week. All FVC and FEV1 actual values were above

the predicted values. There were no significant differences between groups in pulmonary function

parameters. A higher prevalence of respiratory symptoms were reported by controls than exposed workers.

There were no significant changes in FVC values for either exposed or control workers during the work shift

on Monday or during the workweek. FVC values for female workers in the exposed group did increase

during the work shift in comparison to females in the control group, however, this was increase was not

considered significant (p<0.l). This increase resulted in a significant increase in FVC values for the entire

exposed group for Friday in comparison to controls (p<0.05). No significant FEV1 changes were observed.

Based upon the workplace survey, it appeared that exposures were well controlled. The authors suggested

that this may have contributed to the negative findings. In the same report, a group of workers with similar

population characteristics, but with longer-term exposure (minimum of 1 year but not more than 2.5 years)

showed a significant long-term reduction in their FVC (P<0.05) and FEV1 (p<0.00l). Mean air samples

indicated exposure to 0.0010 ±0.0004 ppm HDI, 0.29 ±0.35 ppm HDI polyisocyanate, and 0.00045 ppm in

these isocyanate/solvent exposed workers. These changes were not observed in non-exposed or solvent

exposed groups. A significantly greater proportion of isocyanate/solvent-exposed workers developed

respiratory symptoms than non-exposed (Akbar-Khanzadeh and Rivas 1996).
HEXAMETHYLENE DIISOCYANATE 26

2. HEALTH EFFECTS

Several experimental studies have described the respiratory effects of HDI after acute inhalation

exposures in laboratory animals. The acute inhalation toxicity of the aerosols of HDI and various

pre-polymer products were tested on male and female Wistar rats exposed to 105, 143,259, 341, 383,443,

575,589, or 719 mg HDI/m3

 (15.3,20.7,37.6,49.4,55.5,64.2, 83.4, 85.4, 104.3 ppm) ininhalationchambers

for 4 hours. All HDI-exposed rats exhibited signs of labored breathing at all exposure concentrations.

Lung edema and pneumonia were observed upon necropsy (Kimmerle 1976).

Male English smooth-haired guinea pigs were exposed to 0.5 ppm HDI for 6 hours, 1.8 ppm for 2 hours, or

4 ppm for 3 hours. Animals exposed to 1,8 ppm displayed severe respiratory irritation as evidenced by slowed

respiratory rate and labored breathing, with high death rates at the highest dose (Karol et al. 1984).

The time-response and concentration-response relationships of HDI as sensory irritants was evaluated

in Male Swiss Webster mice (4 per group). Respiratory rates were recorded by plethysmography prior to,

during, and following exposure. With the time-response relationships, the response was gradual with time,

reaching a first maximum within l0-20 minutes of exposure and continuing to increase slowly, reaching a

plateau within 180 minutes. Recovery was rapid with short exposures and very slow for longer exposures,

regardless of the level of response induced in each exposure group. For concentration-response relationships,

values for HDI that produced a 50% decrease in respiration rate (RD50) were 0.96,0.35,0.35,0.22, and

0.17 ppm for the 10-, 30-, 60-, 120-, and 18Ominute exposures, respectively (Sangha et al. 1981).

The mouse sensory irritation potentials of HDI, toluene-2,4diisocyanate (TDI), isocyanatoethyl

methacrylate (IEM), and isocyanatoethyl propionate (IEP) were determined in another study, Male Swiss

albino CD- 1 mice were exposed for a 2minute control period with room air, 3 minutes of exposure to

one of 4 isocyanate vapors, then 2 minutes of recovery with room air. The range of concentrations tested

were: 0-0.82 ppm for HDI, 0-3.44 ppm for TDI, 0-2.5 1 ppm for IEM, and 0 -l.95 ppm for IEP. The

concentration that produced RD50 was determined. HDI was determined to be approximately 3 times

more irritating than TDI, IEM, and IEP, with an RD50 of 0.36 ppm for a 3-minute exposure. Even though

HDI was the most irritating, recovery from exposure was rapid and respiration rate was essentially

normal at all test concentrations 2 minutes post-exposure. TDI, IEM, and IEP vapors were similar in

sensory irritation potential with RD50 values of 1.28 ppm, 1.14 ppm, and 0.98 ppm, respectively, for
HEXAMETHYLENE DIISOCYANATE 27

2. HEALTH EFFECTS

3-minute exposures. Exposed mice recovered slowly from TDI exposure (E.I. DuPont de Nemours

1978).

In another acute study of slightly longer duration, male Fischer 344 rats were exposed to 0.11, 0.18, 0.30,

0.88, 1.75, 2.46, or 5.58 ppm concentration of HDI for 30 minutes. The only clinical signs monitored for

HDI-induced respiratory irritation were changes in the average respiratory rate normalized to control rats.

The concentration associated with an RD50 was calculated using the data obtained from changes in

respiratory rates in all exposure groups. The inhalation 30-minute RD50 of HDI in rats was calculated to

be 1.42 ppm, with 95% confidence intervals from 1.03 to 2.09 ppm, with a correlation coefficient of 0.99.

Overall, the time-response curves showed that the onset of the response was rapid, with a major decrease

in respiratory rate occurring within the first 5 minutes. After this time, tolerance to HDI was observed,

manifested by slow increases in respiratory rates, but still considerably lower than those observed in

control animals. The pattern was most clear at the middle concentration tested (1.75 ppm). Decreases in

average respiratory rates were dose-dependent, ranging from 2 to 66% (Mobay Corporation 1982).

Male albino ChR-CD rats were exposed to 5, 11,26,27, or 370 ppm HDI for 4 or 8 hours. When rats

were exposed to 370 ppm, they died after 2-3 hours of exposure. Prior to death, rats showed signs of

irritation, gasping, and convulsions. Tracheitis, pleural effusion, and small areas of pulmonary

hemorrhage were observed, but were not considered extensive enough to cause death. Rats survived a

4-hour exposure to 72 ppm but showed severe respiratory impairment, cyanosis, and signs of respiratory

irritation during exposure. The respiratory impairment progressed to labored breathing and gasping

during the exposure. Bronchopneumonia and bronchiectasis were observed in all of the rats exposed to

this HDI concentration when sacrificed 14-16 days later. Rats also survived exposure to 27 and 26 ppm

for 4 and 8 hours, respectively, but showed similar, though less severe, clinical and histopathological

signs of toxicity. Rats exposed to 11 ppm for 4 hours showed the same, though less severe, clinical signs

of toxicity seen at higher concentrations without tissue changes (Haskell Laboratory 1961).

In another study of longer duration, 4 male albino ChR-CD rats were exposed to 30 ppm HDI for 4 hours

daily for 10 days over a 2-week period. In the 2 rats that died (one during the 8th exposure and the other

6 days after the last exposure), bronchitis with purulent obstruction of some bronchial branches was

observed in the rat that died during exposure; bronchopneumonia was observed in the other dead rat.

Respiratory impairment was observed, which included labored breathing and irritation (Haskell

Laboratory 1961).
HEXAMETHYLENE DIISOCYANATE 28

2. HEALTH EFFECTS

Male albino Sprague-Dawley rats were exposed to HDI air concentrations of 3, 4, 6, 11,22,44, or

88 ppm for 6 hours. Surviving animals were kept as long as 15 days after the single exposure, with some

animals sacrificed between 0 and 15 days after exposure to determine lung damage due to HDI toxicity.

At all exposure concentrations, except 88 ppm, nasal irritation was observed clinically at the beginning

of exposure but did not progress during the exposure period. Gross necropsies showed hemorrhagic

areas of the lungs in rats exposed to 88 ppm HDI. Animals sacrificed at timed-intervals up to 2 weeks

after exposure found no histopathological changes in the lung related to HDI exposure in the 3,4, 6, 11,

22, or 44 ppm exposure groups; however, in the one rat that died immediately after exposure to 44 ppm

of HDI, lung changes were limited to moderate congestion. The rats that died at 88 ppm exposure to HDI

had moderate to severe pulmonary edema and congestion which may be indicative of acute irritation

and/or heart failure (Dow Chemical Co. 1964).

HDI exposures have also been conducted in dogs. Two female Beagle dogs were exposed to <2 ppm of

HDI 6 times (2 hours each) over a 10-day period (the frequency between exposures was not reported).

Average HDI concentrations were 0.28, 0.32, 0.39, 0.55, 0.89, and 1.07 ppm for each trial. Severe nose,

throat, and eye irritation was observed in the dogs at all concentrations of HDI tested (0.27-l.43 ppm).

Generally, the severity of these signs of irritation was directly correlated to the inhaled HDI

concentration. Recovery was complete by the end of each exposure day (Haskell Laboratory 1961).

A study by Ferguson et al. (1987) reported on HDI polymer exposure for acute-duration periods. In one

study, groups of male English short-haired guinea pigs were exposed to 8-121 mg/lm3

 (0.4-6.2 ppm) HDI

trimer for 3 hours by inhalation. Tidal volume and respiratory frequency were measured during

inhalation of room air (5 minutes before and after challenge) and during a 7-minute challenge with 10%

CO, in 20% O2 and 70% N, as indicators of ventilatory response. Particle sizes had a mass median

diameter of 0.38 pm at low concentrations of HDI polymer and 0.73 pm at high exposures, with 98% of

all particles (by weight) measuring <3 μm. Four animals were exposed to 22 mg/m3

 (1.l ppm) and

84 mg/m3

 (4.3 ppm) HDI trimer for 3 hours per day for one exposure, challenged with CO2 immediately

after exposure; 7 hours after exposure, the animals were sacrificed, and the lungs and trachea removed

and weighed. Another set was exposed identically, sacrificed at 24 hours, and organs extracted and

weighed. The animals displayed a concentration-dependent increase in respiratory rate and decrease in

tidal volume when challenged with CO2 as well as coughing and apnea. Their ventilatory response to

10% CO2 was abnormal and characteristic of a lung restriction response. Some airflow limitation was

seen during expiration, but this occurred more often during air breathing than during CO2 challenge. No
HEXAMETHYLENE DIISOCYANATE 29

2. HEALTH EFFECTS

significant changes in lung weights were noted in either exposure group compared to controls exposed to

acetone only or to controls exposed to air only. Similar decreases in respiratory rates were found in male

Swiss Webster mice acutely exposed to 1.3-6.7 ppm HDI trimer (Weyel et al. 1982).

In the same study (Ferguson et al. 1987), groups of 4 male English short-haired guinea pigs were exposed

to HDI trimer via inhalation for 3 hours per day for 5 or 11 consecutive days. On days 12 and 13, the

animals received no exposure or challenge and then were exposed for 3 hours on day 14. A final

exposure was performed on day 25. The exposure concentration during the 5-day exposure ranged from

27.5 to 34.4 mg/m3

 (1.4 ppm-1.8 ppm) and during the 1l-day exposure ranged from 65.1 to 74.4 mg/m3

((3.3-3.8 ppm). Tidal volume and respiratory frequency were measured during inhalation of room air

(5 minutes before and after challenge) and during a 7-minute challenge with 10% CO2 in 20% O2 and

70% N2, as indicators of ventilator-y response as before. With daily exposures repeated for

11 consecutive days, guinea pigs began to adapt to the exposures as indicated by a return to a normal

ventilator-y response to CO2. This adaptation occurred within the first 5 days of exposure, with a

maximum change in tidal volume and respiratory frequency occurring 24 hours after the first exposure.

From days 6 to 11, there was a demonstrable effect, but the level of response was much less than that

following the first exposure. No cumulative effect could be demonstrated. No significant changes in

lung weights were noted in either exposure group compared to controls.

One clinical report of a human exposed to HDI for an intermediate-duration was reported. A 60-year-old

male automobile paint sprayer was examined following health complaints, which included shortness of

breath, a productive cough, and an intermittent fever (usually about 6 hours after he finished work) of

l-month duration. Symptoms were reported to subside on weekends. He had used paint materials

containing HD1 for about one month and had worked without a protective mask. Clinical signs were

noted to have begun when he started to use a paint containing HDI. A chest X-ray taken after exposure

showed diffuse ground-glass infiltrates, with focal fine nodular infiltrates. A transbronchial lung biopsy

revealed chronic inflammatory cells diffusely infiltrating the lung interstitium and cellular bronchiolitis.

Non-necrotizing granulomas were not found. Increased number of activated cytotoxic T lymphocytes in

the bronchoalveolar lavage fluid (BALF) were also discovered. A gradual improvement in his symptoms

was observed once the worker began wearing a mask containing activated charcoal during exposure to

HDI (Usui et al. 1992).
HEXAMETHYLENE DIISOCYANATE 30

2. HEALTH EFFECTS

Compared to the acute studies, there are fewer reported studies on the toxicity of inhaled HDI for an

intermediate-duration in laboratory animals. Male and female Sprague-Dawley rats were exposed (head-only)

to HDI vapors at average concentrations of 0.005, 0.0175, 0.15, or 0.3 ppm for 5 hours a day,

5 days a week for 3 weeks. Five animals per sex per exposure concentration were sacrificed at the end of

the exposure period; the balance of the animals were allowed a 2-week period to recover from the

exposures and then sacrificed. All animals exposed to all concentrations of HDI exhibited varying

degrees of irritation of eyes and/or noses during exposure and at 1 hour post-exposure, with all animals

appearing normal the following morning. No clinical signs of toxicity were observed during the nonexposure

days (weekends). All animals exposed to 0.15 ppm were sneezing during the last week of

exposure while the animals exposed to 0.3 ppm started to sneeze at the end of the first week of exposure

and then sneezed randomly during the second and third week of exposure. The author attributed the

sneezing to a local and severe irritation of the nasal cavity. The severity of the irritation in the animals

exposed to the 0.005 ppm level was similar to that of controls (slightly irritated eyes and/or noses at

1 hour post-exposure). Histologic changes in the nasal cavity, trachea, and larynx were noted. Changes

in the nasal tract included hemorrhage, inflammatory exudate, and epithelial changes; the epithelial

changes varied from vacuolation and disruption of epithelial cells to a more chronic squamous

metaplasia, characterized by a loss of cilia and change from the normal ciliated pseudostratified columnar

cell type to a more flattened (squamous) type of epithelium with minimal-to-mild keratinization.

Changes in the larynx included focal accumulations of inflammatory cells in the submucosa and a

minimal-to-mild hyperplasia of the epithelium. The nasal changes occurred in a dose-related manner. At

0.3 ppm, 80-90% of the animals were affected with moderate severity, while at 0.15 ppm, 50-70% were

affected with a slightly milder severity. At 0.005 and 0.0175 ppm, the changes were minimal-to-mild in

severity and similar to controls, even though the incidence was slightly higher in the 0.0175 ppm males.

The severity of the changes in the trachea and larynx was not dose-related (Mobay Corporation 1984).

The NOAEL of 0.005 ppm was used to derive an intermediate-duration inhalation MRL of 3.0x10-5 ppm.

Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm HDI in air for 6 hours a day 66-69 days

over a period of approximately 13 weeks. Animals were exposed to HDI in 2-cubic-meter-chambers

(whole body). All rats were sacrificed at week 14. Rhinorrhea and ocular opacity were observed in all

groups of rats tested, including control animals, and animals did not exhibit a concentration-dependent

response. No compound-related toxic effects were noted by changes in lung weights or lung weight to

body weight ratios at gross necropsy. Although many histopathologic lesions were found in the many

organs examined in this study, the only lesions attributable to HDI toxicity (at 0.143 and 0.041 ppm
HEXAMETHYLENE DIISOCYANATE 31

2. HEALTH EFFECTS

doses only) were hyperplasia and/or squamous metaplasia, mucous cell hyperplasia and inflammation in

the anterior nasal cavity of both sexes of rats. The author determined that the health effects of HDI at an

inhaled concentration of 0.011 ppm in rats were very mild, and that this concentration could be

considered to be a threshold level (Mobay Corporation 1988).

Chronic-duration inhalation exposures to HDI in humans are a more frequently reported phenomenon,

exhibiting mixed results on health effects. Exposure to low doses of HDI over long periods of time have

shown no changes in respiratory function. In one study at a plant in Freeport, Texas, a matched casecontrol

epidemiologic study was undertaken to determine if chronic exposure to HDI resulted in an added

decline in respiratory function above what is expected from aging alone. Workers were identified as

having a potential for HDI exposure (n=30) or not (controls, n=30) and then matched according to age,

height, smoking history, sex and race. All subjects were male. The average age for HDI-exposed

workers was 37.7 ± 8.7 years versus 36.9 ±7.8 years for controls. One-third of all HDI-exposed and

control workers fell into the categories of current smokers, previous smokers, or having never smoked.

The mean annual change on pulmonary function tests results, including forced vital capacity (FVC),

forced expiratory volume in 1 second (FEV1), and mean forced expiratory flow during the middle half of

FVC (FEF25-75%) were compared statistically. No estimation of an average exposure for workers

potentially exposed to HDI was reported. However, the authors speculated that the actual average

exposures, when considering the protection from respirators, was below 5 ppb. No statistically

significant differences in pulmonary function tests were observed among the workers with potential HDI

exposure and controls. The authors noted a number of study design flaws, including a small sample, a

large variance in pulmonary function test values, inability to define an exposure dose, and malfunctioning

of the industrial hygiene monitoring devices (interference from high humidity and NOx from welding

fumes). Also, radio frequency emissions from portable radios adversely affected personal dosimetry

measuring devices, which also gave falsely high readings (Shepperly and Hathaway 1991).

In a related study, a matched case-control epidemiologic study to determine if chronic exposure to HDI

resulted in an added decline in respiratory function above what is expected from aging alone was

undertaken again at a plant in Freeport, Texas from 1988 to 1991 (see Shepperly and Hathaway 1991).

This report added data from additional pulmonary function tests obtained in 1991, 1992, and 1993.

Workers were again identified as having a potential for HDI exposure (n=41) or not (controls, n=43) and

were matched according to age and smoking history. All subjects were male. The average age for

HDI-exposed workers was 42.6 years versus 35 years for controls. The smoking history of HDI-exposed
HEXAMETHYLENE DIISOCYANATE 32

2. HEALTH EFFECTS

was 34% current smokers, 37% previous smokers, and 29% never smoked. The smoking history of

control was 40% current smokers, 26% previous smokers, and 35% never smoked. Area monitoring

estimated HDI levels of 7, 5.4, 3.4, 2.3, 4.7, and 0.5 ppb for 1988, 1989, 1990, 1991, 1992, and 1993,

respectively. Personal monitoring of HDI levels ranged from 0.7 to 3.9 ppb in HDI-exposed workers in

1992 and 0.6-l .8 ppb in 1993. Again, no statistically significant differences in incidence of respiratory

complaints or in pulmonary function tests were observed among the workers with potential HDI exposure

and controls. As in the Shepperly and Hathaway (1991) study, a major limitation of this study was the

inability of the authors to define a dose for the HDI-exposed workers during this 5-year period.

According to the authors, workers may have been exposed to air concentrations of HDI ranging from

7 ppb in 1988 to 0.5 ppb in 1993. A number of air samples were taken each year; however, no data were

provided as to the variance between air samples taken each year in the areas where the HDI workers were

exposed; only mean values were supplied. Respirators worn by some of the exposed workers may have

further decreased the actual amounts of HDI breathed in. No estimates of dose was provided by the

authors for the workers using the respirators (DeWilde and Hathaway 1994).

Other studies have indicated respiratory effects from chronic exposure to HDI. The radioallergosorbent

test (RAST) method and skin tests were used to evaluate the significance of type I sensitization, its

incidence, and relationship to respiratory dysfunctions in a large population of isocyanate-exposed

workers. A group of 62 1 workers engaged in isocyanate processing for a period of 2 weeks to 40 years

were studied. Sex of the workers was not reported. Of these workers, 183 had contact with TDI; 66 with

diphenylmethane diisocyanate (MDI); 82 with HDI; 220 with a combination exposure of TDI, MDI, and

“other aromatic isocyanates;” 30 with a combination of TDI, MDI, and HDI; and 32 with other

isocyanates. Air concentration data (where available) tended to range from 0.02 to <0.005 ppm,

indicating very low exposures to these isocyanates. Of the 621 workers in this study, 247 were

considered symptomatic for isocyanate exposure, exhibiting clinical signs such as bronchial asthma,

chronic obstructive pulmonary decrease (COPD), nonobstructive bronchitis, rhinitis, conjunctivitis,

urticaria/erythema, eczema, pyrexia, and hypersensitivity pneumonitis. Most workers had more than one

of these symptoms simultaneously. Of these 247 workers, 212 were RAST negative (i.e., no detectable

levels of IgE antibodies to any of the isocyanates tested). The remaining 35 workers (14% of

symptomatic workers) were symptomatic for isocyanate exposure and were RAST-positive; only

1 worker was RAST-positive and asymptomatic. These 35 workers suffered more frequently than RAST

negative symptomatic workers from bronchial asthma, rhinitis, conjunctivitis (p<0.0l), urticaria/

erythema, and hypersensitivity pneumonitis. Nonobstructive bronchitis was significantly more
HEXAMETHYLENE DIISOCYANATE 33

2. HEALTH EFFECTS

frequently associated with negative RAST results. Frequencies of COPD, eczema, and fever were not

remarkably different in positive and negative RAST groups (Baur et al. 1984).

Alexandersson et al. (1987) studied the clinical signs and changes in lung function parameters of

3 groups of garage workers to HDI and HDI-BT (HDI-biuret trimer). Average duration of employment

was 7 years. Group 1 consisted of 41 male car painters exposed to several diisocyanates, but mostly to

HDI and HDI-BT. The hardener sprayed onto surfaces and containing the HDI compounds contained

40-50% HDI-BT and 0.5-l% unreacted HDI. Car platers were the second group, consisting of 48 males

exposed to high amounts of dust (but not isocyanates) but usually equipped with masks. The third group

was the control group composed of car mechanics (70 males) who were not considered to be exposed to

HDI or other related diisocyanate compounds. All groups were subjected to lung function testing

parameters periodically, including forced vital capacity (FVC), forced expiratory volume after 1 second

(FEV,,, maximum mean expiratory flow (MMV), and nitrogen washout with subsequent calculations of

phase III and closing volume (volume expired after the onset of phase IV, the departure of the nitrogen

level from the alveolar plateau). The mean exposure to HDI-BT through car painting was 115 μg/m3

(0.006 ppm) (range, l0-385 μg/m3 [0.0005-0.0197 ppm]). Nine ofthe 43 painters had exposures below

90 μg/m3

 (0.0046 ppm), 13 had exposure values between 90-180 μg/m3 (0.0046-0.0092 ppm), and 9 had

exposures between 180-360 μg/m3

 (0.0092-0.0184 ppm). Two workers had exposures in excess of

360 μg/m3

 (0.0184 ppm). High short-time exposure peaks of up to 13,500 μg/m3 (0.6897 ppm) were also

noted in this study. Results of a questionnaire indicated that eye, nose, and throat irritation occurred

more frequently in car painter and car platers than in controls, but the difference was significant for car

platers only. Although many lung parameters were measured, the only significant difference in values

was found in the percentage of closing volume (%CV) for car painters compared to controls the Monday

before the work week began, where %CV was significantly higher (p<0.003) for car painters than in

controls. This difference was attributed to an effect on the small airways and could fit with the small

airways disease associated with other diisocyanate exposures. The %CV increased as the work week

progressed (2.6%), lending more validity to this observation.

A follow-up study of these garage workers was performed by Tornling et al. (1990). At the time of this

study, the mean duration of employment was 16.5 years. Group 1 consisted of 36 of the 46 male car

painters examined in 1978. These workers had been exposed to several diisocyanates, but predominately

to HDI and HDI-BT. Within this group, 28 worked as painters during the entire 6-year period between

studies. The second group consisted of 115 of the 142 male controls examined in 1987; these workers
HEXAMETHYLENE DIISOCYANATE 34

2. HEALTH EFFECTS

were mainly car platers and mechanics and may have been exposed to high amounts of dust (but usually

while equipped with masks), but not to HDI or to other related diisocyanate compounds. These groups

were further divided based upon smoking history (current and ex-smokers versus those who never

smoked). Among those who never smoked, 9 were car painters and 27 were controls; among current or

former smokers, 27 were car painters and 115 were controls. Exposure was assessed for the 28 who

worked as painters for the entire 6-year period. Both groups were again subjected to lung function tests,

performed during the first 3 hours of a working day, which included forced vital capacity (FVC), forced

expiratory volume after 1 second (FEV1), FEV% (FEV1/FVC x l00), maximum mean expiratory flow

(MMF), and nitrogen breath washouts with subsequent calculations of phase III and closing volume

(volume expired after the onset of phase IV, the departure of the nitrogen level from the alveolar

plateau). IgG and IgE levels were also analyzed in all workers, with IgE antibodies specific to

isocyanates analyzed only in the group of painters. Exposure calculations indicated that the painters had

a mean exposure of 0.0015 mg/m3

 (0.0002 ppm) HDI and 0.09 mg/m3 (0.005 ppm) HDI-BT. Painters

reported a statistically significant higher frequency of wheezing than did controls for both the neversmoked

category (p<0.01) and the current or ex-smoker category (p<0.05). Other airway and eye

symptoms were reported more frequently among car painters than among controls; however, the

differences were not statistically significant. Among the current and former smokers, decreases in FEV1,

VC, and FVC over the 6-year period were significantly greater in the painter groups versus controls

(p<0.05,p<0.01, and p<0.00l, respectively). Among those who never smoked, the decrease in lung

function over the 6-year period was similar for painters and controls. Among those workers continuously

employed as painters during the 6-year period, the number of yearly peak exposures was significantly

correlated with decrease in FVC (p<0.05); however, the decrease in FVC was correlated with main

exposure levels. None of the painters had IgE specific to isocyanates. Six painters and 20 controls had

IgE levels above reference (122 kilo units/litre), while none of the painters and 10 of the controls had IgG

levels exceeding reference values (19.9 g/L). However, the workers studied were exposed to a

combination of diisocyanates, particularly HDI and HDI-BT, so it was not possible to determine which

chemical form was responsible for the symptomatology and clinical signs.

Another study (Grammar et al. 1988) evaluated (using a questionnaire) a group of 149 men and 1 woman

who worked with HDI to determine any clinical illness associated with HDI exposure and via blood

antibody production to both HDI and the HDI trimer. This population worked in a factory that spraypainted

trucks with paint containing HDI and HDI trimer. The authors classified each person as to a

particular task (laborer, plumber, paint mixer, spray painter, etc.), had each fill out questionnaires about
HEXAMETHYLENE DIISOCYANATE 35

2. HEALTH EFFECTS

clinical symptomology, took blood samples periodically for antibody determination, and sampled the ambient

air in their work environment for HDI and HDI trimer concentrations over an IS-month period. Serum

samples were analyzed via an enzyme-linked immunosorbent assay (ELBA) for antibodies to HDI and/or

HDI trimer. Mean levels of exposure for both HDI and HDI trimer seemed to be extremely variable. For the

HDI, the levels were <0.08-3.8 μg/m3

 (0.00001-0.0006 ppm), while for HDI trimer the mean exposure

levels were 5.3-75 μg/m3

 (0.0003-0.004 ppm) among all classifications of work, with mean duration of

exposure ranging from 24 to 5 1 months. HDI trimer seemed to be the main exposure concern for this group

of workers. Eighteen workers reported at least one respiratory system symptom on their questionnaire;

however, only one person developed symptoms that were compatible with work-related respiratory disease;

that worker also had no antibody response to either HDI or HDI trimer, with the symptoms clearing after

relocating to another area of the plant.

Only one chronic-duration inhalation study was identified in laboratory animals exposed to HDI. In that

study, male and female Fischer 344 rats were exposed to 0,0.005, 0.025, or 0.175 ppm HDI. HDI-related

histopathological changes were limited to the nasal cavity and lungs. Lung lesions included minimal-tomild

focal to multifocal lesions, classified as epithelialization (alveolar lining cell proliferation),

interstitial pneumonia (septal thickening, alveolar cellular content, and increased alveolar ling cell

prominence), and alveolar macrophage accumulation (histiocyte cells in alveolar space). The authors

considered there to be an exposure-related incidence of these lesions in the rats (both sexes) exposed to

0.025 and 0.175 ppm of HDI. Histopathological lesions within the nasal cavity were numerous; however,

only a few were considered to be a direct effect of HDI inhalation exposure. Lesions observed in the

0.175 ppm exposure group included degeneration of the olfactory epithelium, characterized by

hyperkeratosis, occasional atrophy, and focal erosion or ulceration; these lesions were not present at

lower exposure concentrations. Other lesions in the nasal cavity that occurred due to HDI exposure in

the 0.025 and 0.005 ppm exposure groups included hyperplasia/metaplasia, mucus hyperplasia, and

inflammation. Combining information obtained from a satellite group of rats exposed to HDI at identical

concentrations but for a l-year duration instead of 2 years. After 1 year of exposure, an adaptive nasal

epithelial response (mucus secretory cell and epithelial hyperplasia) was observed in females at the

lowest dose (0.005 ppm) and males at the highest dose (0.175 ppm). At the 0.025 and 0.175 ppm

concentrations, a progression from this response occurred, exhibited as hyaline droplet degeneration,

hyperkeratosis, chronic inflammation, and olfactory epithelial damage. After 2 years, an adaptive

response at the lowest concentration occurred, characterized by hyperplasia/metaplasia and hyaline

droplet degeneration. At the 0.025 and 0.175 ppm concentrations, a progression of the lesions noted in
HEXAMETHYLENE DIISOCYANATE 36

2. HEALTH EFFECTS

the l-year exposure group at the same dose of HDI was also noted (Mobay Corporation 1989). A chronic

inhalation MRL of 1.0x10-5 ppm was derived, based on nasal cavity epithelial hyperplasia in female rats

(minimal LOAEL) .

Cardiovascular Effects. No studies were located regarding cardiovascular effects in humans after

inhalation exposure to HDI.

Based on the few laboratory animal studies available, the cardiovascular system does not appear to be a target

organ system for HDI toxicity. No studies were located regarding cardiovascular effects in animals following

acute-duration inhalation exposure. Groups of 10 male and 10 female rats were exposed (head-only) to HDI

vapors at average concentrations of 0.005, 0.0175, 0.15, or 0.3 ppm for 5 hours a day, 5 days a week for

3 weeks. Five animals per sex per exposure concentration were sacrificed at the end of the exposure period;

the balance of the animals were allowed a 2-week recovery period and then sacrificed. No significant changes

in heart weights or histopathology were observed at any dose of HDI (Mobay Corporation 1984).

In another study of intermediate-duration, Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm

HDI in air for 66-69 days for 6 hours per day over a period of approximately 13 weeks. Animals were

exposed to HDI in 2-cubic-meter chambers (whole body). All rats were sacrificed during week 14. No

compound-related toxic effects were noted by changes in heart weights or heart weight to body weight ratios

at gross necropsy. Histopathologic evaluation of the cardiovascular tissue was also conducted, and no

compound-related effects were found (Mobay Corporation 1988).

In a chronic-duration study, groups of 60 male and 60 female Fischer 344 rats were exposed to 0, 0.005,

0.025, or 0.175 ppm HDI over a 2-year period. At gross necropsy at the end of the study, many non-HDI

body organ weight changes were noted; however, there were increases in the relative heart weights in the

0.175 ppm HDI treated females. Although these organs had increased weight compared to controls, the

values were still within accepted control range values and not considered an effect of HDI inhalation

exposure. Histopathologic evaluation of the cardiovascular tissue was also conducted and no-compoundrelated

effects were found (Mobay Corporation 1989).

Gastrointestinal Effects. No studies were located regarding gastrointestinal effects in humans after

inhalation exposure to HDI.
HEXAMETHYLENE DIISOCYANATE 37

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Based on the few laboratory animal studies available, the gastrointestinal tract does not appear to be a target

organ system for HDI toxicity. Two female Beagle dogs were exposed to <2 ppm of HDI 6 times (2 hours

each) over a 10-day period. The length of time between exposures was not reported. Average HDI

concentrations were 0.28, 0.32, 0.39, 0.55, 0.89, and 1.07 ppm for each trial. These dogs were reported to

cough up foamy material (not specified if this material was from the lung or stomach), and vomiting was

observed. Generally, the severity of these signs of toxicity correlated with the inhaled HDI concentration.

Recovery was complete by the end of each exposure day, and no other clinical effects were observed, based

on rectal temperature, weight, or general condition (Haskell Laboratory 1961).

In another acute-duration exposure using higher concentrations of HDI, groups of 4 male albino ChR-CD rats

were exposed to 5, 11, 26, 27, 72, or 370 ppm HDI for 4 or 8 hours. The only pathology attributed to HDI

toxicity observed at sacrifice was chronic gastritis in 2 rats exposed for 4 hours to 26 ppm HDI. Rats

exposed to 11 ppm for 4 hours showed clinical signs of toxicity seen at higher concentrations, but no tissue

changes were noted in the gastrointestinal tract. No histopathological effects were observed in rats exposed

to 5 ppm for 4 hours (Haskell Laboratory 1961).

In an intermediate-duration study, groups of 10 male and 10 female Sprague-Dawley rats were exposed

(head-only) of HDI vapors at average concentrations of 0.005, 0.0175, 0.15, or 0.3 ppm for 5 hours a day,

5 days a week for 3 weeks. Five animals per sex per exposure concentration were sacrificed at the end of the

exposure period; the remaining animals were allowed a 2-week recovery period and then sacrificed. No

clinical signs of toxicity were observed during the non-exposure days (i.e., weekends). No statistically

significant changes in gross pathology or in the gastrointestinal organ weights were observed at any of the

inhalation doses of HDI (Mobay Corporation 1984).

In a chronic-duration study, groups of 60 male and 60 female Fischer 344 rats were exposed to 0, 0.005,

0.025, or 0.175 ppm HDI over a 2-year period. Control rats were sham-exposed rats (conditioned air

exposure). No significant changes in absolute or relative gastrointestinal tract organ weights were found

(Mobay Corporation 1989).

Hematological Effects. No studies were located regarding hematological effects in humans

following intermediate- or chronic-duration inhalation exposure to HDI. Several case reports were

available that described some hematological effects of HDI after acute-duration inhalation exposures. In

one report, a 35-year-old male who sprayed his car with a polyurethane paint containing prepolymerized
HEXAMETHYLENE DIISOCYANATE 38

2. HEALTH EFFECTS

HDI and also 1.6% or less of a monomer of HDI in a poorly ventilated workshop on 3 separate occasions

over the span of about a year, experienced several adverse clinical signs. Within 15 minutes after

beginning one painting, a cough, tight chest, and chills occurred and progressed into a serious asthmatic

reaction, for which he was admitted into an intensive care unit the following day. Respiratory signs, such

as dyspnea, prolonged expirations, and crepitating rales, were observed. Blood gases showed hypoxia;

however, no fever, leucocytosis, or eosinophilia was observed, indicating no hematological effects due to

HDI exposure were detected (Belin et al. 1981).

In another report, the occurrence of respiratory effects in a 34-year-old male working as a spray painter

was investigated. He had no previous history of lung disease and was otherwise in good health. After

ruling out a possible immunologic trimellitic anhydride (TMA) hemorrhagic pneumonitis, the possibility

of HDI-induced asthma was considered. After the paint (containing the monomer HDI, as presumably

the biuret form as well) was sprayed on a warm metal surface, the worker subsequently developed an

acute illness, including hemoptysis, dyspnea, bilateral pleuritic chest pain, and bilateral pulmonary

opacities, which then progressed to respiratory failure. White blood cells were elevated at 14,500, with

the cell differential showing 8 lymphocytes, and 1 mono and 91 segmented neutrophils. Recovery

occurred with the assistance of corticosteroid therapy, suggesting an allergic reaction had occurred

(Patterson et al. 1990).

Another case involved a 56-year-old male who worked as a foreman in a garage where painting was

performed. One of the paints used contained 7% polymeric HDI to which he was exposed intermittently

for 5-6 years. During that time, episodes of shortness of breath, wheezing, malaise, and chills were

reported. Symptoms tended to occur in the late afternoons of working days and lasted for several hours.

An inhalation challenge to the paint the worker was using was performed; HDI concentration was

measured in the room during exposure at 0.02 ppm (polymeric forms of HDI were not measured in this

study). In addition to respiratory signs of an asthmatic reaction beginning to occur 1 hour after exposure

began, an elevated WBC count from a blood sample taken 3 hours after exposure began was noted (WBC

count = 18,700, 60% segs, 1% eosinophils); however, a chest radiograph at that time was normal (Malo

et al. 1983).

Based on these few human case reports, it appears that the major hematological effect, if present,

incurred by inhalation of HDI (either monomer or monomer and polymeric forms) is a mild leucocytosis

without eosinophilia.
HEXAMETHYLENE DIISOCYANATE 39

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In an acute-duration inhalation exposure study, groups of 4 male albino ChR-CD rats were exposed to 5, 11,

26, 27, 72, or 370 ppm HDI for 4 or 8 hours. Rats survived a 4-hour exposure to 72 ppm but showed severe

respiratory impairment and cyanosis during exposure. No other hematologic pathology was described

(Haskell Laboratory 1961).

In studies of intermediate-duration, groups of 10 male and 10 female rats were exposed (head-only) to HDI

vapors at average concentrations of 0.005, 0.0175, 0.15, or 0.3 ppm for 5 hours a day, 5 days a week for

3 weeks. Five animals per sex per exposure concentration were sacrificed at the end of the exposure period;

the remaining animals were allowed a 2-week recovery period and then sacrificed. No significant differences

in blood chemistry and hematology were observed compared to control animals for both male and female rats

(Mobay Corporation 1984). Similarly, Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm HDI

in air for 6 hours per day for 66-69 days over a period of approximately 13 weeks. Animals were exposed to

HDI in 2-cubic-meter chambers (whole body). All rats were sacrificed at week 14. Hematology and blood

chemistry were performed to determine the lesions that might be associated with HDI inhalation exposure at

these doses. No compound-related changes in blood chemistry and hematology were found (Mobay

Corporation 1988).

In a chronic-duration study, groups of 60 male and 60 female Fischer 344 rats were exposed to 0, 0.005,

0.025, or 0.175 ppm HDI over a 2-year period. Control rats were sham exposed rats (conditioned air

exposure). Hematologically, the only effect that HDI may have had was an increase in the number of

reticulocytes at sporadic intervals during the study in both males and females exposed to the 0.164 ppm

concentration of HDI, suggesting anemia. No statistically significant HDI exposure-related changes in serum

chemistry were noted (Mobay Corporation 1989).

Based on the data found in all of these laboratory animals studies, the bone marrow appears not to be a

system significantly affected by inhalation exposure at the low concentrations of HDI tested.

Musculoskeletal Effects. No studies were located regarding musculoskeletal effects in humans after

inhalation exposure to HDI.

No studies were located regarding musculoskeletal effects in animals after acute-duration inhalation

exposure to HDI. A study by Mobay Corporation (1984), using male and female Sprague-Dawley

rats exposed to 0.005-0.3 ppm HDI for 5 hours a day, 5 days a week for 3 weeks, failed to produce
HEXAMETHYLENE DIISOCYANATE 40

2. HEALTH EFFECTS

musculoskeletal lesions at the highest dose tested. Similarly, Mobay Corporation (1989) found that in male

and female Fischer 344 rats exposed to HDI concentrations ranging from 0.005 to 0.175 for 6 hours a day,

5 days a week over a 2-year period, no musculoskeletal lesions could be found at gross necropsy or during

histopathologic examinations at the end of the study period.

Hepatic Effects. No studies were located regarding hepatic effects in humans after inhalation exposure

to HDI.

No studies were located regarding hepatic effects in animals following acute-duration inhalation exposure to

HDI. The only notable change in laboratory animals was decreased liver weights at 0.3 ppm in female rats.

In that study, groups of male and female rats were exposed (head-only) to HDI vapors at average

concentrations of 0.005, 0.0175, 0.15, and 0.3 ppm for 5 hours a day, 5 days a week for 3 weeks. At an HDI

exposure concentration of 0.3 ppm, a statistically significant decrease in liver absolute and relative weights in

female rats only was observed in those animals sacrificed immediately after the 3-week exposure was

completed. Male rats exposed to 0.3 ppm HDI failed to show a significant decrease in the relative and

absolute liver weights. No significant changes in gross pathology or histopathology of the liver were found in

either sex (Mobay Corporation 1984).

In another study of intermediate-duration, Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm

HDI in air 6 hours per day for 66-69 days over a period of approximately 13 weeks. Animals were exposed

to HDI in 2-cubic-meter chambers (whole body). All rats were sacrificed during week 14. No compound related

toxic effects were noted by changes in liver weights or liver weight to body weight ratios at gross

necropsy (Mobay Corporation 1988).

In a chronic-duration study, groups of 60 male and 60 female Fischer 344 rats were exposed to 0,0.005,

0.025, or 0.175 ppm HDI over a 2-year period. At gross necropsy at the end of the study, many non-HDI related

body organ changes were noted; however, there were no increases in the relative liver weights in the

0.175 ppm HDI treated females (Mobay Corporation 1989).

Renal Effects. No studies were located regarding renal effects in humans after inhalation exposure to

HDI.
HEXAMETHYLENE DIISOCYANATE 41

2. HEALTH EFFECTS

No studies were located regarding renal effects in animals following acute-duration inhalation exposure to

HDI. The only notable changes in the kidneys of laboratory animals were in decreased organ weights at

0.3 ppm in both male and female rats and increased urinary ketone concentrations in male rats at a lower dose

of HDI. Groups of male and female Sprague-Dawley rats were exposed (head-only) to HDI vapors at

average concentrations of 0.005, 0.0175, 0.15, or 0.3 ppm for 5 hours a day, 5 days a week for 3 weeks. Five

animals per sex per exposure concentration were sacrificed at the end of the exposure period; the remaining

animals were allowed a 2-week period recovery period and then sacrificed. At an HDI exposure

concentration of 0.3 ppm, a statistically significant decrease in absolute and relative kidney weights in male

and female rats was observed in those animals sacrificed immediately after the 3-week exposure was

completed. No other statistically significant changes in kidney weights were observed at any of the lower

inhalation doses of HDI. No significant changes in the gross pathology or histopathology of the kidney were

found (Mobay Corporation 1984).

Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm HDI in air for 6 hours per day for

66-69 days over a period of approximately 13 weeks. Animals were exposed to HDI in 2-cubic-meter

chambers (whole body). All rats were sacrificed at week 14. After exposures ended, urine analysis in male

rats exposed to 0.041 ppm HDI showed a statistically significant increase in urinary ketone concentration.

No other compound-related induced urine changes were noted. No compound-related toxic effects were noted

by changes in kidney weights or kidney weight to body weight ratios at gross necropsy. No HDI-related

histopathologic lesions were noted in the kidney of the treated rats (Mobay Corporation 1988).

Groups of 60 male and 60 female Fischer 344 rats were exposed to 0, 0.005, 0.025, or 0.175 ppm HDI over a

2-year period. Control rats were sham-exposed rats (conditioned air exposure). No HDI-related lesions were

found at gross necropsy or during histopathologic examination (Mobay Corporation 1989).

Endocrine Effects. No studies were located regarding endocrine effects in humans after inhalation

exposure to HDI.

No studies were located regarding endocrine effects in animals following acute-duration inhalation

exposure to HDI. A study by Mobay Corporation (1984), using male and female Sprague Dawley rats

exposed to exposed to 0.005-0.3 ppm HDI for 5 hours a day, 5 days a week for 3 weeks, failed to

reduce endocrine organ lesions at the highest dose tested. Similarly, Mobay Corporation (1989) found
HEXAMETHYLENE DIISOCYANATE 42

2. HEALTH EFFECTS

that in male and female Fischer 344 rats exposed to HDI concentrations ranging from 0.005 to 0.175 for

6 hours a day, 5 days a week over a 2-year period, no endocrine organ lesions could be found at gross

necropsy or during histopathologic examinations at the end of the study period.

Dermal Effects. No studies were located regarding dermal effects in humans after inhalation exposure to

HDI.

No studies were located regarding dermal effects in animals following acute-duration inhalation exposure to

HDI. A study by Mobay Corporation (1984), using male and female Sprague Dawley rats exposed to

0.005-0.3 ppm HDI for 5 hours a day, 5 days a week for 3 weeks, failed to produce dermal lesions at the

highest dose tested. Similarly, Mobay Corporation (1989) found that in male and female Fischer 344 rats

exposed to HDI concentrations ranging from 0.005 to 0.175 for 6 hours a day, 5 days a week over a 2-year

period, no dermal lesions could be found at gross necropsy or during histopathologic examinations at the end

of the study period.

Ocular Effects. No studies were located regarding ocular effects in humans after inhalation exposure to

HDI.

Ocular toxicity via vapor exposure to HDI has been reported and is somewhat milder than when HDI is

placed directly into the eyes (see Section 2.2.3). Two female Beagle dogs were exposed to <2 ppm of HDI

6 times (2 hours each) over a lo-day period. The length of time between exposures was not reported.

Average HDI concentrations were 0.28,0.32,0.39,0.55,0.89, and 1.07 ppm for each exposure. Severe nose,

throat, and eye irritation (including lacrimation) was observed in both dogs at all concentrations of HDI tested

(0.27-1.43 ppm) with the severity of these signs of irritation generally correlated to the inhaled HDI

concentration. Recovery was complete by the end of each exposure day, and no effects were observed based

on rectal temperature, weight, or general condition (Haskell Laboratory 1961).

In other acute-duration studies, groups of 4 male albino ChR-CD rats were exposed to 30 ppm HDI for

4 hours daily for 10 days over a 2-week period. A slit-shaped opacity of the cornea (clinically interpreted

to be a comeal ulcer) of one eye was reported in one rat that died after exposure had ended (Haskell

Laboratory 1961). In another study, male rats (strain not specified) were exposed for 6 hours to an

unlmown air concentration of HDI. The investigators estimated that 0.4% of the HDI in a bubbler was
HEXAMETHYLENE DIISOCYANATE 43

2. HEALTH EFFECTS

potentially evaporated, but total air flow through the chamber was not measured, so that it is not possible to

precisely calculate the air concentration of HDI inhaled by the test animals. Animals were observed for

behavioral changes for 10 days after exposure. All animals survived exposure and the lo-day observation

period. The authors concluded that HDI was mildly toxic. The fumes were moderately irritating to the

conjunctiva of the eye soon after the start of exposure (Mobay Corporation 1966).

For intermediate-duration studies, Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm HDI in air

for 6 hours per day for 66-69 days over a period of approximately 13 weeks. Animals were exposed to HDI

in 2-cubic-meter chambers (whole body). All rats were sacrificed at week 14. Ocular opacity was observed

in all groups of rats tested, including control animals, and did not exhibit a concentration-dependent response;

the only compound-related clinical sign was ocular irritation with subsequent lacrimation (Mobay

Corporation 1988).

Male and female Sprague-Dawley rats were exposed (head-only) to HDI vapors at average concentrations of

0.005,0.0175,0.15, or 0.3 ppm for 5 hours a day, 5 days a week for 3 weeks. Five animals of each sex per

exposure concentration were sacrificed at the end of the exposure period; the remaining animals were allowed

a 2-week recovery period and then sacrificed. All animals exposed to all concentrations of HDI exhibited

varying degrees of irritation of eyes and/or noses during exposure and at 1 hour post-exposure, with all

animals appearing normal the following morning (Mobay Corporation 1984).

In a chronic-duration study, groups of 60 male and 60 female Fischer 344 rats were exposed to 0,0.005,

0.025, or 0.175 ppm HDI over a 2-year period. Control rats were sham-exposed rats (conditioned air

exposure). HDI caused eye irritation in males exposed to the 0.175 ppm dose only during the first year of the

study but not during the second year. No other HDI-related eye lesions were detected during ophthalmologic

examinations performed during the 2-year study (Mobay Corporation 1989).

Metabolic Effects. No studies were located regarding metabolic effects in humans following

intermediate or chronic-duration inhalation exposure to HDI.

One report described a case of a 56-year-old man who worked as a foreman in a garage where automobile

painting was performed and consequently was exposed intermittently to paints containing HDI for

5-6 years. He reported having developed respiratory and systemic reactions after exposure to paints

(which contained 7% polymeric HDI) used in the garage. Episodes of shortness of breath, wheezing,
HEXAMETHYLENE DIISOCYANATE 44

2. HEALTH EFFECTS

malaise, and chills were reported, with symptoms occurring in the late afternoons of working days and

lasting for several hours thereafter. In an attempt to confirm that HDI was the compound responsible, the

man was removed from the garage environment for several weeks and then exposed to an inhalation test

exposure to one of the spray paints in which the HDI concentration was measured to be 0.02 ppm

(polymeric forms of HDI were not measured. No abnormalities in clinical signs were noted during the

first hour after exposure. During the third hour of exposure, chills, headache, and malaise were noted,

with the man prostrate at 6 hours after exposure (Malo et al. 1983).

No studies were located regarding metabolic effects in animals following intermediate- or chronicduration

inhalation exposure to HDI. No effect on rectal temperature was observed in 2 female Beagle

dogs exposed to <2 ppm of HDI 6 times (2 hours each) over a 10-day period. The length of time between

exposures was not reported. Average HDI concentrations were 0.28, 0.32, 0.39, 0.55, 0.89, and 1.07 ppm

for each exposure (range, 0.27-l .43 ppm) (Haskell Laboratory 1961).

Body Weight Effects. No studies were located regarding body weight effects in humans after

inhalation exposure to HDI.

The body of information available suggests that HDI does little to affect body weight at the

concentrations of 0.3 ppm or less, while changes in body weight are marginal at inhaled concentrations of

3 ppm or higher for l-time exposures. To demonstrate this, male albino Sprague-Dawley rats were

exposed to HDI air concentrations of 3,4, 6, 11, 22, 44, or 88 ppm for 6 hours. Surviving animals were

kept as long as 15 days after the single exposure, with some animals sacrificed between 0 and 15 days

after exposure to determine lung damage. Animals exposed to 3 and 4 ppm had an initial weight loss of

10 g when sacrificed at 24 hours after exposure. After exposure to 3-l 1 ppm of HDI, rats showed a

slight weight gain of approximately 10 g (about 3%) during the first week after exposure. Rats exposed

to 22 and 44 ppm showed a 15-20 g loss of weight (about 6%) during the first week, followed by a

recovery to more than their original weight when sacrificed 2 weeks after exposure (Dow Chemical Co.

1964). No effect on body weight was observed in male albino Chrl-CD rats exposed to 300 ppm HDI

4 hours per day for 10 days over a 2-week period (Haskell Laboratory 1961). Similarly, no body weight

effects were observed in female Beagle dogs exposed to 0.27 ppm HDI via whole-body inhalation

2 hours per day for 6 days over a 10-day period (Haskell Laboratory 1961). However, severe body

weight loss was observed in male albino Chrl-CD rats exposed to 72 ppm HDI after a single 4-hour

exposure (Haskell Laboratory 1961).
HEXAMETHYLENE DIISOCYANATE 45

2. HEALTH EFFECTS

Male and female Sprague-Dawley rats were exposed (head-only) to HDI vapors at average concentrations of

0.005, 0.0175, 0.15, or 0.3 ppm for 5 hours a day, 5 day a week for 3 weeks. Five animals of each sex per

exposure concentration were sacrificed at the end of the exposure period; the remaining animals were allowed

a 2-week period recovery period and then sacrificed. No significant differences in body weights were

observed compared to control animals for both male and female rats (Mobay Corporation 1984).

Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm HDI in air for 6 hours per day for

66-69 days over a period of approximately 13 weeks. Animals were exposed to HDI in 2-cubic-meter

chambers (whole body). All rats were sacrificed during week 14. Statistically significant increases in body

weight were noted after exposure ended in female rats dosed at all 3 concentrations of HDI and in male rats

dosed at 0.143 ppm HDI. During the exposure period, no statistically significant weight differences were

noted. Since similar findings were noted for control rats, these findings were not considered to be related to

HDI treatment (Mobay Corporation 1988).

Male and female Fischer 344 rats were exposed to 0, 0.005, 0.025, or 0.175 ppm HDI over a 2-year period.

Decreases in body weight (compared to control animals) were small (only a 5% decrease) but consistent, and

were considered to be related to the toxicity of HDI in female rats exposed to the 0.175 ppm dose during the

second year of the study only. There were also no statistically significant differences in terminal body weight

between controls and exposed male rats at the end of the study (Mobay Corporation 1989).

2.2.1.3 Immunological and Lymphoreticular Effects

In addition to their local irritant effects on the respiratory tract, the diisocyanates also have a propensity to

induce an immunological response in some individuals, which is characterized by an asthma-like respiratory

reaction, and will induce the formation of antibodies to both the monomeric and polymer forms of HDI. A

few studies have examined the immunological effects of HDI toxicity in humans, with some data available

from laboratory animal studies as well.

Several studies have reported antibodies being produced in response to an HDI inhalation exposures. In

one study, 149 men and 1 woman were selected to prospectively evaluate any clinical signs of illness

associated with HDI exposure and, by blood antibody production, to both HDI and HDI trimer. These

workers were employed in a factory that spray-painted trucks with paint containing HDI and HDI trimer.
HEXAMETHYLENE DIISOCYANATE 46

2. HEALTH EFFECTS

Questionnaires were distributed that asked about clinical symptomology, blood samples were taken

periodically for antibody determination, and the ambient air in their work environment was sampled for

HDI and HDI trimer concentrations over an 18-month period. Serum samples were analyzed via an

ELISA for antibodies to HDI and/or HD1 trimer. Mean levels of exposure for both HDI and HDI trimer

were found to be extremely variable. For the HDI monomer, the levels were between <0.08 and

3.8 μg/m3

 (0.00001-0.0006 ppm), while for HDI trimer the mean exposure levels were 5.3-75 μg/m3

(0.0003-0.0038 ppm) among all classifications of work, with mean duration of exposure ranging from

24 to 51 months. HD1 trimer seemed to be the main exposure concern for this group of workers. The

mean indices of IgG and IgE to HDI-human serum albumin (HSA) antibodies were 1.65 and 1.22,

respectively; the mean indices of IgG and IgE to HDI trimer-HSA antibodies were 1.63 and 1.19,

respectively. Approximately 21% of all workers had a positive antibody response to either of these

2 antigens. There was no significant correlation between any mean antibody levels and mean duration of

isocyanate exposure; however, among the plumber/painter exposure group, there were significant

positive correlations between exposure duration and IgG antibody to HDI-HSA and HDI trimer-HSA.

Eighteen workers reported at least one respiratory system symptom on their questionnaire; however, only

one person developed symptoms that were compatible with work-related respiratory disease; that worker

also had no antibody response to either HDI or HDI trimer, with the symptoms clearing after relocating to

another area of the plant (Grammar et al. 1988).

Another report described a 35-year-old male who used a polyurethane paint containing prepolymerized

HDI and 1.6% or less of the monomer of HDI in a poorly ventilated workshop on 3 separate occasions

over the span of about 1 year. Cough, dyspnea, prolonged expirium and crepitating rales, and chest

tightness, progressing into a serious asthmatic reaction (after the third exposure), were observed. No

fever, leucocytosis, eosinophilia, or wheezing was observed. The patient’s serum was analyzed with the

radioallergosorbent test (RAST) method, and IgE antibodies, particularly to HDI-HSA and to MDI-HSA

and, to a lesser extent, TDI-HSA were found. The authors stated that the positive result with MDI and

TDI was probably due to cross-reactivity. Six months later, the patient was skin-tested with the prick test

method. Common allergens gave negative results. Conjugates of HDI-HSA and MDI-HSA elicited

significant wheal and flare reactions (Belin et al. 1981).

High levels of IgG and IgE antibodies were detected against HDI-HSA and TDI-HSA in a 34-year-old

male working as a spray painter. Exposure to spray paint that containing HDI and an aliphatic

polyisocyanate 1 week prior to the onset of respiratory symptoms was noted, so immunoassays for MDI,
HEXAMETHYLENE DIISOCYANATE 47

2. HEALTH EFFECTS

HDI, and toluene diisocyanate (TDI) conjugated to human serum albumin (HSA) were carried out.

Radioimmunoassay RIA) results for IgG and IgE against HDI-HSA were noted: IgG titres were

>l:1,000 for HDI-HSA; IgE antibodies against HDI-HSA were also present at a 1:1,000 dilution; results

for IgG and IgE against MDI-HSA were negative. Based on the blood antibody data collected, the

authors proposed that the pathogenesis of this case of hemorrhagic pneumonitis this man experienced

was immunologic in nature because of uncontrolled exposures to HDI and TDI (Patterson et al. 1990).

Relative amounts of specific IgE and IgG in challenge-positive and challenge-negative were determined

in workers in another study to determine the specificity of the isocyanate antibodies for hapten-protein

conjugates. The study examined 55 workers (sex not specified) who had respiratory symptoms while

working with TDI, MDI, or HDI. Sera was obtained from each person and analyzed via an ELISA using

human serum albumin conjugates to each isocyanate for relative amounts IgG and IgE antibodies to the

above isocyanates using an IgG or IgE index. Each isocyanate was tested against human serum albumin

conjugate carrier molecules. Index values of >2 were considered positive to that antibody. Crossreactivity

with other isocyanate-protein conjugates (dog serum albumin, ovalbumin, etc.) was also

demonstrated and the degree of cross-reactivity varied with the individual. None of the TDI workers

were found to have a positive IgE index for TDI-HSA, and only two of the six workers were found to

have positive IgG indices for TDI-HSA. None of the 11 MDI workers had a positive IgE index, but 4 of

the 11 workers had positive IgG indices. Eight of the 38 HDI workers had a positive IgE index, and 21 of

38 workers had positive IgG indices to HDI-HSA (Grammar et al. 1990). These results suggest that the

antibody formed is directed against the combined complex formed by HDI and tissue protein, rather than

against either fraction alone.

RAST and skin tests were used to evaluate the significance of type I sensitization, its incidence, and its

relationship to respiratory dysfunctions in a large population of isocyanate-exposed workers. A group of

621 workers (sex not specified) engaged in isocyanate processing for a period of 2 weeks to 40 years was

studied. Of these workers, 183 had contact with TDI; 66 with MDI; 82 with HDI; 220 with a

combination exposure of TDI, MDI, and “other aromatic isocyanates;” 30 with a combination of TDI,

MDI and HDI; and 32 with other isocyanates. Air concentration data (where available) tended to range

from <0.005 to 0.02 ppm, indicating very low exposures to these isocyanates. Of the 247 workers with

clinical symptomology (symptomatic workers) related to HDI toxicity, 212 were RAST negative (i.e., no

detectable levels of IgE antibodies to any of the isocyanates tested); the remaining 35 workers (14% of

symptomatic workers) were symptomatic for isocyanate exposure and were RAST-positive (only
HEXAMETHYLENE DIISOCYANATE 48

2. HEALTH EFFECTS

1 worker was RAST-positive and asymptomatic). The 35 RAST-positive workers suffered more

frequently than RAST negative symptomatic workers from bronchial asthma, rhinitis, conjunctivitis

(p<0.0l), urticaria/erythema, and hypersensitivity pneumonitis. Nonobstructive bronchitis was

significantly more frequently associated with negative RAST results. Frequencies of chronic obstructive

pulmonary disease (COPD), eczema, and fever were not remarkably different in positive and negative

RAST groups. Of the 35 RAST-positive workers, 27 cases were positive for HDI; all but one worker

(worker #12) showed positive RAST results to other isocyanates in addition to HDI. Fifty-three

symptomatic workers underwent skin testing for specific isocyanate testing; five workers gave positive

skin test results (wheal-and-flare reaction) for the HDI-HSA conjugate. The authors concluded that: (1)

the existence of an immunologically mediated type 1 sensitization to isocyanate components is supported

by the observed clinical symptomatology after inhalation challenge and RAST results, and (2) for routine

investigations and for follow-up studies, RAST and skin testing with isocyanate-HSA conjugates appear

to be suitable methods for detecting immunologically sensitized workers. The failure to detect

isocyanate antibodies in the sera of symptomatic workers may indicate the involvement of other

non-immunologic mechanisms, such as a local effect of the isocyanate on the lung tissue (binding to

certain proteins and enzymes) that alters lung function and may induce the clinical symptoms associated

with isocyanate-induced lung disease (Baur et al. 1984).

A 60-year-old male automobile paint sprayer was examined following health complaints of shortness of

breath, productive cough, and intermittent fever of l-month duration, about when he began using paint

containing HDI. Increased number of activated cytotoxic T lymphocytes in the bronchoalveolar lavage

fluid (BALF) and an increased percentage and absolute number of non-major histocompatibility

complex-restricted natural killer cells in the peripheral blood during the recovery phase of

hypersensitivity pneumonitis were discovered. The total number of cells was markedly elevated and the

differential counts of lymphocytes and neutrophils were increased. ELISA revealed that IgG and IgA

antigen-specific antibodies to TDI and HDI were present in BALF and the serum (Usui et al. 1992).

Increased levels of specific IgG antibodies against HDI-HSA and MDI-HSA were demonstrated in a

56-year-old male, who worked as a foreman in a garage where one or more of several paints containing

7% polymeric HDI were used. The man reported that he had developed respiratory and systemic

reactions after exposure to HDI after a history of being intermittently exposed to paint containing HDI

for 5-6 years. During that period of time, episodes of shortness of breath, wheezing, malaise, and chills

were reported. Symptoms tended to occur in the late afternoons of working days and lasted for several
HEXAMETHYLENE DIISOCYANATE 49

2. HEALTH EFFECTS

hours. Upon initial physical exam, no chest anomalies were found. To confirm that HDI was the compound

responsible, the man was removed horn the garage environment for several weeks and lung parameters were

measured, including FEV1, FVC, VC, FRC, and TLC. Body temperature and blood samples were also

collected. Inhalation HDI challenge was performed. No reactions were noted when the man was challenged

with enamel or air. After being challenged with HDI for 5 minutes (air concentrations measured to be

0.02 ppm), he developed general malaise, cough, fever, and leukocytosis beginning 3 hours after exposure,

together with a mixed restrictive and obstructive breathing defect (Malo et al. 1983).

No studies were located regarding immunological and lymphoreticular effects in animals following acuteduration inhalation exposure to HDI. For laboratory animal studies, the data is mainly limited to the

investigation of changes in lymphoreticular organs. Male and female Sprague-Dawley rats were exposed

(head-only) to HDI vapors at average concentrations of 0.005, 0.0175, 0.15, or 0.3 ppm for 5 hour a day,

5 day a week for 3 weeks. No statistically significant difference in the weight or gross pathology of the

spleen was observed, when compared to control animals, for both male and female rats (Mobay Corporation

1984). Similarly, male and female Fischer 344 rats were exposed to 0, 0.011, 0.041, or 0.143 ppm HDI in

air for 6 hours per day for 66-69 days over a period of approximately 13 weeks. All rats were sacrificed at

week 14. There were no changes in the relative or absolute weight of spleen at gross necropsy (Mobay

Corporation 1988).

In a chronic-duration study, male and female Fischer 344 rats were exposed to 0, 0.005, 0.025, or 0.175 ppm

HDI for a 2-year period. At gross necropsy, many non-HDI-related body organ changes were noted;

however, there were increases in the relative weight of the spleen in the 0.175 ppm HDI treated females, with

an increase in absolute spleen weight as well. Although the spleen had an increased weight compared to

controls, the values were still within the accepted control range and not considered an effect of HDI inhalation

exposure (Mobay Corporation 1989).

The highest NOAEL values and all LOAEL values from each reliable study for immunological and

lymphoreticular effects in each species and duration category are recorded in Table 2-l and plotted in

Figure 2-1.
HEXAMETHYLENE DIISOCYANATE 50

2. HEALTH EFFECTS

2.2.1.4 Neurological Effects

No studies were located regarding neurological effects in humans after intermediate-duration inhalation

exposure to HDI. One report described a case of a 56-year-old man who worked as a foreman in a garage

where automobile painting was performed and consequently was exposed intermittently to paints

containing HD1 for 5-6 years. He reported having developed respiratory and systemic reactions after

exposure to paints (which contained 7% polymeric HDI) used in the garage. Episodes of shortness of

breath, wheezing, malaise, and chills were reported, with symptoms occurring in the late afternoons of

working days and lasting for several hours. In an attempt to confirm that HDI was the compound

responsible, the man was removed from the garage environment for several weeks. The man was then

exposed to an inhalation test exposure to one of the spray paints in which the HDI concentration was

measured to be 0.02 ppm (polymeric forms of HDI were not measured). No abnormalities in clinical

signs were noted during the first hour after exposure. At three hours, a productive cough with headache

and malaise was reported. The man was prostrate at six hours after exposure (Malo et al. 1983).

Few neurological toxicities after inhalation exposures to HD1 could be identified in laboratory animals.

In an acute-duration study, groups of 4 male albino ChR-CD rats were exposed to various concentrations

of HDI for 4 or 8 hours. When rats were exposed to 370 ppm from a bubbler of HDI warmed to

40-50 oC, they died after 2-3 hours of exposure, with irritation and convulsions observed prior to death.

However, mechanical difficulties with the exposure apparatus may have contributed other factors that

might have been responsible for the convulsions and eventual death of these animals (Haskell Laboratory

1961).

Other neurological aberrations have been reported in laboratory animals. Groups of 4-6 male English

smooth-haired guinea pigs were exposed to 0.5 ppm HDI for 6 hours, 1.8 ppm for 2 hours, or 4 ppm for

3 hours. Erythrocyte acetylcholinesterase and plasma cholinesterase were determined prior to and during

HDI exposures. Pulmonary cholinesterase was determined from bronchial lavage fluid after animals

were sacrificed. Enzyme levels were not significantly different (P<0.05) from controls. Although some

of the animals exposed to HDI displayed severe respiratory irritation, slowed respiratory rate and labored

breathing, and 50% of the animals died at the 4 ppm dose level, no inhibition of serum cholinesterase or

erythrocyte acetylcholinesterase activity was detected following any of the exposures (Karol et al. 1984).
HEXAMETHYLENE DIISOCYANATE 51

2. HEALTH EFFECTS

A study by Mobay Corporation (1984), using male and female Sprague Dawley rats exposed to

0.005-0.3 ppm HDI for 5 hour a day, 5 day a week for 3 weeks, failed to produce any neurological lesions at

the highest dose tested. In a study by Mobay Corporation (1988), male and female Fischer 344 rats dosed

with HDI in concentrations ranging from 0.011 to 0.143 ppm for 6 hour a day, 5 day a week for 66-69 days

showed no clinical neurological effects or neurological lesions at gross necropsy or during histopathological

examinations. In a later study by Mobay Corporation (1989), male and female Fischer 344 rats were exposed

to HDI concentrations ranging from 0.005 to 0.164 ppm for 6 hour a day, 5 day a week over a 2-year period.

Again, no clinical neurological effects or neurological lesions could be found at gross necropsy or during

histopathologic examinations at the end of the study period.

The highest NOAEL values and all LOAEL values from each reliable study for neurological effects in each

species and duration category are recorded in Table 2- 1 and plotted in Figure 2-l.

2.2.1.5 Reproductive Effects

No studies were located regarding reproductive effects in humans after inhalation exposure to HDI.

No studies were located regarding reproductive effects in animals after acute-duration inhalation exposure to

HDI. No reproductive tract effects could be identified in laboratory animals exposed to inhalation doses of

HDI. A study by Mobay Corporation (1984), using male and female Sprague Dawley rats exposed to

0.005-0.3 ppm HDI for 5 hour a day, 5 day a week for 3 weeks, failed to produce lesions in any of the male

or female reproductive organs at the highest dose tested. In a study by Mobay Corporation (1988), male and

female Fischer 344 rats dosed with HDI in concentrations ranging from 0.011 to 0.143 ppm for 6 hour a day,

5 day a week for 66-69 days showed no reproductive organ lesions at gross necropsy or during

histopathological examinations. In a later study by Mobay Corporation (1989), male and female Fischer 344

rats were exposed to HDI concentrations ranging from 0.005 to 0.175 ppm for 6 hour a day, 5 day a week

over a 2-year period. Again, no reproductive organ lesions could be found at gross necropsy or during

histopathologic examinations at the end of the study period.

All LOAEL values from each reliable study for reproductive effects in each species and duration category are

recorded in Table 2-1 and plotted in Figure 2-l.
HEXAMETHYLENE DIISOCYANATE 52

2. HEALTH EFFECTS

2.2.1.6 Developmental Effects

No studies were located regarding developmental effects in humans or animals after inhalation exposure to

HDI.

2.2.1.7 Genotoxic Effects

No studies were located regarding genotoxic effects in humans or animals after inhalation exposure to HDI.

2.2.1.8 Cancer

No studies were located regarding cancer in humans after inhalation exposure to HDI.

Only one study was identified that described the potential carcinogenic activity in laboratory animals. In that

study, groups of 60 male and 60 female Fischer 344 rats were exposed 6 hour a day, 5 day a week for 2 years

to 0, 0.005, 0.025, or 0.175 ppm HDI via inhalation. Control rats were sham-exposed (conditioned air

exposure). At the end of the 2-year study period, none of the 3 inhaled concentrations of HDI was shown to

have an effect on the incidence of cancer in treated rats when compared to control animal populations (Mobay

Corporation 1989).

2.2.2 Oral Exposure

There is considerably less information available on the toxicology of HDI after oral exposure compared

to the data available on the inhalation toxicology of HD1 discussed in the previous section of this profile.

Clearly, inhalation is the major route of occupational exposure to HDI; however, given exposure routes

such as the lung mucocilliary clearance pathways, a very small amount of HDI could eventually enter

the gastrointestinal tract and be presented for absorption, with possible systemic effects. Most of the

information available on the oral absorption of HDI is about relatively large doses of HDI

administered to laboratory animals, with no information located on the health effects of HDI in humans after

oral exposure.
HEXAMETHYLENE DIISOCYANATE 53

2. HEALTH EFFECTS

2.2.2.1 Death

No studies were located regarding death in humans after oral exposure to HDI.

Death in laboratory animals has been reported in studies of acute-duration; however, no studies were

located for oral exposures of an intermediate and chronic-duration to HDI. Based on the information

available, large, single megadoses of HDI (i.e., >940 mg/kg) administered to rats orally were associated

with increased mortality, while lower single doses (<620 mg/kg) or lower multiple doses were associated

with little or no mortality in rats.

Reports of death after an acute oral exposure to HDI in laboratory animals appear in some older

toxicological studies on HDI. Rats (sex and strain not specified) received a single oral dose of 280, 420,

620, 940, 1,400, or 2,100 mg/kg of HDI. A single rat was used for each dose level. Rats at the 3 highest

doses died within 24 hours of exposure; however, rats at the 3 lower doses survived and were sacrificed

10 days after exposure and examined for lesions. The estimated minimum lethal dose in these rats was

calculated to be 940 mg/kg (Haskell Laboratory 1946).

In a later study by the same laboratory, HDI, undiluted or as a 5% solution in peanut oil, was

administered via gavage to male albino ChR-CD rats, in single doses from 12 to 3,400 mg/kg. Animals

receiving 3,400,2,250, and 1,500 mg/kg died within 2-21 hours. Prior to death, these animals developed

pallor, cyanosis, slow and deep breathing, and diarrhea. The approximate lethal dose (ALD) in that study

was determined to be 1,500 mg/kg (Haskell Laboratory 1961).

Male albino ChR-CD rats were administered 300 mg/kg HDI in peanut oil (as a 5% solution) via gavage

for 10 days over a 2-week period. All rats survived the treatments (Haskell Laboratory 1961).

The LOAEL values resulting in mortality in all species are recorded in Table 2-2 and plotted in

Figure 2-2.

2.2.2.2 Systemic Effects

No studies were located regarding cardiovascular, musculoskeletal, hepatic, renal, endocrine, dermal,

ocular, or metabolic effects in humans or animals after oral exposure to HDI. The LOAEL values from


HEXAMETHYLENE DIISOCYANATE 56

2. HEALTH EFFECTS

each reliable study for each systemic effect in each species in the acute-duration category are recorded in

Table 2-2 and plotted in Figure 2-2.

Respiratory Effects. No studies were located regarding respiratory effects in humans after oral

exposure to HDI.

No studies were located regarding respiratory effects in animals following intermediate- or chronicduration

oral exposure to HDI. Based on the available information at hand, the respiratory tract seems to

be a target organ of HDI toxicity after oral exposure. Rats (sex and strain not specified) received a single

oral dose of HDI, at a dose of 280, 420, 620, 940, 1,400, or 2,100 mg/kg of HDI, with one rat dosed at

each dose level. Rats at the 3 highest doses died within 24 hours of exposure; however, the rats at the

3 lower doses survived and were sacrificed 10 days after exposure. The rats that died showed congestion

of the lungs and spleen. Rats given the 420 and 620 mg/kg doses showed slight peribronchial edema, but

the authors doubted the significance of this finding (Haskell Laboratory 1946).

In another study of acute-duration, HDI, undiluted or as a solution with peanut oil, was administered via

gavage to male albino ChR-CD rats, in single doses from 12 to 3,400 mg/kg, one rat per dose level. Rats

receiving high sublethal doses of 1,000 and 670, as well as those receiving 450 and 300 mg/kg, were

observed to have slowed respiration after dosing (Haskell Laboratory 1961).

Gastrointestinal Effects. No studies were located regarding gastrointestinal effects in humans after

oral exposure to HDI.

No studies were located regarding gastrointestinal effects in animals following intermediate- or chronicduration

oral exposure to HDI. Male albino ChR-CD rats were administered 300 mg/kg HDI in peanut

oil via gavage for 10 days over a 2-week period. Half the animals were sacrificed after the final exposure

and half were sacrificed 10 days later. All rats survived treatment; however, some rats showed signs of

diarrhea and salivation during treatment. No clinical signs of toxicity were observed during the 10-day

post-treatment observation period. Ulcerative gastritis was observed in rats sacrificed immediately after

treatment, and healing gastritis was observed in rats sacrificed after the 10-day observation period

(Haskell Laboratory 1961). Rats administered a single dose of HDI in peanut oil also showed

inflammation of the stomach mucosa and diarrhea at the 60 and 40 mg/kg doses, respectively (Haskell

Laboratory 1961).
HEXAMETHYLENE DIISOCYANATE 57

2. HEALTH EFFECTS

Hematological Effects. No studies were located regarding hematological effects in humans after

oral exposure to HDI.

No studies were located regarding hematological effects in animals following intermediate- or chronic-duration

oral exposure to HDI. Pallor, cyanosis, slow and deep breathing, and diarrhea were observed

prior to death in male albino CD rats that were administered a single gavage dose of 1,500, 2,250, or

3,400 mg/kg HDI in peanut oil. These animals died within 2-21 hours of dosing (Haskell Laboratory

1961).

Body Weight Effects. No studies were located regarding body weight effects in humans after oral

exposure to HDI.

No studies were located regarding body weight effects in animals following intermediate- or chronicduration

oral exposure to HDI. An unspecified decrease in body weight was observed in male albino CD

rats that were administered a single gavage dose of 670 mg/kg HDI in peanut oil (Haskell Laboratory

1961). However, no body weight effects were observed in male albino CD rats administered 300 mg/kg

HDI in peanut oil by gavage for 10 days over a 2-week period (Haskell Laboratory 1961).

Other Systemic Effects. No studies were located regarding other systemic effects in humans after

oral exposure to HDI.

An unspecified increase in water consumption during the second week of exposure was observed in male

albino CD rats administered 300 mg/kg HDI in peanut oil by gavage for 10 days over a 2-week period

(Haskell Laboratory 1961).
HEXAMETHYLENE DIISOCYANATE 58

2. HEALTH EFFECTS

No studies were located regarding the following effects in humans or animals after oral exposure to HDI:

2.2.2.3 Immunological and Lymphoreticular Effects

2.2.2.4 Neurological Effects

2.2.2.5 Reproductive Effects

2.2.2.6 Developmental Effects

2.2.2.7 Genotoxic Effects

2.2.2.8 Cancer

2.2.3 Dermal Exposure

Dermal exposure to HDI, like oral exposure, is considered to be a secondary route of exposure in

humans. Little information is available on the toxicity of HDI applied to skin in either humans or in

animals.

2.2.3.1 Death

No studies were located regarding death in humans after dermal exposure to HDI.

A study by Thorne et al. (1987) described the effects of diisocyanates after topical exposure. The dermal

sensitization capabilities HDI and several other isocyanates (TDI, MDI, HDI) in BALB/cBy mice

exposed to a variety of topical doses of each isocyanate was performed. Groups of 4-32 male mice were

exposed to dermal doses of HDI in acetone. A topical dose of HDI at 2,800 mg/kg was shown to be

lethal to 100% of the mice within 16 hours of exposure. No other reports of death after topical exposure

to HDI were located.

The LOAEL values resulting in mortality in all species are recorded in Table 2-3.

2.2.3.2 Systemic Effects

No studies were located regarding respiratory, cardiovascular, gastrointestinal, hematological,

musculoskeletal, hepatic, renal, endocrine, metabolic, or body weight effects in humans or animals after



HEXAMETHYLENE DIISOCYANATE 62

2. HEALTH EFFECTS

acid (HDI) in acetone on the shaved, intact and non-occluded shoulder skin of 10 male albino guinea

pigs. HDI caused slight to no irritation at 24 hours as a 0.1% acetone solution and no irritation at

48 hours. As a 0.01% solution, no irritation was observed (E.I. DuPont de Nemours 1977a, 1977b).

Toxic cutaneous reactions also have been demonstrated in rabbits. Male New Zealand albino rabbits

were topically exposed to HDI by covering shaved skin with 2.5x2.5 cm cloths saturated with 500 μL of

undiluted HDI and covered with an inert PVC film (i.e., occluded dermal exposure). Duration of

exposure was for 4 hours. The skin under the patch was observed immediately following exposure and

24, 48, and 72 hours and 8 days later. HDI was found to be a direct irritant and severely irritating to the

skin of rabbits. Moderate to severe erythema and slight scabbing or corrosion was observed in all

animals 4 hours after exposure. Severe cutaneous edema was also observed in all but one animal 4 hours

after exposure. The application area showed severe congestion and severe skin thickening within 4 hours

after topical exposure. The epidermis subsequently underwent a parchment-like change (dry surface

necrosis). No gross cutaneous changes were reported 8 days after exposure (Mobay Corporation 1981b).

In an intermediate-duration study, HDI was applied 9 times over a period of 3 weeks to abraded skin of

adult male albino guinea pigs in solutions of 0.05-0.5% in 1:1 acetone-dioxane containing 13% guinea

pig fat. After a 3-week rest period, a repeat topical challenge test showed that 8 of 9 guinea pigs tested

had developed an allergic contact dermatitis to HDI at these concentrations (Haskell Laboratory 1961).

Ocular Effects. No studies were located regarding ocular effects in humans after topical exposure to

HDI.

HDI has been demonstrated to be an ocular irritant in laboratory animals in several studies. HDI

(0.1 mL, undiluted) was instilled into both eyes of a male albino rabbit. One eye was washed 20 seconds

later with large amounts of water, whereas the other eye was not washed. The animal was sacrificed

8 days after treatment. Initially, the exposure caused severe conjunctival inflammation accompanied by

serous and hemorrhagic exudates of both eyes, with severe (unwashed eye) or moderate (washed eye)

cornea1 injury. When the rabbit was sacrificed 8 days after treatment, the corneas of both eyes appeared

dull and the eyelids were inflamed and still showed the hemorrhagic and serous exudates. Healed

cornea1 lesions of both eyes and inflammation of the eyelids of the unwashed eye were also observed

8 days after treatment (Haskell Laboratory 1961).
HEXAMETHYLENE DIISOCYANATE 63

2. HEALTH EFFECTS

In a similar study, a single application of 100 μL of HDI was placed into the conjunctival sac of the

lower lid of both eyes in 6 male New Zealand albino rabbits. The lids were then held together for

1 second after application. The right eye was flushed 30 seconds after the application with saline and the

left eye was left unflushed. Eyes were examined for irritation with an ophthalmoscope and fluorescein

test (after 1 hour for the right eye and after 24 hours for the left eye). A high level of damage occurred to

the cornea, iris, and conjunctiva of both eyes. Detectable gross damage was caused to the eye after only

30 seconds of material contact. Damage to the cornea and iris of both the right and left eyes became

more severe as time since exposure increased. Seventy-two hours after exposure, no reflex reaction of

the eye to light stimulation was noted, with hemorrhaging, and/or gross destruction of the iris also

observed. The conjunctiva of the right and left eyes was inflamed and swollen, and discharge was

observed with damage becoming most severe by 24-48 hours and remaining severe throughout the

remainder of the study; inflammation was less severe in the left eye than in the right eye. Eight days

after exposure, complete cornea1 opacity was observed, with the iris not visible. As in the previous

study, HDI as a neat solution was confirmed to be corrosive to the eye when applied directly to the eye

and conjunctiva (Mobay Corporation 1981a).

2.2.3.3 Immunological and Lymphoreticular Effects

No studies were located regarding immunological and lymphoreticular effects in humans after topical

exposure to HDI.

In addition to its toxicity to the respiratory tract in both humans and animals, HDI has also been

demonstrated to be a contact skin sensitizer after dermal exposure in laboratory animals. Having seen

that other chemicals produce a dose-response relationship to dermal sensitization, Stadler and Karol

(1985) determined that such an approach could also be used to observe sensitizing potencies of HDI for

simulation of human occupational exposures. Male guinea pigs were first exposed to a topical

sensitizing dose of HDI (total dose approximately 105 mg). Seven days after the initial dose, another

topical dose of either 0, 0.01, 0.1, 1, or 10 mg was placed on the skin and examined for erythema for up

to 48 hours after this second challenge. Erythema was noted by 8 hours following challenge and reached

a maximum intensity at 24 hours after topical exposure. It was found that as the topical dose of HDI

increased, the intensity of the erythema scores also increased in a dose-responsive manner. Erythema

scores were lowest in the 0.1 mg treatment group and highest in 10 mg treatment group. Erythema

ranged from a pale pink color to a bright red cutaneous reaction. The relationship between dose and
HEXAMETHYLENE DIISOCYANATE 64

2. HEALTH EFFECTS

response was found to be statistically significant (p<0.05). No erythema response was noted in the

lowest dose treatment group, 0.01 mg.

A similar study also conducted by Stadler and Karol (1985) was performed in mice. Groups of

BALB/cBy mice were again exposed to a sensitizing dose of HDI placed on the skin (total dose

approximately 105 mg). Five days after the initial topical dose, another dose of 1, 10, 100, or 1,000 μg

was placed on the ear, followed by examination for an increase in ear thickness (i.e., swelling) 24 hours

later. A general dose-response curve was found for topical doses of HDI from 1 pg up to and including

100 μg HDI, although not as clearly defined as with the companion guinea pig study. As the dose

continued to increase above 100 μg, the mean ear thickness began to decrease. Further analysis showed a

clear dose-response relationship between the topical doses of 5 and 10 pg. The dose of chemical

required to sensitize 50% of the mice (SD50) was calculated to be approximately 0.20 mg/kg.

The dermal sensitization capabilities of HDI and several other isocyanates (MDI, HMDI, and TDI) were

determined using BALB/cBy mice exposed to a variety of topical doses of each isocyanate. The study

also attempted to determine if one isocyanate would confer dermal reactivity to another isocyanate (i.e.,

cross-reactivity). Groups of male mice were exposed to dermal doses of HDI and other diisocyanates

(separately) in acetone. Animals exposed only to topical doses of acetone served as controls and no

dermal reactions were noted in these animals. Dermal reactivity of each dose of HDI was determined by

using the Mouse Ear Skin Test (MEST). A dose-dependent increase in ear swelling was observed for

mice as the dose of HDI increased. The SD50 was calculated to be 0.088 mg/kg (60 times more potent

than TDI). The maximum sensitization dose of HDI was 2.80 mg/kg. Exposure of mice to 28 mg/kg

gave a comparable dermal response. This study also found that the order of potency for dermal

sensitization of the isocyanates tested was: HDI>HMDI>MDI>TDI. With respect to the crosssensitization potential of HDI with other isocyanates, mice sensitized to a specific isocyanate

demonstrated cross-reactions with all dermally applied aromatic or aliphatic isocyanates (including HDI).

For all isocyanates tested in this study, the severity of the dermal reactions was greatest when

rechallenged with the same isocyanate used for sensitization. Heterologous isocyanate challenges

elicited significantly smaller responses than homologus challenges. TDI was the least potent sensitizer,

and was the compound least able to evoke a dermal response in mice sensitized to other isocyanates.

This study also noted that the aromatic isocyanates (associated most frequently with respiratory

sensitization) induced less severe dermal reactions when compared to the aliphatic isocyanates, which are

most frequently associated with dermal sensitization (Thorne et al. 1987).
HEXAMETHYLENE DIISOCYANAT 65

2. HEALTH EFFECTS

The immunologic activity of phenyl isocyanate (PI) was investigated in Balb/c (6-8 weeks old) mice and

compared with responses to TDI, MDI, and HDI. The MEST was determined by adding 100 pL

containing 0.03-250 pg of the chemical diluted in acetone to the shaven abdomen of mice (n=6/group;

sex not specified). Doses of 0.001, 0.004, and 0.024 mmol/kg HDI were used. Four days later, 20 uL of

the chemical was applied to each side of the right ear (40 μL) total and an equal volume of acetone was

applied to the left ear. The doses used for the challenge application, 40 μg PI and 100 μg HDI, were

determined to be nonirritant in pilot studies. Ear thickness was measured at 24 hours following the

challenge. A significant response was defined as an increase in ear thickness >2 standard deviations

(SD) above the mean response of control animals which had received acetone on the abdomen and were

challenged on the ear with isocyanate. Serum IgE was assessed in mice exposed to PI and to TDI, and

the specific antibody response to PI was evaluated using ELISA. PI was found to be the most potent

isocyanate tested yielding an SD50 of 0.04 μmol/Kg, compared with SD50 values of 0.5, 2.1, and

30.4 μmol/kg for HDI, MDI, and TDI, respectively. PI was 12 times more potent than HDI (the second

most potent chemical), and 760 times more potent than TDI. Antibody titers to PI were more than

10-fold greater than those induced by TDI. The results suggest that PI is a potent inducer of both cellular

and humoral immune responses (Karol and Kramarik 1996).

HDI has also been identified as a skin sensitizer in studies of intermediate-duration. To test for

sensitization potential, a series of 4 four intradermal injections, once each week over a 3-week period,

were administered over the sacral skin area of male guinea pigs. Injection consisted of 0.1 mL of a 1%

solution (vol/vol) of HDI in dimethyl phthalate. Following a 2-week rest period, the test animals were

challenged for sensitization by applying and lightly rubbing in 1 drop each of a 0.1, 0.05, 0.01, and

0.005% solution (vol/vol) of HDI in acetone on non-occluded, shaved, intact shoulder skin. A group of

10 previously unexposed guinea pigs received similar applications at the time of challenge to provide a

direct comparison of the challenge reactions on skin of similar age. After a 2-week rest period, these test

animals were rechallenged for sensitization by applying and lightly rubbing in 1 drop of a 0.5 and 0.1%

solution of HDI in acetone. Of the animals tested, 50% showed sensitization responses when challenged

with a 0.1% acetone solution; mild (5 of 10) and moderate (5 of 10) erythema was observed. Challenge

with 0.05, 0.01, and 0.005% did not elicit a sensitization response. A rechallenge with the 0.5% HDI

solution showed sensitization in 8 of 10 animals; mild (1 of l0), moderate (1 of l0), and strong (2 of 10)

erythema and erythema plus edema (6 of 10) was observed 24 hours after challenge and mild (1 of l0),

moderate (2 of l0), and strong (3 of 10) erythema and erythema plus edema (4 of 10) was observed

48 hours after challenge. Rechallenge with 0.1% HDI did not elicit a sensitization response; mild
HEXAMETHYLENE DIISOCYANATE 66

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erythema (5 of 10) was observed at 24 and 48 hours post-challenge. The author concluded that HDI is

both a strong skin irritant and sensitizer (E.I. DuPont de Nemours 1977b).

Skin sensitization reactions can also occur with polymers of HDI. To test for HDI’s sensitization

potential to polymers, a series of 4 intradermal injections was given over the sacral area of the back of

male guinea pigs, one each week over a 3-week period. Each injection consisted of 0.1 mL of a 1%

solution (vol/vol) of the HDI monomer in dimethyl phthalate. Following a 2-week rest period, the test

animals were challenged for sensitization by applying and lightly rubbing in 1 drop each of a 0.1, 0.05,

0.01, and 0.005% solution (vol/vol) of HDI in acetone on the shaved intact shoulder skin. A group of

10 previously unexposed guinea pigs received similar applications at the time of challenge to provide a

direct comparison of the challenge reactions on skin of similar age. After a rest period of 1 week, the

animals were treated with the HDI polymer, Desmodur N-75, which contained 74% 1,3,5-tris (normalhexylisocyanate)-biuret and 0.45% free HDI monomer. The polymer had been diluted with acetone to

contain 0.1, 0.05, and 0.01% residual monomer. A rechallenge was done after an additional 3-week rest

period, and an additional group of previously untreated control animals were added for comparison with

previously treated control animals to determine if their previous exposures had caused sensitization.

When the polymer containing 0.45% residual monomer (HDI) was diluted with acetone to contain 0.1,

0.05, and 0.01% residual monomer, sensitization responses were elicited in 8 of 9 test animals; 0.01%

appeared to be a marginal “no response” level. Mild (4 of 9), moderate (2 of 9), and strong (3 of 9)

erythema was observed 24 hours after challenge; and mild (4 of 9), moderate (4 of 9), and strong (1 of 9)

erythema was observed 48 hours after challenge with 0.1%. Mild (6 of 9) and strong (1 of 9) erythema

was observed 24 hours after challenge and mild (8 of 9) erythema was observed 48 hours after challenge

with 0.05%. At this time, 4 of 10 control animals had become sensitized by the single topical exposure to

the monomer during the first challenge; mild, moderate, or strong erythema was observed. When

rechallenged and compared to previously untreated controls, the sensitization response ratio had

increased to at least 6 of 10 of the original controls, with questionable reactions observed in the

remaining 4 animals; mild or moderate erythema or erythema plus edema was observed in affected

animals.

In rechallenged test animals, mild (3 of 9), moderate (2 of 9) and strong (3 of 9) erythema and erythema

plus edema (1 of 9) was observed 24 hours after rechallenge and mild (3 of 9), moderate (5 of 9), and

strong (1 of 9) erythema was observed 48 hours after rechallenge with 0.1%. Mild (5 of 9) and moderate

(2 of 9) erythema and erythema plus edema (1 of 9) was observed 24 hours after rechallenge and mild
HEXAMETHYLENE DIISOCYANATE 67

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(5 of 9) and moderate (2 of 9) erythema was observed 48 hours after rechallenge with 0.05%. Mild (2 of

9) and moderate (1 of 9) erythema was observed 24 hours after rechallenge and mild (3 of 9) erythema

was observed 48 hours after rechallenge with 0.01% (E.I. DuPont de Nemours 1977a). This study

demonstrated that HDI monomer may not be the only component in a Desmodur N-75 that can elicit

cutaneous sensitization reactions in laboratory animals.

The highest NOAEL values and all LOAEL values from each reliable study for immunological and

lymphoreticular effects in each species in the acute-duration category are recorded in Table 2-3.

No studies were located regarding the following effects in humans or animals after dermal exposure to

HDI:

2.2.3.4 Neurological Effects

2.2.3.5 Reproductive Effects

2.2.3.6 Developmental Effects

2.2.3.7 Genotoxic Effects

2.2.3.8 Cancer

2. 3 TOXICOKINETICS

Very little information exists on the toxicokinetics of HDI in animals. More information is available on

the toxicokinetic and pharmacokinetics of aromatic diisocyanates. Since HDI is an aliphatic

diisocyanate, no useful toxicokinetic extrapolations can be applied to derive information about the

absorption, distribution, metabolism, and excretion behavior of HDI in animals.

Based on some known properties of HDI in some in vitro studies, HDI can be expected to hydrolyze in

aqueous media. This hydrolysis process is fairly slow, but is accelerated in the presence of carboxylic

acid-containing neutral buffers (Berode et al. 1991), such as are present in biological matrices.

Hydrolysis probably begins in the aqueous media lining the trachea and bronchi (inhalation route) or by

water and acid hydrolysis (oral route) and continues at an accelerated rate. Because of this hydrolytic

action, the toxic potential of HDI (particularly in acute exposures) is directly applicable to its

concentration and direct interaction with cellular components at the site of exposure. Absorption of

significant amounts of HDI into the general circulation would, therefore, not be expected. Any free HDI
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that may reach the blood may bind to serum proteins and not be available as a free form in the blood (Tse

and Pesce 1979). No reports were found in the literature that reported detecting blood levels of HDI in

humans or animals. The major urinary metabolite of HDI reported in the literature is 1,6-hexamethylene

diamine (HDA) (Brorson et al. 1990a, 1990b), with several reported methods for its detection available

(Dalene et al. 1990, 1994; Rosenberg and Savolainen 1986). Since little HDI is absorbed, the only

toxicokinetic parameters that are readily available in the literature are the absorption and excretion data.

No reports on the absorption or metabolism of HDI after topical exposure were available in the literature.

2.3.1 Absorption

Little information was located in the available literature on the absorption of HDI after inhalation, oral,

or dermal exposure. Information is limited to one report of oral administration to men (Brorson et al.

1990a). The metabolite of HDI (1,6-hexamethylene diamine, HDA) was not detected in the serum of

these men after a 7.5-hour inhalation exposure to HDI (Brorson et al. 1990b).

2.3.2 Distribution

No information was located in the available literature on the distribution of HDI after inhalation, oral, or

dermal exposure.

2.3.3 Metabolism

As stated earlier, the major metabolite of HDI in humans appears to be 1,6-hexamethylene diamine

(HDA). No information was located in the available literature that specifically addressed the metabolism

of HDI after inhalation, oral or dermal exposures.

2.3.4 Elimination and Excretion

2.3.4.1 Inhalation Exposure

Only one study was located in the literature that described the elimination and excretion of HDI after an

inhalation exposure. In that study, 5 men (age, 36-50 years; mean age, 42 years) inhaled 95-l 15 μg

(0.01-0.02 ppm) of HDI in air (range, 25-29 μg/m3

 or approximately 0.004 ppm) for 7.5 hours. Blood
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and urine samples were taken at 2-hour intervals. Beginning almost immediately after exposure, urinary

levels of 1,6-hexamethylene diamine (HDA) began to accumulate in the urine. The urinary elimination

rates for all men ranged from 1.1 to 1.7 μg/hr, with the average urinary level of I-IDA ranging from

0.01-0.03 mmol/mol creatinine for the S-hour sample and 0.006 mmol/mol creatinine for the 10-hour

sample. HDA levels were undetectable by 15 hours after exposure began (or 7.5 hours after exposure

ended). The cumulative excretion of HDA over a 28hour period was 8.0-14 μg, which is about 1l-21%

of the inhaled dose of HDI. The half-life of HDA in the urine in these men ranged from 1.1 to 1.4 hours

(mean = 1.2 hours) (Brorson et al. 1990b).

2.3.4.2 Oral Exposure

Little information was located in the available literature on the elimination of HDI after oral exposure.

One study by Brorson et al. (1990a) was located that described the elimination and excretion of HDA in

the urine of men after oral dosing with HDA. Six males were administered an oral dose of 0.1 mg/kg

HDA on 2 occasions 3 months apart, and urine was collected for several hours after dosing. Peak

amounts of free HDA in single urine samples ranged from 0.080 to 0.19 mg 2-5 hours after dosing. Four

of the 6 men tested had no detectable levels of HDA in their urine 10 hours after dosing; however,

2 subjects had detectable levels of HDA in the urine 15 hours after dosing. The elimination half-life was

calculated to be approximately 1.5 hours; l-6% of the total HDA dose was recovered in the urine. The

authors also noted two pathways by which HDA could be metabolized: (1) to N-acetyl1,6-hexamethylene diamine, via N-acetyl transferase, and (2) 6-aminohexanoic acid, via diamine oxidase.

Both HDA metabolites may appear in the urine.

2.3.4.3 Dermal Exposure

No information was located in the available literature on the elimination of HDI after dermal exposure.

2.3.5 Physiologically Based Pharmacokinetic (PBPK)/Pharmacodynamic (PD) Models

Physiologically based pharmacokinetic (PBPK) models use mathematical descriptions of the uptake and

disposition of chemical substances to quantitatively describe the relationships among critical biological

processes (Krishnan et al. 1994). PBPK models are also called biologically based tissue dosimetry
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models. PBPK models are increasingly used in risk assessments, primarily to predict the concentration

of potentially toxic moieties of a chemical that will be delivered to any given target tissue following

various combinations of route, dose level, and test species (Clewell and Andersen 1985). Physiologically

based pharmacodynamic (PBPD) models use mathematical descriptions of the dose-response function to

quantitatively describe the relationship between target tissue dose and toxic end points.

PBPK/PD models refine our understanding of complex quantitative dose behaviors by helping to

delineate and characterize the relationships between: (1) the external/exposure concentration and target

tissue dose of the toxic moiety, and (2) the target tissue dose and observed responses (Andersen and

Krishnan 1994; Andersen et al. 1987). These models are biologically and mechanistically based and can

be used to extrapolate the pharmacokinetic behavior of chemical substances from high to low dose, from

route to route, between species, and between subpopulations within a species. The biological basis of

PBPK models results in more meaningful extrapolations than those generated with the more conventional

use of uncertainty factors.

The PBPK model for a chemical substance is developed in four interconnected steps: (1) model

representation, (2) model parametrization, (3) model simulation, and (4) model validation (Krishnan and

Andersen 1994). In the early 1990s, validated PBPK models were developed for a number of

toxicologically important chemical substances, both volatile and nonvolatile (Krishnan and Andersen

1994; Leung 1993). PBPK models for a particular substance require estimates of the chemical substancespecific

physicochemical parameters, and species-specific physiological and biological parameters. The

numerical estimates of these model parameters are incorporated within a set of differential and algebraic

equations that describe the pharmacokinetic processes. Solving these differential and algebraic equations

provides the predictions of tissue dose. Computers then provide process simulations based on these

solutions.

The structure and mathematical expressions used in PBPK models significantly simplify the true

complexities of biological systems. If the uptake and disposition of the chemical substance(s) is

adequately described, however, this simplification is desirable because data are often unavailable for

many biological processes. A simplified scheme reduces the magnitude of cumulative uncertainty. The

adequacy of the model is, therefore, of great importance, and model validation is essential to the use of

PBPK models in risk assessment.
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PBPK models improve the pharmacokinetic extrapolations used in risk assessments that identify the

maximal (i.e., the safe) levels for human exposure to chemical substances (Andersen and Krishnan 1994).

PBPK models provide a scientifically sound means to predict the target tissue dose of chemicals in

humans who are exposed to environmental levels (for example, levels that might occur at hazardous

waste sites) based on the results of studies where doses were higher or were administered in different

species. Figure 2-3 shows a conceptualized representation of a PBPK model.

If PBPK models for HDI exist, the overall results and individual models are discussed in this section in

terms of their use in risk assessment, tissue dosimetry, and dose, route, and species extrapolations.

No PBPK/PD models were identified for HDI in the open literature.

2.4 MECHANISMS OF ACTION

No specific information on the pharmacokinetic mechanisms, mechanisms of toxicity, or animal-toHuman extrapolations of these parameters for HDI were located in the available literature. In general,

both aliphatic and aromatic isocyanates are considered to be pulmonary, oral, and dermal irritants.

Several studies discussed earlier have reported respiratory irritation, which included burning and

irritation to the nasal tract, throat, and the chest after inhalation exposure (Baur et al. 1984; Cockcroft

and Mink 1979; Grammer et al. 1988; Patterson et al. 1990; Tornling et al. 1990; Usui et al. 1992). Oral

exposure can also produce irritation of the mouth, pharangeal region, and the gastrointestinal tract. Eye

contact produced severe eye irritation, resulting in moderate-to-severe lacrimation, photophobia, and

edema, as well as severe dermatitis after topical skin exposure (Patterson et al. 1990; Haskell Laboratory

196 1; Mobay Corp. 198la; Von Burg 1993). The exact mechanism of action for producing irritation by

all of these routes are unknown, but it is likely related to their high reactivity with biological macromolecules and various body proteins (Karol 1986; Von Burg 1993). Most isocyanates are also

considered to be potential respiratory tract sensitizers (E.I. DuPont de Nemours 1977b; Malo et al. 1983;

Tornling et al. 1990) and, although many investigators have attempted to elucidate the immunological

mechanisms behind this response, the mechanism(s) involving sensitization are likely quite complex and

are still unknown.

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2.5 RELEVANCE TO PUBLIC HEALTH

Overview

There is a large body of information available in the open literature on the toxicological and occupational

hazards of diisocyanate compounds (particularly on toluene diisocyanate); however, relatively little

information is available specifically for HDI or HDI polymers. It would be convenient to be able to use

the available data on other diisocyanates, such as toluene diisocyanate, to extrapolate any missing

information on HDI toxicity; however, fundamental differences in chemical properties and metabolism

have precluded that possibility.

Based on the limited human and animal data at hand, it is possible to draw a few conclusions about the

toxicity of HDI in both humans and animals. HDI is an occupational health hazard to some individuals,

especially people employed in the automotive paint industry who come in contact with paint hardeners

containing HDI monomers and polymers (Belin et al. 1981; Grammar et al. 1988; Patterson et al. 1990;

Tornling et al. 1990). The target organ of HDI toxicity in humans is the respiratory tract, with most

exposures resulting from inhaling vapors from HDI or its prepolymers (Alexandersson et al. 1987; Malo

et al. 1983; Tornling et al. 1990; Tulane Medican 1982a, 1982b). All the data suggest that HDI is a

direct irritant to the respiratory tract. A significant immune component is also present in HDI-induced

respiratory toxicity, resulting in asthma-like symptoms (Belin et al. 1981; Grammar et al. 1988; Patterson

et al. 1990). Other organs may also be affected in humans; however, the only supporting data available

for this conclusion comes from laboratory animals exposed to HDI or its prepolymers (Haskell

Laboratory 1961; Mobay Corporation 1984, 1989). Toxic effects of HDI exposure via the oral and

dermal routes of exposures have been demonstrated in laboratory animals as well; however, this does not

seem to be a major route of exposure for humans (Haskell Laboratory 1961). HDI has been demonstrated

to be a strong dermal sensitizer in mice, with dermal cross-reactivity demonstrated with other aliphatic

and aromatic isocyanates as well (Thorne et al. 1987). If unreacted HDI were present at hazardous waste

sites, the major concern would be exposure via the inhalation route; thus, respiratory protection would be

required in those situations. HDI has not been detected at any hazardous waste site. It is unlikely that

unreacted HDI would be present at any such site, due to HDI’s propensity to react quickly to form other

compounds. Those persons involved in the clean-up at the site should also be aware of the skin irritant

potential of this compound.
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Minimal Risk Levels for Hexamethylene Diisocyanate.

Inhalation MRLs.

• An MRL of 3.0x 10-5 ppm has been derived for intermediate-duration inhalation exposure

(15-364 days) to hexamethylene diisocyanate.

The intermediate-duration inhalation MRL was based on a NOAEL of 0.005 ppm administered to rats for

5 hours a day, 5 days a week for 3 weeks (doses were 0.005 ppm, 0.0175 ppm, 0.15 ppm, 0.3 ppm) (Mobay

Corporation 1984). At 0.3 ppm, decreased kidney weights were observed in both male and female rats, with

decreased liver weights observed in females only. Hepatic and renal effects were not seen at an inhaled dose

of 0.15 ppm and lower. Nasal lesions occurred in 80-90% of the animals exposed at the 0.3 ppm level, while

only 50-70% of the animals were affected at the 0.15 ppm concentration. No significant lower respiratory

tract alterations were noted at the 0.0175 ppm inhalation dose; however, hemorrhage, inflammatory exudate,

and epithelial changes were observed in the nasal cavity. Subsequently, these effects at the 0.0175 ppm dose

were classified as minimal and used to base the inhalation intermediate-duration MRL. The NOAEL was

placed at 0.005 ppm. For purposes of calculating human equivalency concentrations, these effects were also

classified as extra-thoracic respiratory tract effects (EPA 1994b). Since there is no reported threshold for

HDI immunological hypersensitivity in humans, this MRL may not be protective for persons with

hypersensitivities to HDI. Respiratory tract lesions in laboratory animals have been described in other studies

(Dow Chemical 1964; E.I. DuPont de Nemours 1978; Haskell Laboratory 1961; Karol et al. 1984); however,

the doses tested were much higher than those tested in the Mobay Corporation (1984) study. More

information on the Mobay Corporation (1984) study and the calculations used to derive this MRL are

available in Appendix A.

No human studies were located that described any of the nasal tract lesions which occurred in the

HDI-exposed rats. However, the available studies did not specifically examine this particular

endpoint in HDI-exposed human populations. In addition, anatomical differences exist between

rodents and humans (e.g. complex nasal turbinates and close apposition of the epiglottis and soft

palate in rodents). It has been suggested that because of these differences, nasal epithelial changes

observed in rats may roughly translate into effects in the more distal sections of the human respiratory

tract (Haschek and Witschi 1991). This may be a significant point that may require further

investigation. Several human studies indicated other toxicological properties that can be attributed to

either HDI, HDI polymers, or a combination of both of these chemical forms functioning in tandem to

produce the reported health effects in humans. It is clearly preferable to utilize human exposure studies when
HEXAMETHYLENE DIISOCYANATE 75

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deriving MRLs; however, given the typical exposure scenarios reported in Section 2.2 of this profile, the

human data suggest that auto painters (the population most likely to be exposed to HDI) are exposed to

mixtures of both HDI and HDI polymers, making it difficult to definitively state that effects (LOAELs)

such as coughing, alterations in pulmonary function parameters, chills, chest tightness; alterations in

immune function, or any number of other health effects, are due exclusively to exposure to HDI and not

its polymers, and vice-versa (Alexandersson et al. 1987; Grammar et al. 1988; Malo et al. 1983; Tornling

et al. 1990). This mixture exposure (HDI, HDI polymers, volatile organics, etc.) disqualified many of the

human studies reported in this profile that could otherwise have been used for MRL derivation. A study

by Shepperly and Hathaway (1991) reported a NOAEL for workers exposed to HDI at concentrations of

5 ppb (0.005 ppm) or less at a plant in Freeport, Texas. These workers had been chronically exposed to

HDI for >l year with no statistically significant differences in pulmonary function test data, nor any

significant increase in the frequency of respiratory complaints observed in these exposed workers versus

the control (unexposed) population. A later study by DeWilde and Hathaway (1994), again using

chronically exposed workers at the Freeport, Texas plant, found no statistically significant differences in

pulmonary function data among HDI-exposed individuals and the control group. The dose in that study

was estimated to be between 0.5 and 7 ppb (0.0005-0.007 ppm). Both the Shepperly and Hathaway

(1991) and the DeWilde and Hathaway (1994) studies provided estimates of HDI doses to which workers

were exposed, but neither study could provide definitive exposure doses to the worker populations. Both

studies also had difficulties with some of the industrial hygiene monitoring devices and personal

dosimetry devices, which may have provided inaccurate exposure data. In addition to occasional high

short-term exposures (10-20 ppb), there were also some large variations in pulmonary function test

results, which varied markedly from year to year and were attrihuted to human error. Again, these study

limitations precluded either of these reports from being used to derive an intermediate-duration MRL

based on human exposures, but do lend some support to the MRL based on results found using the rat

model.

• An MRL of 1.0x10-5 ppm has been derived for chronic-duration inhalation exposure (365 days

or more) to hexamethylene diisocyanate.

The chronic-duration inhalation MRL was based on the study by Mobay Corporation (1989).

Fischer 344 rats were exposed to 0, 0.005, 0.025, or 0.175 ppm HDI for 5 day a week, 6 hour a day for

2 years. A satellite group was also exposed for 1 year. Reticulocytosis (less serious LOAEL), as well as

eye irritation (observed in males only, first year only), were noted at the 0.175 ppm dose. At the
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0.025 ppm dose, nasal cavity hyperplasia/metaplasia, lung epithelialization, alveolar macrophage

accumulation (less serious LOAEL) were observed. Nasal cavity epithelial hyperplasia (minimal LOAEL)

was also reported at the 0.005 ppm dose level in female rats and was used to derive the chronic inhalation

MRL for HDI.

It should be noted that the EPA Reference Concentration (RfC) for HDI was based on the same study as this

chronic-duration inhalation MRL (Mobay 1989) and was also calculated to be 1.0×10-5 ppm. A report by

Foureman et al. (1994) described how this RfC was derived using the 0.005 ppm as the NOAEL dose end

point; for purposes of chronic-duration inhalation MRL derivation, the MRL was based on the same dose end

point, but was classified as a minimal LOAEL. Foureman et al. (1994) argue that although an effect was

seen at the 0.005 ppm dose (nasal epithelial hyperplasia), this response should be classified as an adaptive

response (as noted with many types of other irritants) and not a true toxic response. They concluded that the

olfactory degenerative response should be considered the significant effect in these rats, because it followed a

concentration-response relationship for both incidence and severity. In contrast, the hyperplastic and

inflammatory responses followed the traditional dose-response for incidence, but not for severity of the

lesions. The ATSDR Minimal Risk Level Workgroup carefully reviewed this data and the arguments

presented by the Foureman et al. (1994) report and concluded that the hyperplasia and hyperkeratosis were,

indeed, adverse (toxic) effects and warranted a classification as a minimal LOAEL. After uncertainty factors

were applied, the RfC and the MRL concentration values resulted in the same value, 1.0x10-5 ppm, despite

the differences in end point classification. This study involving the exposure of rats to HDI demonstrates that

the line between an adaptive and toxic response in not always clearly defined, and it may be a matter of

scientific judgement as to whether the effects are true adverse toxic responses.

A few human studies (Alexandersson et al. 1987; Cockcroft and Mink 1979; Grammar et al. 1988) were

identified that described the respiratory toxicity associated with the inhalation of HDI and, as such, were

considered for use in deriving the chronic-duration inhalation MRL. The human studies had many limitations

and generally did not adequately define concentrations and chemical compositions of the inhaled vapor. The

Alexandersson et al. (1987) study was determined not to be suitable for use in deriving an MRL, due to the

fact that workers were simultaneously exposed to both the monomeric as well as the trimer forms of HDI, as

discussed earlier. The authors of that study were unable to determine if the toxicological effects described

were solely due to the monomeric or trimer form of HDI, or if the combination of the two chemicals were

responsible for the observed changes in respiratory function.
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As discussed earlier, a study by Shepperly and Hathaway (1991) reported a NOAHL for workers exposed

to HDI at concentrations of 5 ppb (0.005 ppm) or less at a plant in Freeport, Texas. A later study by

DeWilde and Hathaway (1994), again using chronically exposed workers at the Freeport, Texas plant,

found no statistically significant differences in pulmonary function data among HDI-exposed individuals

and the control group. The dose in that study was estimated to be between 0.5 and 7 ppb

(0.0005- 0.007 ppm). Both the Shepperly and Hathaway (1991) and the DeWilde and Hathaway (1994)

studies provided estimates of doses that the workers were exposed to, but could not provide definitive

exposure doses to the worker populations. Both studies also had a number of difficulties (discussed

above), which precluded their use in deriving a chronic-duration MRL based on human exposures, but do

lend some support to the chronic-duration MRL based on results found using the rat model.

The Mobay Corporation (1989) study was the only animal study identified that defined the chronic

toxicity of inhaled monomeric HDI in laboratory animals. More information on the Mobay Corporation

(1989) study and the calculations used to derive this MRL are available in Appendix A.

Oral MRLs.

MRLs for oral exposure to HDI were not derived for any duration category because the available data in

the open literature were considered to be insufficient. No reports of humans orally exposed to HDI were

found in the open literature; very few reports of oral exposure in laboratory animals exist (Haskell Laboratory

1946, 1961).

Death. No reports of death were found in humans exposed to HDI by any route of exposure; however,

several dated reports of death occurring in laboratory animals were found for all three primary routes of

exposure. When inhaled, HDI causes death in animals at doses as low as 4 ppm when exposed for

4 hours (Karol et al. 1984). Death was related to respiratory impairment. Higher inhaled doses resulted

in death as well (Dow Chemical Co. 1964; Haskell Laboratory 1961). Some of the studies that described

exposure to HDI for acute durations used small numbers of animals for each exposure concentration, so firm

LC50, values (lethal concentration, 50% kill) may be difficult to derive (Dow Chemical 1964, Haskell

Laboratory 1961). Kimmerle (1976) reported an LC50, of 45 ppm in male Wistar rats; however, this may

be the LC,, for the mixture (HDI + solvent) and not for the HDI itself. Animals exposed via inhalation to

lower concentrations of HDI (<1 ppm) for intermediate and chronic durations had high survival rates,
HEXAMETHYLENE DIISOCYANATE 78

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indicating that HDI seems to be tolerated at low doses for long periods of time with no outward adverse

effects (Mobay Corporation 1984, 1988, 1989).

Deaths due to oral exposures were reported to occur at very high doses in laboratory animals

(>940 mg/kg). The cause of death in many cases was not reported; however, respiratory impairment may

have been responsible (Haskell Laboratory 1961). Due to the limited data available, it is not known if

the vehicle plays a role in oral HDI toxicity in laboratory animals. As with the inhalation studies

described above, using small numbers of animals in testing lethal oral concentrations makes deriving a

firm LD50 difficult. Larger doses of HDI placed topically also resulted in death (Thorne et al. 1987).

Systemic Effects.

Respiratory Effects. Several studies have described the respiratory toxicity of HDI monomer and HDI

polymers after inhalation exposures in both humans and laboratory animals. A few reports indicated that

no respiratory effects after prolonged exposure to HDI at concentrations as low as 0.5 ppb (0.0005 ppm )

and as high as 7 ppb (0.007 ppm) could be found (DeWilde and Hathaway 1994; Shepperly and

Hathaway 1991). Unfortunately, these studies suffered from small sample sizes and a number of

difficulties in determining an accurate exposure dose. Conversely, in the majority of the reports in which

humans were exposed to either the same or similar range of doses as those in the Shepperly and

Hathaway (1991) and DeWilde and Hathaway (1994) studies (<l-20 ppb), the predominant clinical sign

is an asthma-like syndrome, appearing soon after an exposure to a commercial product containing HDI

and its polymers. Clinical signs indicate respiratory compromise, including shortness of breath,

wheezing, tightness of the chest, bronchitis, and coughing (non-productive and productive). When

individuals with these clinical signs were subjected to respiratory function tests, total lung capacity, vital

capacity, forced expiratory volume (1 set), and PaO2 were noted to markedly decrease, with an increase

in residual lung capacity (Akbar-Khanzadeh and Rivas 1996; Bauer et al. 1984; Cockcroft and Mink

1979; Malo et al. 1983; Tulane Medican 1982a). A bronchoconstrictive response may or may not be

demonstrated with the use of bronchoconstrictive agents (acetylcholine, histamine, etc.) (Alexandersson

et al. 1987; Malo et al. 1983; Tulane Medican 1982a). These studies clearly indicate that HDI (either in

the monomeric or polymeric form), either through some local, direct effect or via immune system

modulation, adversely affects the ability of the lungs to function during and after HDI exposure

(Alexandersson et al. 1987; Tornling et al. 1990).
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Many of these reports imply that monomeric HDI is the causative principal agent responsible for these

respiratory and immunologic reactions; however, other HDI prepolymers (dimeric, biuret, or trimer) and

other diisocyanates may also be responsible for inducing an asthmatic reaction in sensitized individuals

(Vandenplas et al. 1993), but this possibility was overlooked in those studies. It seems likely that HDI

prepolymers are also responsible for these reactions because most commercial products containing HDI

(i.e., paint hardeners) contain as little as 0.2% monomeric HDI and up to 50-70% (estimates vary) of the

prepolymer forms, as well as other organics (xylene, etc.) (Alexandersson et al. 1987). Most of these

human reports are based on cases in which a worker who is suspected of having HDI intolerance (usually

with a history of being an automobile painter or similar factory worker) is subjected to a controlled

exposure to the paint vapor for a small amount of time, and the elicited response (clinical signs, blood

gas, respiratory parameters, etc.) is recorded. Although a significant number of responses are incurred in

these studies in this way, it is obvious that some component(s) of the paint induce an asthmatic reaction

in these individuals. However, it is by no means clear that the HDI monomer is the specific chemical

responsible for these reactions (Alexandersson et al. 1987; Tomling et al. 1990; Tulane Medican 1982a).

Only one study (Vandenplas et al. 1993) has demonstrated that some individuals do have asthmatic

reactions to only the HDI monomer, some to only the prepolymer form, and others to both monomer and

prepolymer. It has also been demonstrated that there is significant cross-reactivity among aromatic

diisocyanates (TDI, MDI) in their ability to induce asthmatic reactions in humans. This may occur

through IgE-hapten formation, a local effect of the isocyanates on the lung tissue (due to irritant or

pharmacological actions on the airways), or a combination of specific hypersensitivity to diisocyanates

with an increase in non-specific bronchial airway reactivity (International Isocyanate Institute 1987a).

The conclusion to be drawn from all of these studies is that the HDI monomer may be responsible for

eliciting the asthmatic reactions observed in sensitized individuals; however, other compounds

aerosolized with the monomer (in particular the HDI polymers) may also elicit these reactions by

themselves or in concert with the HDI monomer.

While the human studies describe the allergic component of HDI toxicity, most of the animal studies

describe the direct irritant effects of HDI and HDI prepolymers after inhalation. Laboratory animals

exposed to HDI via inhalation showed such adverse signs as respiratory irritation, tracheitis, pleural

effusion, pulmonary hemorrhage, bronchitis, and bronchopneumonia, mostly at concentrations >l ppm

(Dow Chemical 1964; E.I. DuPont de Nemours 1978; Haskell Laboratory 1961; Karol et al. 1984).

These studies clearly demonstrated that HDI is irritating to the respiratory tract and may be responsible

for the decreased respiratory rates noted in two studies (E.I. DuPont de Nemours 1978; Mobay
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Corporation 1982). The RD50 was calculated in mice to be 0.36 ppm during a 3-minute exposure and

1.42 ppm for a 30-minute exposure, with evidence to suggest that rats develop tolerance to HDI after short

periods of exposure (Mobay Corporation 1982). HDI was also demonstrated to be much more of a

respiratory irritant than other more commonly used diisocyanates (e.g., TDI and MDI) (E.I. DuPont de

Nemours 1978).

When tested in laboratory animals at < l ppm concentrations, the animals exhibited varying degrees of

respiratory tract irritation and degenerative nasal responses. Nasal lesions tended to occur in a dose- related

manner, which included varying degrees of hemorrhage, inflammatory exudate, epithelial changes, loss of

cilia, and changes in cell type, which strongly indicate cellular damage induced by HDI vapor in rats over a

3-week exposure period (Mobay Corporation 1984). The lungs of rats exposed for a 2-year period also

sustained histologic damage. Lung anomalies induced over a 2-year period in rats included epithelialization

and interstitial pneumonia with macrophage accumulation in the lung; however, the nasal lesions, which

included hyperplasia/metaplasia, atrophy, ulceration of the olfactory epithelium, hyaline droplet degeneration,

hyperkeratosis and chronic inflammation, were still the most outstanding lesions found, with an adaptive

nasal response occurring in the lowest dose group (0.005 ppm) after 1 year (Mobay Corporation 1989).

These studies indicate that, at least in laboratory animals, HDI induces a highly toxic response in the nasal

cavity and in the lungs when inhaled at very low (0.005-0.175 ppm) concentrations over a long period of

time (2 years). The exact mechanisms responsible for these nasal lesions are unknown, but may be related to

the nose’s ability to remove a large percentage of some organic compounds for metabolism or for temporary

storage and removal into the exhaled air-stream (Dahl and Hadley 1991; Dahl and Lewis 1993; Dahl et al.

1991; Gerde and Dahl 1991; Lewis et al. 1992; Snipes et al. 1990). Increasing the interaction time of HDI

with the nasal mucosal tissue possibly resulted in the gross and histological lesions observed in these animals.

Although these reports find compelling evidence for the induction of nasal lesions due to long-term HDI

exposure in laboratory animals, there have been no reports of similar nasal lesions found in humans exposed

to HDI.

It is unclear what the precise effects of HDI are on the respiratory tract after oral exposure. In the studies that

were examined, any effects on the respiratory tract were a result of megadoses of HDI. In many cases, these

doses resulted in death, with congestion of the lungs, peribronchial edema, altered respiration, and other nonspecific respiratory symptoms (Haskell Laboratory 1946,196l). No histopathology was performed on the

lungs in these studies, so it is not possible to speculate about what was occurring at the cellular and molecular

level in these tissues.
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Cardiovascular Effects. Limited information on the cardiovascular toxicity of HDI was available. Three

inhalation studies conducted in large numbers of rats exposed to doses ranging from 0.005 to 0.3 ppm for

intermediate- and chronic-durations did examine the heart tissue post-mortem and failed to produce any

biologically significant gross anatomical or histopathological evidence of HDI-induced toxicity (Mobay

Corporation 1984, 1988, 1989). It is not known if there are significant changes in heart function

parameters (ECG, heart rate, stroke volume, etc.) during HDI exposures. No oral or dermal exposure

studies were located that described the cardiac toxicity of HDI in either humans or laboratory animals.

Gastrointestinal Effects. Reports of gastrointestinal effects of HDI toxicity have been found in

laboratory animals only. Ulcerative gastritis was reported in male rats after they received HDI in peanut

oil via gavage; however, it is not known whether this was a side-effect of gavage administration

technique (Haskell Laboratory 1946, 1961) or if the data were skewed by the small numbers of animals

used during that study. One inhalation study did report chronic gastritis in male rats exposed to 72 ppm

HDI (Haskell Laboratory 1961), but 2 other studies using much lower exposure concentrations (<1 ppm)

failed to produce any gastrointestinal tract lesions. Based on this limited data, it is unclear whether HDI

has ulcerogenic properties at higher doses in laboratory animals.

Hematological Effects. HDI appears to produce some mild hematological effects in both humans and

laboratory animals. If an allergic component was producing adverse effects, elevations in circulating IgE

(as well as other immunoglobulins) and increased numbers of eosinophils can reasonably be expected in

peripheral blood. Following inhalation of HDI-containing vapors in humans, mild leucocytosis but no

eosinophilia were noted in two cases that occurred after an asthmatic reaction began to occur (Malo et al.

1983; Patterson et al. 1990). In both cases, the workers were not exposed to the pure form of HDI, but

rather to a vapor and particulates produced by paints containing HDI, HDI prepolymers, and other

organics normally found in automotive paints. Allergic reactions to either of these forms of HDI, in

addition to any of the other myriad of organics found in these paints, may have elicited this mild

elevation of leukocytes. No data were available on changes in hematology after oral or dermal exposure

routes.

Decreased acetylcholinesterase activity has been reported with other diisocyanates (Manno and Lotti

1976; Trevisan and Moro 1981). However, no significant changes in plasma cholinesterase have been

noted in laboratory animals exposed to HDI via inhalation (Karol et al. 1984).
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Hepatic Effects. No information is available on the hepatic effects of HDI in humans. Limited

information exists on these effects in laboratory animals and is confined to inhalation studies. One study

of intermediate-duration showed decreased liver weights in female rats dosed at 0.3 ppm (Mobay

Corporation 1984); however, 2 studies of longer durations and slightly lower inhaled doses showed no

changes in liver weights attributable to HDI toxicity (Mobay Corporation 1988, 1989). It appears that the

changes in liver weights are a transitory phenomenon in laboratory animals.

Renal Effects. No information is available on the renal effects of HDI in humans. Renal changes

appeared to be mild when rats were exposed to HDI. Decreased kidney weights were noted in one study

of intermediate-duration in both male and female rats (Mobay Corporation 1984), while two other studies

of longer duration noted no significant changes in kidney weights (Mobay Corporation 1988, 1989). The

changes in kidney weights, like the changes in liver weights, appear to be a transient phenomenon. Male

rats experienced an increase in urinary ketone concentration in two separate studies (Mobay Corporation

1984, 1988). No changes in urine ketones were noted in one study of chronic-duration (Mobay

Corporation 1989). Anorexia, resulting in decreasing body weight and mobilization of fat stores, may be

a reasonable cause for observing increased ketone bodies in the urine. However, both of the studies that

noted ketonuria also reported no significant changes in body weight throughout the study. It is unclear

why urinary ketone bodies increased in HDI-exposed rats.

Dermal Effects. Dermal effects of HDI are limited to those cases of topical exposure. HDI has been

demonstrated to be a topical irritant in several studies in laboratory animals at topical (non-occluded)

doses as low as 0.l%, resulting in erythema, edema, and, in some cases, frank skin necrosis (Haskell

Laboratory 1961). Studies that dosed HDI on the skin of rabbits, with the dosing site occluded, resulted

in more severe cutaneous reactions (Mobay Corporation 1981a). In addition to its local irritation effect,

HDI also induces an allergic contact dermatitis in guinea pigs (Haskell Laboratory 1961). Neither direct

irritant or allergic contact dermatitis effects of HDI have been documented in humans.

Ocular Effects. No ocular effects due to HDI toxicity have been reported in humans. Ocular effects due

to HDI toxicity have been documented in dogs, rats, and rabbits (Haskell Laboratory 1961; Mobay

Corporation 1966, 1988, 1989). When HDI was placed directly into the eyes of laboratory animals,

direct irritation resulted in the form of severe conjunctivitis, damage to the cornea and iris, and an

inflammatory reaction (Haskell Laboratory 1961; Mobay Corporation, 1981a). Clinical signs of ocular

irritation have been observed when animals were exposed to vapor concentrations as low as 0.01 ppm
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(Mobay Corporation 1988), but reactions were limited to lacrimation and conjunctivitis. The severity of

signs was generally proportional to the air concentration (Haskell Laboratory 1961). At air

concentrations ≥ 0.164 ppm, the clinical signs were observed during and shortly after the HDI exposure,

with a full recovery observed by the following day (Mobay Corporation 1989). Ophthalmologic or

histopathological examination after two years of exposure revealed no compound-related ocular effects

(Mobay Corporation 1989). These studies demonstrate the HDI, even at very low concentrations,

functions as a direct irritant to the eye and surrounding structures, and as a result are considered to be

transient physiological responses.

Body Weight Effects. HDI does not appear to have an appreciable effect on the body weights of animals,

based on inhalation dosing. Only one study showed a mild drop in body weight within 1 day or 1 week

after exposure began (Dow Chemical 1964); however, the effect appeared transient, was accompanied by

a rebound weight gain, and was probably related to the relatively high concentrations of HDI used in that

study. Other studies using doses of HDI at <0.3 ppm for intermediate- and chronic-durations failed to

elicit a significant change in body weights (Mobay Corporation 1984, 1988, 1989).

Neurological Effects. Little information was available to determine the neurotoxicity or the

mechanism of neurotoxicity of HDI after inhalation, oral, or dermal exposure. Headache was reported in

only one human exposure case (Malo et al. 1983). Neurotoxic effects (convulsions) may occur in

laboratory animals if concentrations reach high levels in the air (Haskell Laboratory 1961); however,

since HDI is metabolized quickly in a biological matrix (Berode et al. 1991), little intact HDI is expected

to reach the nervous tissue to elicit a toxic response, except possibly at very high concentrations. No

neurological effects have reported in laboratory animals, or in humans exposed chronically to low

concentrations of HDI (Mobay Corporation 1989). HDI, in addition to other isocyanates, have been

shown to inhibit acetylcholinesterase in human erythrocytes (Dewair et al. 1983), human serum

acetylcholinesterase (Brown et al. 1982), as well as equine serum, bovine erythrocyte, and eel

acetylcholinesterase (Brown et al. 1982).

Immunological and Lymphoreticular Effects. Many reports confirmed that both HDI monomer

and prepolymers can elicit an immunological reaction in both humans and laboratory animals after

inhalation and dermal exposures. There is clear evidence that in mice and guinea pigs, HDI and HDI

prepolymers can induce sensitization reactions after one sensitizing dermal exposure (E.I. DuPont de

Nemours 1977b, 1977a; Stadler and Karol 1985; Thorne et al. 1987), although there have been no reports
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of human dermal sensitization. The information on the immunological reactions in humans is limited to

inhalation data; however, these reports indicate that the immune system responds to HDI exposure by

producing IgG, IgE (Belin et al. 1981; Grammar et al. 1988, 1990; Patterson et al. 1990), and IgA antibodies

(Usui et al. 1992) after inhalation exposure to very small doses (<0.2 ppm) in some individuals. IgG is the

prevalent antibody produced in humans exposed to HDI (Grammar et al. 1990). Antibody detection in the

serum and BALF is usually performed using an RIA or ELISA utilizing the diisocyanate conjugated to human

senm albumin (HSA).

Presently, there is no one specific test to detect antibodies produced exclusively in response to HDI exposure,

although HDI-HSA antigens are available to detect immunoglobulins produced in response to HDI exposure.

It has been demonstrated that some cross-reactivity does occur with the HDI-HSA antigen and other aromatic

isocyanates, such as TDI and MDI (Belin et al. 1981), making serum or skin antibody measurements of

limited value as a biomarker of HDI exposure. In addition, most reports indicate that both presence and

quantity of antibodies found in the serum or after RAST and skin prick tests do not always correlate to the

occurrence of respiratory symptoms experienced in many exposed workers (Baur et al. 1984; Grammar et al.

1988, 1990). In other words, the presence of respiratory symptoms attributed to HDI exposure does not

always produce a detectable antibody response to HDI, and vice versa. Although current data are admittedly

scant in this area, it appears that in addition to a pharmacologic mechanism(s) of pulmonary toxicity to HDI,

there is an immunologic component involved in inducing HDI respiratory toxicity. The immune system’s

specific role in HDI-induced pulmonary toxicity is unclear and requires further study to properly elucidate

these immunologic and pharmacologic mechanisms.

Genotoxic Effects. HDI was demonstrated to be non-mutagenic against some Salmonella typhimurium

strains with or without metabolic activation (Anderson et al. 1980). HDI also inhibited the growth of Ehrlich

ascites tumor cells in female mice (Moos et al. 1971) and decreased the mutation frequency in Escherichia

coli (Kawazoe et al. 1981). Calf thymus DNA incubated in vitro with 10.4 or 52 μmol of HDI for 10 or 20

minutes produced no evidence of intrastrand cross-links or DNA strand breaks (Peel et al. 1997). No studies

were located that studied the genotoxic effects of HDI on human cells or that described the ability of

prepolymer forms of HDI to induce genotoxicity.”

Cancer. No reports of HDI-induced cancer in humans were retrieved. One study in rats showed no

increase in the incidence of cancer at the concentrations tested (Mobay Corporation 1989).
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2.6 BIOMARKERS OF EXPOSURE AND EFFECT

Biomarkers are broadly defined as indicators signaling events in biologic systems or samples. They have

been classified as markers of exposure, markers of effect, and markers of susceptibility (NAS/NRC

1989).

Due to a nascent understanding of the use and interpretation of biomarkers, implementation of

biomarkers as tools of exposure in the general population is very limited. A biomarker of exposure is a

xenobiotic substance or its metabolite(s), or the product of an interaction between a xenobiotic agent and

some target molecule(s) or cell(s) that is measured within a compartment of an organism (NRC 1989).

The preferred biomarkers of exposure are generally the substance itself or substance-specific metabolites

in readily obtainable body fluid(s) or excreta. However, several factors can confound the use and

interpretation of biomarkers of exposure. The body burden of a substance may be the result of exposures

from more than one source. The substance being measured may be a metabolite of another xenobiotic

substance (e.g., high urinary levels of phenol can result from exposure to several different aromatic

compounds). Depending on the properties of the substance (e.g., biologic half-life) and environmental

conditions (e.g., duration and route of exposure), the substance and all of its metabolites may have left

the body by the time samples can be taken. It may be difficult to identify individuals exposed to

hazardous substances that are commonly found in body tissues and fluids (e.g., essential mineral

nutrients such as copper, zinc, and selenium). Biomarkers of exposure to hexamethylene diisocyanate

are discussed in Section 2.6.1.

Biomarkers of effect are defined as any measurable biochemical, physiologic, or other alteration within

an organism that, depending on magnitude, can be recognized as an established or potential health

impairment or disease (NAS/NRC 1989). This definition encompasses biochemical or cellular signals of

tissue dysfunction (e.g., increased liver enzyme activity or pathologic changes in female genital epithelial

cells), as well as physiologic signs of dysfunction such as increased blood pressure or decreased lung

capacity. Note that these markers are not often substance specific. They also may not be directly

adverse, but can indicate potential health impairment (e.g., DNA adducts). Biomarkers of effects caused

by hexamethylene diisocyanate are discussed in Section 2.6.2.

A biomarker of susceptibility is an indicator of an inherent or acquired limitation of an organism’s ability

to respond to the challenge of exposure to a specific xenobiotic substance. It can be an intrinsic genetic
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or other characteristic or a preexisting disease that results in an increase in absorbed dose, a decrease in

the biologically effective dose, or a target tissue response. If biomarkers of susceptibility exist, they are

discussed in Section 2.8, Populations That Are Unusually Susceptible.

2.6.1 Biomarkers Used to Identify or Quantify Exposure to Hexamethylene Diisocyanate.

Few biomarkers are available for determining exposure to HDI. Detection of HDI in the blood, serum,

urine and other body fluids would be difficult, given the accelerated rate at which hydrolysis of HDI

probably occurs in biological matrices (see Figure 5-1) (Berode et al. 1991; Brorson et al. 1990b).

According to surveyed literature, parent HDI has not been detected following exposure in humans or

animals. The hydrolysis product of HDI, 1,6-hexamethylene diamine (HDA) has also not been detected

in the blood after HDI exposure; however, it has been detected in the urine of humans exposed by the

inhalation route (Brorson et al. 1990b) and via the oral route several hours after ingestion of HDA

(Brorson et al. 1990a). The average half-life of HDA in the urine after inhalation exposure to HDI for

8 hours at concentration levels ranging from 25 μg/m3

 to 29 μg/m3 (63% to 73% of the Swedish TLV) was

reported to be 1.2 hours. The half-life after oral ingestion of 0.1 mg/kg of I-IDA was 1.5 hours. Urine levels

of HDA (after oral ingestion or after inhalation of HDI) were generally undetectable after 13-15 hours,

indicating that HDA may be a suitable biomarker for determining acute exposure to HDI when air

concentrations are near TLV. Urinary HDA assessment would be of little value in determining exposures

occurring at air concentrations far below the TLV, or >12-15 hours post exposure. The use of two known

urinary metabolites of HDA (N-acetyl-1,6-hexamethylene diamine and 6-aminohexanoic acid) as biomarkers

is unclear; however, given their probable shorter half-lives (compared to HDA), these metabolites would

probably be of little value.

The study by Brorson et al. (1990b) suggests an additional feature that may be important in biological

monitoring. On the basis of the ability to acetylate an oral dose of HDA, Brorson determined the

phenotypes of 6 individuals as either rapid or slow acetylators. The rapid acetylators excreted

approximately twice as much acetylated HDA over the subsequent 15 hours as did the slow acetylators.

The potential importance of this difference in excretory rates with respect to toxicity has not been

investigated. However, the author suggests that after measurements of urinary metabolites have been

made in conjunction with determinations of acetylation phenotypes, it would be worth considering the

possibility of biological monitoring of occupation exposure to HDA and HDI.
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HDI exposure has also been reported to induce the production of immunoglobulins, mainly IgG and IgE

(Belin et al. 1981; Grammar et al. 1988, 1990; Patterson et al. 1990), making this response a potential for

use as a biomarker of exposure. Several difficulties arise when attempting to use blood immunoglobulin

levels specifically as an HDI biomarker of exposure. As discussed earlier, there is no one specific test to

detect the antibodies produced exclusively in response to HDI exposure. Cross-reactivity does occur

with the HDI-HSA antigen and other aromatic isocyanates, such as TDI and MDI (Belin et al. 1981),

making serum or skin antibody measurements of limited value as a biomarker of HDI exposure when

workers may have been exposed to more than one diisocyanate. The presence and quantity of antibodies

found in the serum or after RAST and skin prick tests do not always correlate to the occurrence of ocular,

nasal, and respiratory tract symptoms experienced in many exposed workers (Baur et al. 1984; Grammar

et al. 1988, 1990). Furthermore, it has been documented that many exposed workers will not mount an

immune response (i.e., IgG, IgE, or IgA production) after exposure to HDI, yielding false negatives for

exposure (Baur et al. 1984; Grammar et al. 1988, 1990). Given these difficulties, the use of blood

immunoglobulins as a biomarker of exposure to HDI may be of limited use. With the current tests

available, immunoglobulin levels may be of more use in determining an individual’s exposure to

diisocyanates in general, although a positive titre to the HDI antigen may indicate exposure to HDI itself.

Exposure history to diisocyanates would be a useful tool for assessing the validity of the test data.

Immunoglobulins may also be more useful than urinary HDA levels because the immunoglobulins will

persist in the blood for an extended length of time after an exposure has occurred.

2.62 Biomarkers Used to Characterize Effects Caused by Hexamethylene Diisocyanate.

The primary target organ for HDI toxicity is the respiratory tract. The signs and symptoms of exposure

to HDI (burning and irritation of the respiratory tract, headache, bronchitis, asthmatic reactions,

obstructive breathing defects, tightness of the chest, pulmonary edema, etc.) are easily recognizable;

however, none are specific for exposure to HDI. No specific biomarkers used to characterize effects

caused by HDI were located in the literature.

For more information on biomarkers for renal and hepatic effects of chemicals see ATSDR/CDC

Subcommittee Report on Biological Indicators of Organ Damage (1990) and for information on

biomarkers for neurological effects see OTA (1990).
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2.7 INTERACTIONS WITH OTHER CHEMICALS

There were no reliable reports available in the surveyed literature that described the interaction of HDI

with other chemicals.

2.8 POPULATIONS THAT ARE UNUSUALLY SUSCEPTIBLE

A susceptible population will exhibit a different or enhanced response to hexamethylene diisocyanate

than will most persons exposed to the same level of hexamethylene diisocyanate in the environment.

Reasons may include genetic makeup, age, health and nutritional status, and exposure to other toxic

substances (e.g., cigarette smoke). These parameters may result in reduced detoxification or excretion of

hexamethylene diisocyanate, or compromised function of target organs affected by hexamethylene

diisocyanate. Populations who are at greater risk due to their unusually high exposure to hexamethylene

diisocyanate are discussed in Section 5.6, Populations With Potentially High Exposure.

People who have developed hypersensitization to HDI are likely to be most susceptible to the toxic

effects of HDI. People may develop a hypersensitization to HDI after only one exposure, either at a very

low concentration for many hours or to a high concentration for just a few seconds. The first exposure

may induce only the local irritant effects of HDI, depending on the exposure concentration and duration

of exposure. However, upon re-exposure at very low concentrations (TLV or lower), sensitized persons

may exhibit respiratory symptoms resembling an asthma attack (e.g., shortness of breath, difficulty in

breathing, burning sensation in the chest, bronchoconstriction). Individuals with pre-existing lung

disease who are also sensitized to HDI (or other diisocyanates) are another population unusually

susceptible to the effects of HDI. HDA, the metabolite of HDI, is known to be excreted in the urine of

humans after inhalation exposure (Brorson et al. 1990b) and is moderately toxic in fasted rats (Dashiell

and Kennedy 1984). It is not known whether severely impaired renal functions in humans exposed to

HDI has an impact on HDA-induced toxicity.

2.9 METHODS FOR REDUCING TOXIC EFFECTS

This section will describe clinical practice and research concerning methods for reducing toxic effects of

exposure to hexamethylene diisocyanate. However, because some of the treatments discussed may be

experimental and unproven, this section should not be used as a guide for treatment of exposures to
HEXAMETHYLENE DIISOCYANATE 89

2. HEALTH EFFECTS

hexamethylene diisocyanate. When specific exposures have occurred, poison control centers and

medical toxicologists should be consulted for medical advice. The following texts provide specific

information about treatment following exposures to hexamethylene diisocyanate:

• Ellenhom, MJ and Barceloux, DG. 1988. Medical Toxicology: Diagnosis and Treatment of

Human Poisoning. Elsevier Publishing, New York, NY.

• Dreisback, RH. Handbook of Poisoning 1987. Appleton and Lange., Norwalk, CT.

• Haddad, LM and Winchester, JF (ed.) 1990. Clinical Management of Poisoning and Drug

Overdose. 2nd edition, WB Saunders, Philadelphia, PA.

• Aaron, CK and Howland, MA (ed.) 1994. Goldfrank’s Toxicologic Emergencies. Appleton

and Lange, Norwalk, CT.

2.9.1 Reducing Peak Absorption Following Exposure

Few specific recommendations can be made for reducing the absorption of HDI after exposure. To avoid

exposure, persons handling or transporting products containing it should ensure that all devices

containing the HDI are sealed and intact. HDI should be used in a well ventilated area at normal room

temperatures. Owing to the low molecular weight of HDI, increased room temperatures may increase the

vaporization of HDI into the room air, increasing the risk of human exposure. Adequate ventilation

should always be provided when using products containing HDI; respiratory equipment may also be

necessary, depending on working conditions. If splashes or contact with aerosols are likely to occur in

the working environment, workers should protect themselves by wearing rubber or polyvinyl chloride

gloves, aprons, rubber boots, goggles, and respiratory equipment as needed to prevent exposure (NIOSH

1978).

If the skin comes into contact with HDI or products containing HDI, workers should flush their skin with

water to remove the agent and wash the contaminated area with soap and water. Isopropyl alcohol can

also be used to neutralize any remaining HDI after washing with soap and water, provided the skin

barrier is intact. If HDI comes into contact with the eyes or conjunctiva, copious amounts of water

should be used to gently flush the eyes for at least 15-20 minutes. To avoid oral exposure to HDI,

persons should thoroughly wash their hands after handling products containing HDI prior to eating,

drinking, or smoking (NIOSH 1978).
HEXAMETHYLENE DIISOCYANATE 90

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2.9.2 Reducing Body Burden

No reports were found in the open literature on methods to reduce the body burden of HDI after

inhalation, oral or dermal exposures. No blood, tissue or urine concentrations of HDI have been reported

in the surveyed literature. Since HDI is easily hydrolyzed in biological media (Berode et al. 1991;

Brorson et al. 1990b), little if any HDI is expected to accumulate in the tissues of humans after acute or

chronic exposures.

2.9.3 Interfering with the Mechanism of Action for Toxic Effects

The mechanism of action of HDI has not been elucidated to any great extent in the surveyed literature.

No information is available to determine what action, if any, can be taken to interfere with the

mechanism of action of HDI toxicity.

2.10 ADEQUACY OF THE DATABASE

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with

the Administrator of EPA and agencies and programs of the Public Health Service) to assess whether

adequate information on the health effects of hexamethylene diisocyanate is available. Where adequate

information is not available, ATSDR, in conjunction with the National Toxicology Program (NTP), is

required to assure the initiation of a program of research designed to determine the health effects (and

techniques for developing methods to determine such health effects) of hexamethylene diisocyanate.

The following categories of possible data needs have been identified by a joint team of scientists from

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean

that all data needs discussed in this section must be filled. In the future, the identified data needs will be

evaluated and prioritized, and a substance-specific research agenda will be proposed. 

2.10.1 Existing Information on Health Effects of Hexamethylene Diisocyanate

The existing data on health effects of inhalation, oral, and dermal exposure of humans and animals to

hexamethylene diisocyanate are summarized in Figure 2-4. The purpose of this figure is to illustrate the

HEXAMETHYLENE DIISOCYANATE 92

2. HEALTH EFFECTS

existing information concerning the health effects of hexamethylene diisocyanate. Each dot in the figure

indicates that one or more studies provide information associated with that particular effect. The dot

does not necessarily imply anything about the quality of the study or studies, nor should missing

information in this figure be interpreted as a “data need.” A data need, as defined in ATSDR’s Decision

Guide for Identifyng Substance-Specific Data Needs Related to Toxicological Profiles (ATSDR 1989),

is substance-specific information necessary to conduct comprehensive public health assessments.

Generally, ATSDR defines a data gap more broadly as any substance-specific information missing from

the scientific literature.

2.10.2 Identification of Data Needs

Acute, Intermediate and Chronic-Duration Exposures. Inhalation exposures in both humans

and laboratory animals predominate in the available information on acute, intermediate, and chronic

effects of HDI, and will be considered here as a group. Information on laboratory animals describes the

direct irritant effects of HDI, which was usually inhaled in large doses (>4 ppm); however, no

information on the allergic component of HDI toxicity at low doses, the type of dose most commonly

encountered in humans, was provided. Information on acute inhalation exposure of humans may be

misleading. In most cases of acute exposure, the workers had been exposed to HDI and HDI prepolymers

in their workplace for several months or several years (doses often not available). These workers were

then tested with a small dose of either HDI or a product containing HDI with the HDI prepolymers and

other organics. Workers were tested for an acute duration (<1 hour) (Belin et al. 1981; Cockcroft and

Mink 1979; Malo et al. 1983; Patterson et al. 1990; Tulane Medican 1982a, 1982b) and a chronic

duration (Alexandersson et al. 1987). Allergic reactions in these workers were often reported. From

these data, it is unclear whether it is the HDI component or the HDI prepolymers of these products that

are responsible for eliciting the observed allergic reactions (Malo et al. 1983; Tulane Medican 1982a).

Better designed studies are needed to determine if humans never exposed to HDI and then given small

doses of HDI (<0.02 ppm) or HDI prepolymers for an acute duration, can develop these

hypersensitivities, as well as at what inhaled concentrations these sensitivities can be expected to occur

or not occur. It is also important to determine if it is the HDI component, the HDI prepolymers, or an

additive and synergistic effect of these components that elicit the allergic reactions observed in those

individuals exposed chronically to products containing these components. Finally, studies are also

necessary to determine if respiratory and dermal allergic reactions can be induced in humans after dermal

exposure only, as was observed in laboratory animals.
HEXAMETHYLENE DIISOCYANATE 93

2. HEALTH EFFECTS

Genotoxicity. HDI was demonstrated to be non-mutagenic against some S. typhimurium strains with or

without metabolic activation (Anderson et al. 1980). HDI also inhibited the growth of Ehrlich ascites tumor

cells in female mice (Moos et al. 1971) and decreased the mutation frequency in E. coli (Kawazoe et al.

1981). No studies were located that studied the genotoxic effects of HDI on human cells or that described the

ability of the prepolymer forms of HDI to induce genotoxicity. Although the limited data suggest that HDI is

not genotoxic, a data need exists here to confirm that both HDI and the prepolymer of HDI are not capable of

inducing genotoxic effects in human cell lines.

Reproductive Toxicity. No reproductive toxicological studies were located in the surveyed literature for

HDI. Only a few animal studies examined the reproductive organs of both male and female animals, with no

gross or histological results evident (Mobay Corporation 1984, 1988,1989); none of the human studies of

acute, intermediate, or chronic durations directly addressed the issue of reproductive toxicity. The majority of

studies used male humans and animals, presumably because human males are presently the predominant sex

in the automotive painting industry and, therefore, more likely to be exposed to HDI. It is not known if HDI

affects reproductive tissues in males or females; however, given its short half-life in biological fluid, this

seems unlikely. HDI has been reported to bind to biological tissues (protein) (Ted and Pesce 1979); however,

the relevance of this observation to reproductive toxicity is not known. The toxicity of the HDI metabolite

HDA) is not known. Toxicological studies should be designed to answer questions about the potential

reproductive toxicity of HDI or its prepolymers in both male and female humans and laboratory animals.

Developmental Toxicity. No developmental toxicological studies were located in the surveyed literature

for HDI. It is not known if HDI exerts an effect on reproductive tissues iu males or females or on the

developing fetus; however, given its short half-life in biological fluid, this seems unlikely0 HDI has been

reported to bind to biological tissues (protein) (Ted and Pesce 1979); however, the relevance of this

observation to reproductive toxicity is not known. The toxicity of the HDI metabolite (HDA) is not known.

Toxicologic studies should be devised to answer questions about HDI’s potential developmental toxicity or

its prepolymers in the developing human or laboratory animal.

Immunotoxicity. No immunotoxicity induced by HDI was observed in the studies found in the open

literature. HDI can, however, elicit immunological reactions iu both humans and animals. There appears to

be an immunological component involved in HDI respiratory toxicity. The immune system’s specific role in

HDI-induced pulmonary toxicity may be useful.
HEXAMETHYLENE DIISOCYANATE 94

2. HEALTH EFFECTS

Neurotoxicity. Little information was available to determine the neurotoxicity of HDI after inhalation,

oral or dermal exposure. Neurotoxic effects may occur if concentrations reach high levels in the air (Haskell

Laboratory 1961); however, since HDI is metabolized quickly in a biological matrix (Berode et al. 1991),

little intact HDI is expected to reach the nervous tissue to elicit a toxic response, except possibly at very high

concentrations. No neurological effects have reported in laboratory animals, or in humans exposed

chronically to low concentrations of HDI (Mobay Corporation 1989); therefore the data need for determining

the neurotoxicity of HDI is a low priority.

Epidemiological and Human Dosimetry Studies. The target population for HDI toxicosis is the

worker using products that contain both HDI and/or HDI in combination with the HDI prepolymers, usually

in the form of automobile paint hardeners. One flaw in these reports is that the dosimetry data were not well

described in many cases (Baur et al. 1984; Grammar et al. 1990; Malo et al. 1983; Patterson et al. 1990;

Usui et al. 1992); often concentrations were not stated or a wide-range of exposure concentrations reported.

The usual scenario noted from the majority of these reports was that a worker was exposed to products

containing both HDI and HDI prepolymers for a period of several weeks or several years with accompanying

allergic (asthmatic) symptoms. The worker was administered an inhalation challenge to the paint he was

using and subsequently developed the clinical symptomatology, with HDI assumed to be the causative agent,

although there was no conclusive proof that it was the chemical responsible for eliciting the reaction. In some

of these reports, the possibility of the prepolymeric form inducing an allergic reaction was not considered

(Bauer et al. 1984; Belin et al. 1981; Grammar et al. 1988; Patterson et al. 1990; Tulane Medican 1982a;

Usui et al. 1992), while in other reports this was addressed to some extent (Alexandersson et al. 1987;

Grammar et al. 1988; Malo et al. 1983). A strong data need in this area is to determine definitively if it is the

HDI, the HDI prepolymer, a combination of the HDI and HDI prepolymer, or (less likely) other organic

components in these products that are eliciting the allergic and irritant reactions observed in these chronically

exposed workers.

Biomarkers of Exposure and Effect.

Exposure. Only one biomarker of exposure, HDA, was located in the surveyed literature (Brorson et al.

1990a,b). This biomarker may be some use for acute-duration exposures, but only if urine is collected from

the exposed person within 6-12 hours after exposure. No reliable biomarkers of exposure are available for

chronic, low-level exposures in humans, although blood immunoglobulins (in particular IgG) may be useful

in determining exposures to the diisocyanates as a group, and not a specific exposure to HDI.
HEXAMETHYLENE DIISOCYANATE 95

2. HEALTH EFFECTS

Studies to determine other biomarkers that would be sensitive enough to detect chronic, low-level

exposures to HD/HDI prepolymers and be specific to HDI only, with low cross-reactivity to other

diisocyanates, would be extremely useful, and would enhance the database.

Effect. No studies were found in the open literature that used a biomarker of effect to HDI toxicity. The

target organ of HDI toxicity is the respiratory system, with significant effects on the eyes if present in

high concentrations (Haskell Laboratory 1961; Mobay Corporation 1981 a). More effort to identify

subtle biochemical changes to serve as biomarkers of effects of HDI would be useful in detecting early,

subtle signs of HDI-induced toxicity.

Absorption, Distribution, Metabolism, and Excretion. There is an obvious data need to

determine the pharmacokinetic and toxicokinetic behavior of HDI in both humans and laboratory

animals. Determination of blood levels of inhaled, ingested and dermally absorbed HDI would be

difficult, given the very short half-life in biological matrices (Berode et al. 1991) and the rate at which

HDI binds to proteins in the blood. Although some information is known about the metabolism of HDI

in humans inhaling a known quantity of HDI (Brorson et al. 1990), the rate at which absorption occurs,

where the majority of the metabolism of HDI occurs (in the water in the mucous layer of the bronchi as

opposed to the blood or the kidney), and the distribution patterns and toxic effects of the metabolite (if

any) are not well described. Information in these areas of toxicokinetics and toxicodynamics could also

be useful in developing a PBPK/PD model for HDI. Research should focus on the respiratory and dermal

routes of exposure.

Comparative Toxicokinetics. Little information is present on the comparative toxicokinetics of

HDI, both between laboratory animal species and between humans and laboratory animals. As discussed

earlier in this chapter, the majority of the laboratory animal studies have focused on the direct irritant

effects of HDI after inhalation exposure (E.I. DuPont de Nemours 1978; Haskell Laboratory 1961; Karol

et al. 1984; Mobay Corporation 1982, 1989), while the human studies have described the allergic

components of HDI exposure (Alexandersson et al. 1987; Bauer et al. 1984; Grammar et al. 1988; Malo

et al. 1983; Tulane Medican 1982a; Usui et al. 1992). The allergic component of HDI toxicity has been

described in laboratory animals after dermal exposure (E.I. DuPont de Nemours 1977a, 1977b; Haskell

Laboratory 1961; Stadler and Karol 1985; Thorne et al. 1987), but no reports of such reactions have been

located for humans. Efforts should focus on finding a laboratory animal that would serve as a suitable
HEXAMETHYLENE DIISOCYANATE 96

2. HEALTH EFFECTS

model for studying the allergic respiratory system reactions seen in humans and in in vitro studies that

would outline the mechanism of action of the toxic effects of HDI on a cellular and molecular level.

Methods for Reducing Toxic Effects. No studies were located that described methods for

reducing the toxic effects of HDI after exposure has occurred. A data need exists here to determine the

mechanistic pathways of HDI toxicity, followed by research that determines the best way to reduce these

toxic effects (i.e., the allergic reactions) observed.

2.10.3 Ongoing Studies

A few research projects are in progress that investigate the health effects of HDI. The projects relevant

to HDI are summarized in Table 2-4.


HEXAMETHYLENE DIISOCYANATE 99

3. CHEMICAL AND PHYSICAL INFORMATION

3.1 CHEMICAL IDENTITY

Information regarding the chemical identity of HDI is located in Table 3-1. Most of the HDI

manufactured in, or imported into, the United States is converted into HDI prepolymers

(polyisocyanates). These prepolymers are biurets and trimers. Information for those prepolymers is

shown in Table 3-2.

3.2 PHYSICAL AND CHEMICAL PROPERTIES

Information regarding the physical and chemical properties of HDI is located in Table 3-3.

HDI is a monomer used in the production of polyurethane foams and other related products, and is found

in some industrial paints and spray painting operations. It is a compound which reacts readily with water

and alcohols (Von Burg 1993). It has a vapor pressure of 0.05 mm Hg at room temperature, but can be

present in aerosol form allowing a potentially higher exposure to individuals. The HDI-BT trimer is

often present for similar industrial uses. It would be expected to have a lower vapor pressure; however,

the aerosol form can also be present, allowing potentially higher exposure of HDI-BT to individuals.

HDI reacts slowly with water to form carbon dioxide (HSDB 1996). The base-catalyzed reaction of HDI

with alcohols should be carried out in inert solvents; the reaction may occur with explosive violence in

the absence of solvents (NFPA 1994).



HEXAMETHYLENE DIISOCYANATE 103

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

4.1 PRODUCTION

Isocyanates are produced almost exclusively by the reaction of amines with phosgene (COCl2), with the

specific reaction conditions varying particularly for aromatic and aliphatic isocyanates (Chadwick and

Cleveland 1981; Codd et al. 1972; Uhich 1989). Aliphatic diisocyanates are produced by reaction of

phosgene with either a shury of the carbamate salts obtained in the reaction of the aliphatic diamines with

carbon dioxide, or with a slurry of the amine hydrochloride (Ulrich 1989). Hexamethylene diisocyanate

(HDI) is produced by the reaction of phosgene with the amine salt (Chadwick and Cleveland 1981). The

trimeric HDI biuret (HDI-BT), which has a low monomer content and is widely used in the formulation of

exceptionally high quality polymer coatings, is produced by controlled reaction of HDI with water, a water

generator, or an amine (Chadwick and Cleveland 1981).

Current U.S. producers of HDI are Arco Chemical Company, Lake Charles, Louisiana and Bayer

Corporation, Baytown, Texas (formerly, Mobay Corporation and Miles Incorporated) (Chemical

Manufacturers Association 1997). Data on U.S. production volumes of HDI could not be located in the

available literature.

No information is available in the Toxics Release Inventory database for facilities that produce HDI because

this compound is not included under SARA, Title III and, therefore, is not among the chemicals that facilities

are required to report (EPA 1995).

4.2 IMPORT/EXPORT

Rhone-Poulenc, Inc. imports HDI (Chemical Manufacturers Association 1997); however, no information on

import volume was found in the available literature. No information on export volumes of HDI was found in

the available literature.

4.3 USE

HDI is one of the most commercially important isocyanate compounds currently used in the United States,

HDI, toluene diisocyanate (TDI) and 4,4’diphenyhnethane diisocyanate (MDI) are widely used in the
HEXAMETHYLENE DIISOCYANATE 104

4. PRODUCTION, IMPORT/EXPORT, USE, AND DISPOSAL

production of polyurethane foams, elastomers, and coatings, which account for more than 90% of the

commercial use of isocyanates (Kennedy and Brown 1992). Specific products include plastics, synthetic

rubber, adhesives and glues, cable and wire insulations, anti-corrosive agents, varnishes, lacquers, and paints

(Codd et al. 1972; Parmeggiani 1983; Plunkett 1987). One of the primary uses of HDI is as a polymerizing

agent in polyurethane spray paint formulations (Butcher et al. 1993) and in other light-stable polyurethane

coatings (Uhich 1989). Other uses of HDI are as solid rocket fuel binder or as paint thickener (Chemical

Manufacturers Association 1997). Because of the potentially high exposures to HDI resulting from its high

vapor pressure (see Table 3-3), a prepolymeric form of HDI, which has a much lower vapor pressure, is

finding increasing use in industrial applications (Chadwick and Cleveland 1981; Dalene et al. 1994a). For

example, HDI-biuret (HDI-BT), a trimeric condensation product of HDI, is widely used as a hardener in

automobile and airplane paints. Automobile paint hardeners typically contain 0.5-1.0% monomeric HDI

(Alexandersson et al. 1987). Consequently, use of these spray paints is one of the most significant sources of

exposure to monomeric HDI and its prepolymers (e.g., HDI-BT) (Butcher et al. 1993).

4.4 DISPOSAL

There are no regulations specifically governing the treatment and disposal of wastes containing HDI.

Toluene diisocyanate is, however, regulated under the Resource Conservation and Recovery Act (RCRA)

(EPA 1989). The recommended technology-based standards for treatment of waste containing TDI are

carbon adsorption or incineration for waste waters, and fuel substitution or incineration for non-waste waters

(EPA 1989). Toxic fumes of nitrogen oxides are emitted when HDI is heated to decomposition (Lewis

1992). Because oxides of nitrogen are criteria pollutants, the potential for their release to the atmosphere

must be controlled during heating or incineration of HDI or waste containing HDI. Bicarbonate-catalyzed

hydrolysis of HDI to 1,6-hexamethylene diamine (HDA) has been suggested as a possible treatment method

in scrubbers used to purify HDI-contaminated atmospheres (Berode et al. 1991). No data were found in the

available literature on the amount of HDI disposed of in the United States.
HEXAMETHYLENE DIISOCYANATE 105

5. POTENTIAL FOR HUMAN EXPOSURE

5.1 OVERVIEW

Hexamethylene diisocyanate is a highly reactive synthetic chemical that is widely used in the production

of polyurethane materials. There is no natural source of HDI. All of the potential exposures to this

compound are associated with the production, handling, use, and disposal of HDI and HDI-containing

products or materials. Exposures to HDI are often associated with exposures to its prepolymers,

especially to a trimeric biuretic prepolymer of HDI (HDI-BT) (see Figure 5-1a), which is widely used as

a hardener in automobile and airplane paints, and which typically contains 0.5-l% unreacted HDI

(Alexandersson et al. 1987; Hulse 1984; Karol and Hauth 1982). There is evidence that diisocyanate

prepolymers may induce asthma at the same or greater frequency as the monomers (Seguin et al. 1987);

therefore, there is a need to assess the potential for human exposure to prepolymeric HDI as well as

monomeric HDI. Except for limited data on occupational exposures, no information was found in the

available literature related to the potential for human exposure to prepolymers of HDI.

Little information is available about the potential for human exposure specifically to HDI. Some human

exposure data have been published by Shepperly and Hathaway (1991) and DeWilde and Hathaway

(1994); those study results and limitations have been discussed at length in Chapter 2. As a result, some

of the information in this section has been extrapolated from the results of studies on the more widely

used diisocyanates, particularly toluene diisocyanate (TDI) (see Figure 5-lb) and methylene

bis(4-phenylisocyanate) (MDI) ( see Figure 5-1c). Information on the environmental fate of TDI and

MDI is relevant to HDI because these diisocyanates undergo many of the same chemical reactions as

HDI, particularly those such as hydrolysis, which involve reaction with active hydrogen compounds and

addition to the carbon-nitrogen double bond of the highly reactive isocyanate group. In most of these

reactions, the aromatic diisocyanates are more reactive than the aliphatic HDI (Chadwick and Cleveland

1981) so that direct quantitative extrapolations cannot be made.

No quantitative estimates of the volume of HDI or HDI prepolymers released to the environment were

found in the available literature. HDI and HDI prepolymers may be released to the atmosphere during

spray applications of polymer paints containing residual amounts (≤1%) of HDI (Alexandersson et al.

1987; Hulse 1984; Karol and Hauth 1982). Waste streams from HDI or HDI polymer production

facilities may release HDI or HDI prepolymers to air, water, and soil. There is also a potential for release

HEXAMETHYLENE DIISOCYANATE 107

5. POTENTIAL FOR HUMAN EXPOSURE

of HDI to air, water, and soil at hazardous waste sites. HDI has not been found in any of the

1,445 current or former EPA National Priorities List (NPL) hazardous waste sites (HazDat 1996).

However, the number of sites evaluated for HDI is not known.

In the atmosphere, HDI will exist entirely in the vapor phase (Bidleman 1988; Eisenreich et al. 1981).

Partitioning to soil or water by wet or dry deposition are not expected to be significant fate processes for

HDI. HDI degrades relatively rapidly in the atmosphere by reaction with hydroxyl radicals (half-life,

≈2 days), and may also undergo hydrolysis. Therefore, it is not expected that HDI will be transported

long distances in the atmosphere. HDI is expected to hydrolyze rapidly (aqueous hydrolysis half-life,

<10 minutes) in water and moist soil or sediment to form an amine (i.e., 1,6-hexamethylene diamine) and

polyurea compounds. As a result, physical partitioning processes such as volatilization, leaching, and

adsorption from water onto suspended particles or sediments will not be significant.

Except for occupational atmospheres, no information was found in the available literature on

concentrations of HDI or HDI prepolymers in air, water, soil, or sediment. Because of the relatively

rapid reaction of HDI with hydroxyl radicals in the atmosphere and its high reactivity with water,

significant environmental concentrations of HDI are not expected to occur except near emission sources.

The general population may be exposed to HDI and HDI prepolymers during the nonoccupational use of

polyurethane paints (Musk et al. 1988), primarily through inhalation of vapors and aerosols, and, to a

much lesser extent, by dermal absorption. Occupational exposures to HDI and HDI prepolymers also

occur via these routes. Estimates from the National Occupational Exposure Survey (NOES) conducted

by the National Institute of Occupational Health (NIOSH) indicate that approximately 20,000 workers

were potentially exposed to HDI in the United States from 1981 to 1983 (NIOSH 1989). This may be an

underestimate because the numbers do not include workers potentially exposed to trade name compounds

containing HDI. Professional painters and paint spraying-machine operators, aircraft engine and other

mechanics, and aircraft machinists were among the occupations with the greatest potential for exposure

to HDI. Similar data were not reported for HDI prepolymers; however, many of the potential HDI

exposures may involve concurrent exposure to HDI prepolymers.
HEXAMETHYLENE DIISOCYANATE 108

5. POTENTIAL FOR HUMAN EXPOSURE

5.2 RELEASES TO THE ENVIRONMENT

5.2.1 Air

HDI and HDI prepolymers can be released to the atmosphere during spray applications of polymer paints

containing residual amounts (0.5-l.%) of monomeric HDI (Alexandersson et al. 1987; Hulse 1984;

Karol and Hauth 1982). These substances could also be released to the atmosphere from waste streams

from sites of HDI or polymer production. No information is available in the Toxic Chemical Release

Inventory database on the amount of HDI released to the atmosphere from facilities that produce or

process HDI because this compound is not included under SARA, Title III, and therefore, is not among

the chemicals that facilities are required to report (EPA 1995). There is also a potential for atmospheric

release of HDI from hazardous waste sites; however, no information was found on detections of HDI in

air at any NPL or other Superfund hazardous waste sites (1996). Because of the relatively rapid reaction

of HDI with hydroxyl radicals in the atmosphere an possible hydrolysis (see Section 5.3.2.1), significant

atmospheric concentrations are not expected to occur except near emission sources.

Releases of HDI and prepolymeric HDI to the atmosphere in occupational settings and available

information on workplace exposure levels are discussed in Section 5.5.

5.2.2 Water

Waste streams from sites of HDI or HDI polymer production may release HDI or HDI prepolymers to

water. No information is available in the TRI database on the release of HDI to water from facilities that

produce or process HDI because this compound is not included under SARA, Title III, and therefore, is

not among the chemicals that facilities are required to report (EPA 1995). HDI and HDI prepolymers

may also be released to water at hazardous waste sites; however, no information was found on detections

of HDI in water at any NPL or other Superfund hazardous waste sites (HazDat 1996). Because of its

reactivity with water to form amine or polyurea derivatives (Chadwick and Cleveland 1981; Hulse 1984;

Kennedy and Brown 1992), monomeric HDI is not likely to be found in waste water streams or in other

aquatic environments except near sources of release. Small amounts of HDI that have become

encapsulated in water-insoluble polyurea agglomerates may persist in water (see Section 5.3.2.2).
HEXAMETHYLENE DIISOCYANATE 109

5. POTENTIAL FOR HUMAN EXPOSURE

5.2.3 Soil

Waste streams from sites of HDI or HDI polymer production may release HDI and HDI prepolymers to

soil. No information is available in the TRI database on the release of HDI to soil from facilities that

produce or process HDI because this compound is not included under SARA, Title III, and therefore, is

not among the chemicals that facilities are required to report (EPA 1995). HDI and HDI prepolymers

may also be released to the soil at hazardous waste sites; however, no information was found on

detections of HDI in soil at any NPL or other Superfund hazardous waste sites (HazDat 1996). Because

of its expected reactivity with water in moist soil to form amine or polyurea derivatives, monomeric HDI

is not likely to be found in soil in significant concentrations except near sources of release. Small

amounts of HDI that have become encapsulated in water-insoluble polyurea agglomerates may persist in

soils and sediments (see Section 5.3.2.3).

5.3 ENVIRONMENTAL FATE

53.1 Transport and Partitioning

No studies of the transport and partitioning of HDI in the environment were found in the available

literature. Based on its vapor pressure of 0.05 mm Hg at 25 oC (see Table 3-3), HDI will exist entirely in

the vapor phase in the atmosphere (Bidleman 1988; Eisenreich et al. 1981). Although the atmospheric

hydrolysis of HDI with condensed water has not been investigated, wet deposition is probably not an

important atmospheric removal process for HDI because of its reactivity with water (see Section 5.3.2.1).

Because HDI exists as a vapor in the atmosphere, its removal from air by dry deposition is also likely to

be negligible, although no estimates of the partition coefficient Koc, for HDI are available to allow further

evaluation of the potential for HDI to adsorb to airborne particles. Laboratory studies have shown that

the highly adsorptive TDI vapor is not significantly removed from the atmosphere by dry deposition via

adsorption on ammonium sulfate particles (reportedly the world predominant aerosol) (Duff 1985).

Although TDI has a vapor pressure similar to that of HDI, the relevance of these results tcrthe removal of

HDI from the ambient atmosphere by dry deposition is not clear. Because of its relatively short

atmospheric half-life of ≈2 days (SRC 1995a), and possible rapid hydrolysis (see Section 5.3.2), it is not

expected that HDI will be transported long distances in air.
HEXAMETHYLENE DIISOCYANATE 110

5. POTENTIAL FOR HUMAN EXPOSURE

Because of the rapid hydrolysis of HDI in water and moist soil or sediment (see Sections 5.3.2.2 and

5.3.2.3), neither volatilization from these media nor leaching from soil or sediment should be important

partitioning processes. HDI would also not be expected to partition onto suspended solids and sediment

in water. Henry’s law constant(H) for HDI has been estimated to be 4.80x10-5 atm-m3

/mol (SRC 1994a),

which indicates a relatively slow rate of volatilization from water (Thomas 1990) and further suggests

that with rapid hydrolysis occurring this would not be an important partitioning process. Estimates of KOC

(see Table 3-3) are not available to allow further evaluation of the possible importance of sorption

partitioning processes. Also, because of the rapid hydrolysis of HDI in water and the ease with which

this substance is metabolized in higher trophic animals (see Section 2.3), it is not expected that HDI will

bioconcentrate in aquatic organisms or bioaccumulate in the food chain (Chadwick and Cleveland 1981;

HSDB 1995; Hulse 1984). This conclusion is supported by the results of a study in which no

accumulation of TDI, MDI, or their respective diamine hydrolysis products, TDA and MDA, was found

in the whole bodies of carp (Cyprinus carpio) after 8 weeks of exposure in a river model system with

initial TDI and MDI concentrations of 0.1 ppm (International Isocyanate Institute 198 1). No

bioconcentration factors (BCFs) for HDI in aquatic organisms were found in the available literature

(ASTER 1995). A BCF of approximately 100 was calculated for HDI using the method of Veith et al.

(1979), further indicating a very low bioaccumulation potential for HDI; however, the estimated log Kow

value of 3.20 used for this calculation is questionable because of the rapid hydrolysis of isocyanates

(SRC 1995b).

No information was found in the available literature on the transport and partitioning of HDI

prepolymers. Because of their low vapor pressures (Rosenberg and Tuomi 1984), HDI prepolymers will

exist in the atmosphere primarily as aerosols. Because of their reactive isocyanate groups, HDI

prepolymers would not be expected to persist unchanged in the environment. Hydrolysis to form amines

and higher molecular weight polyureas would be expected to be a controlling reaction in water and moist

soil. However, additional studies are required to determine the environmental fate of HDI prepolymers.

5.3.2 Transformation and Degradation

5.3.2.1 Air

No studies of the transformation and degradation of HDI in air were located in the available literature.
HEXAMETHYLENE DIISOCYANATE 111

5. POTENTIAL FOR HUMAN EXPOSURE

Based on a vapor pressure of 0.05 mm Hg at 25 °C (see Table 3-3), HDI is expected to exist entirely in

the vapor phase in the ambient atmosphere (Bidleman 1988; Eisenreich et al. 1981). The aliphatic

isocyanates do not absorb light in the near ultraviolet region (Simons 1979); therefore, direct photolysis

is not a probable atmospheric degradation mechanism for HDI. Based on a structure-reactivity

relationship method (Atkinson 1987), the rate constant for the reaction of HDI with photochemically

produced hydroxyl radicals in the atmosphere is estimated to be 7.95x10-12 cm3

/molecule-see at 25 °C,

which corresponds to an estimated atmospheric half-life of approximately 2 days at 25 °C and an

atmospheric concentration of hydroxyl radicals of 5x105

/cm3 (SRC 1995a). The products of the reaction

of HDI with hydroxyl radicals have not been identified. The estimated rate constant for the reaction of

HDI with hydroxyl radicals is in good agreement with the experimentally determined rate constant of

7.4±0.2xl0-I2 cm3

/molecule-set for the reaction of TDI with hydroxyl radicals (International Isocyanate

Institute 1987b).

Because isocyanates, including HDI, react readily with water to form amines and polyureas (Chadwick

and Cleveland 1981; Hulse 1984; Kennedy and Brown 1992) (see Section 5.3.2.2) atmospheric

hydrolysis of HDI may also occur. However, no estimates of the rate of atmospheric hydrolysis of HDI

were found in the available literature. Laboratory studies indicate that reaction of TDI with water vapor

in the atmosphere is not an important removal process (Duff 1983, 1985; Holdren et al. 1984); however,

these studies did not investigate the condensed phase atmospheric hydrolysis of TDI (e.g., reaction with

rain drops, fog, clouds). The typical half-life for aqueous hydrolysis of isocyanates, such as HDI, has

been estimated to be less than 10 minutes (SRC 1994b), which suggests that the heterogeneous

condensed phase atmospheric hydrolysis of HDI may proceed rapidly. Additional research is needed to

determine the significance of atmospheric hydrolysis of HDI. No information was found in the available

literature characterizing the atmospheric hydrolysis products of HDI. 1,6-Hexamethylene diamine

(HDA) would be an expected atmospheric hydrolysis product, and this compound has been found in

appreciable quantities in association with some HDI occupational exposures (Skarping et al. 1988).

Results of laboratory studies indicate that the diamine (i.e., TDA) is not a significant product of gas

phase hydrolysis of TDI (Duff 1983, 1985; Holdren et al. 1984). However, under simulated atmospheric

conditions, the reactions of the diamine hydrolysis products of TDI and MDI with photochemically

generated hydroxyl radicals have been found to proceed more rapidly than those of the parent

compounds, suggesting that there would be no atmospheric accumulation of these diamines even if they

were significant hydrolysis products (Gilbert 1988; International Isocyanate Institute 1987b). By
HEXAMETHYLENE DIISOCYANATE 112

5. POTENTIAL FOR HUMAN EXPOSURE

analogy, significant atmospheric accumulation of HDA may not occur, except perhaps near sources of

HDI emissions.

5.3.2.2 Water

Rapid hydrolysis is expected to be the only major transformation pathway for HDI in water. Typically,

estimated aqueous hydrolysis half-lives of isocyanates such as HDI are less than 10 minutes (SRC

1994b). Although HDI is essentially insoluble in water (see Table 3-3), in the presence of excess water it

can undergo competing two-phase reactions to form: (1) a complete hydrolysis product, HDA; (2) di-,

tri-, or tetra-ureaisocyanates; and/or (3) higher molecular weight polyureas (Chadwick and Cleveland

198 1; Hulse 1984; Kennedy and Brown 1992). The complex hydrolysis reactions of isocyanates usually

involve a mechanism in which an unstable carbamic acid intermediate is initially formed, with

subsequent decomposition to the amine and release of carbon dioxide; further reaction of the amine with

isocyanate may occur to yield polyurea compounds (Chadwick and Cleveland 1981; Gilbert 1988;

Kennedy and Brown 1992). A partial schematic of the possible hydrolysis reactions of HDI is shown in

Figure 5-2. Studies on the environmental fate of TDI in water have shown that the polyurea hydrolysis

products may form inert, water-insoluble agglomerates encapsulating small amounts of unreacted

monomeric isocyanate (Brochagen and Grieveson 1984; Gilbert 1988) and it would be expected that this

would also be the case for HDI. Laboratory studies of the hydrolysis of TDI in aqueous media have

shown that the competing isocyanate hydrolysis reactions depend on several factors, including ionic

strength, temperature, concentration of reactants, hydrophilic/hydrophobic nature of the reaction

environment, mixing rate, and pH, with the formation of the diamine favored under basic or acidic

conditions (Saunders and Frisch 1962). A single study of the hydrolysis of HDI (Berode et al. 1991) was

found in the available literature, in which the reaction of HDI vapor with water in a dynamic system was

found to be very slow without catalysts (<1% in 10 minutes at 30 oC; pH 7.4). However, under more

typical physiologic conditions (i.e., in the presence of neutral buffers containing carboxylic acids), the

hydrolysis of HDI vapor to HDA was markedly catalyzed, with a 20 mmol bicarbonate buffer being the

optimum catalyst (95% in 10 minutes at 30 oC; pH 7.4). Results of experiments in a staticsystem with

liquid-phase HDI in water also indicated that the addition of simple carboxylic-acid-containing neutral

buffers markedly increased the formation of HDA, with less acidic catalysts (pKa >6), such as carbonic

and citric acid, much more effective than those with higher acidity (pKa<5), such as formic or oxalic

acid. Because the experimental conditions of this study are not typical of those found in ambient or

.

HEXAMETHYLENE DIISOCYANATE 114

5. POTENTIAL FOR HUMAN EXPOSURE

waste waters, it is not possible to draw any meaningful conclusions from the results regarding the rate of

hydrolysis of HDI in these aquatic environments.

HDI is expected to be hydrolyzed much more quickly than it would undergo biodegradation in water,

although the resulting amines should be subject to biodegradation (HSDB 1995). From initial

concentrations of 50 ppm, both TDI and MDI (pure methylene bis[4-phenylisocyante] and prepolymeric

MDI consisting of short chain oligomers of MDI with reactive isocyanate terminal groups) were reported

to be completely biodegraded (detection limits 0.02 ppm) within 15 days at 25 °C in a fresh water model

river system with bottom sludge; whereas, in a similar salt water system, TDI could not be detected

within 4 days and MDI disappeared after only one day (International Isocyanate Institute 1983, 1990).

However, the role of hydrolysis in this process, which should be predominant, was not considered. The

formation of TDA and MDA was observed, with maximum concentrations in fresh water of 0.3 and

0.1 ppm, respectively; and in marine water, of 4.0 and 0.02 ppm, respectively. TDA was not detected

(detection limit 0.02 ppm) after 30 days in the fresh water system, and after 15 days in the marine water

system; whereas MDA disappeared (detection limit 0.02 ppm) after only 4 days in both systems. In both

systems, approximately 0.2% of the initial TDI was recovered as TDA from precipitation crusts after

30 days. Less than 0.02% of the initial MDI was recovered as MDA from precipitation crusts after

30 days in the fresh water system, and no MDA was detected in precipitation crusts in the marine water

system after 30 days.

5.3.2.3 Sediment and Soil

No studies of the transformation and degradation of HDI in soil were located in the available literature.

Isocyanates react readily with water to form amines and polyureas (Chadwick and Cleveland 1981; Hulse

1984; Kennedy and Brown 1992) and hydrolysis of HDI is expected to occur much more rapidly than

biodegradation (HSDB 1995). Consequently, reaction with water is expected to be the only significant

fate process of HDI in moist soil or sediment. The HDA resulting from hydrolysis, however, should be

subject to various types of biodegradation (HSDB 1995). Gilbert (1988) has summarized the results of

laboratory experiments on TDI in undisturbed moist sand, which indicate that TDI is converted to

polyureas at a rapidly decreasing rate, with 5.5 and 3.5% of unreacted TDI remaining after 24 hours and

8 days, respectively. The toluene diamine hydrolysis product was not found above the detection limit of
HEXAMETHYLENE DIISOCYANATE 115

5. POTENTIAL FOR HUMAN EXPOSURE

0.01 ppm. These results were interpreted as an indication of encapsulation of unreacted TDI within a

rapidly forming water-insoluble polyurea crust. Similar results may be expected for HDI.

5.4 LEVELS MONITORED OR ESTIMATED IN THE ENVIRONMENT

54.1 Air

Except for occupational settings, no information was found in the available literature on concentrations

of HDI or HDI prepolymers in air. Because of the relatively short atmospheric half-life (approximately

2 days) from reaction with hydroxyl radicals (see Section 5.3.2. l), significant atmospheric concentrations

of HDI would be expected to be found only near sources of this substance (e.g., waste streams from

manufacturing or processing facilities, hazardous waste sites, occupational settings). Atmospheric

concentrations of HDI and HDI-BT found in occupational settings are summarized in Section 5 5.

5.4.2 Water

No information was found in the available literature on concentrations of HDI or HDI prepolymers in

water. Because of the expected rapid hydrolysis of HDI, significant concentrations may not be found in

water, except near sources of this substance (e.g., industrial waste streams, hazardous waste sites). Small

amounts of unreacted HDI may persist in water if encapsulated in water-insoluble polyurea crusts formed

during hydrolysis (Gilbert 1988).

5.4.3 Sediment and Soil

No information was found in the available literature on concentrations of HDI or HDI prepolymers in

sediment and soil. Because of the expected rapid hydrolysis of HDI in moist soil or sediment, significant

concentrations may not be found in these media, except near sources of this substance (e.g., industrial

waste streams, hazardous waste sites). Small amounts of unreacted HDI may persist in sediment and soil

if encapsulated in water-insoluble polyurea crusts formed during hydrolysis (Gilbert 1988).
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5. POTENTIAL FOR HUMAN EXPOSURE

5.4.4 Other Environmental Media

Biuret modified HDI (HDI-BT), a trimeric condensation product of HDI and water, which is commonly

used as a hardener in 2-component coatings, typically contains unreacted HDI at concentrations below

1% (Alexandersson et al. 1987; Hulse 1984); however, after 3-6 months storage, the free monomer

content may increase to approximately 1.6% (Hulse 1984). Polyurethane paints from 5 different

manufacturers in Finland were found to contain HDI and HDI-BT at average concentrations of 0.24%

(range, 0.19-0.32%) and 34% (range, 30-36%), respectively (Rosenberg and Tuomi 1984). Similar HDI

concentrations (<1%) were found in a polyurethane varnish (Desmodur N®, Bayer AG) (Nielsen et al.

1985). HDI-BT (DES-N®, Mobay Corporation), which is commonly used in formulations of automobile

and airplane coatings contains between 0.6 and 2.0% monomeric HDI (Karol and Hauth 1982). A

polyisocyanate activator which was mixed 1:3 with an enamel contained 7% HDI-BT (Malo et al. 1983).

In a Swedish study, the HDI-BT used in polyurethane paints contained 0.5-l.0% unreacted, monomeric

HDI; the applied paint contained approximately 10% HDI-BT in the surface paint layer and varnish

layer, compared to 3-6% HDI-BT in the primary paint layer (Alexandersson et al. 1987). No data on

levels of HDI in other environmental media, including food, were found in the available literature.

Because of the rapid hydrolysis of HDI (see Section 5.3.2) and the evidence against bioaccumulation of

HDI in the food chain (see Section 2.3), it is not expected that HDI will be found in any significant

concentrations in foods.

5.5 GENERAL POPULATION AND OCCUPATIONAL EXPOSURE

General population exposures to HDI may occur during the nonoccupational use of polyurethane coatings

(Musk et al. 1988), primarily through inhalation of aerosols and vapors (Alexandersson et al. 1987;

Grammar et al. 1988; Malo et al. 1983; Tulane Medican 1982a), and to a much lesser extent via dermal

absorption (E.I. DuPont De Nemours 1977b; Haskell Laboratory 1961; Mobay Corporation 1981b;

Stadler and Karol 1985). No information could be found in the available literature on levels of HDI in

the environment, or of general population exposures; however, there were several reports uf individuals

with asthma-like hypersensitivities (see Chapter 2). Because of the expected rapid hydrolysis of HDI in

the environment (see Section 5.3), significant general population exposures from air, water, or soil do not

appear to be likely. Additionally, the rapid metabolism of HDI by higher trophic animals (see

Section 2.3) indicates that this substance will not bioaccumulate in the food chain and, consequently, that

general population exposures to HDI from foods will be insignificant.
HEXAMETHYLENE DIISOCYANATE 117

5. POTENTIAL FOR HUMAN EXPOSURE

Occupational exposure to HDI occurs primarily through inhalation of aerosols and vapors and through

dermal absorption (Alexandersson et al. 1987; Grammar et al. 1988; Hulse 1984; NIOSH 1992) with

inhalation being the predominant route of exposure (Parmeggiani 1983). The NIOSH-recommended

exposure limits (RELs) for HDI, expressed as a 0-hour time-weighted average (TWA) and 10-minute

ceiling value, are 35 and 140 μg/m3

, respectively (NIOSH 1992). The American Conference of

Governmental Industrial Hygienists (ACGIH) has set an S-hour TWA of 0.005 ppm as a threshold limit

value (TLV) for HDI (ACGIH 1994).

Preliminary data from the NOES conducted by NIOSH from 1980 to 1983 indicates that an estimated

19,181 workers were potentially exposed to HDI in the United States from 1981 to 1983 (NIOSH 1989).

These numbers do not include workers potentially exposed to trade name compounds that contain HDI,

so that the actual number of exposed workers may be substantially higher than reported. Among the

occupations with the greatest potential for exposure to HDI were painters and paint spraying-machine

operators (6,139 potentially exposed workers), aircraft engine and other mechanics (7,516 potentially

exposed workers), and aircraft machinists (3,317 potentially exposed workers) (NIOSH 1989). Other

occupations with potential for exposure to HDI include construction laborers, chemical technicians,

mixing and blending machine operators in the chemical industry, plumbers, pipefitters, steamtitters,

metal plating machine operators, miscellaneous machine operators in the aircraft equipment industry, and

production workers and supervisors in the fabricated structural metal industry (NIOSH 1989).

Occupational exposures to HDI may also occur in the areas of spills; however, such exposures can be

minimized by the rapid application of an aqueous surfactant solution, which has been shown to result in

the complete mineralization of isocyanates such as TDI and MDI within minutes (Mobay Corporation

1980). The surfactant appears to act by solubilizing the isocyanate in an aqueous medium, thus

facilitating the complete hydrolysis of the compound. The effectiveness of this treatment has been

demonstrated in the laboratory and in soil under controlled conditions.

Prepolymeric HDI-BT paint formulations, which generally contain <1% monomeric HDI, are now widely

used for coatings applications, particularly in the automobile and airplane industries (Alexandersson et

al. 1987; Karol 1986; Key-Schwartz 1992; Nielson et al. 1985; Rosenberg and Savolainen 1986;

Rosenberg and Tuomi 1984). Consequently, many reported occupational exposures to HDI are actually

exposures predominantly to HDI-BT (Karol 1986; Karol and Hauth 1982). Although isocyanate

prepolymers are safer to use than the free monomers, primarily because of their lower vapor pressures,

they can still pose a health risk to workers when inhaled in the aerosol form (Karol and Hauth 1982;
HEXAMETHYLENE DIISOCYANATE 118

5. POTENTIAL FOR HUMAN EXPOSURE

Rosenberg and Tuomi 1984). There is evidence that isocyanate prepolymers may induce asthma at the

same or greater frequency as diisocyanate monomers (Seguin et al. 1987); therefore, characterization of

both monomeric HDI and HDI-BT exposures is important to adequately assess the overall potential for

health risk.

Most occupational isocyanate exposure studies have focused on toluene diisocyanate because of its

widespread industrial use in the manufacture of flexible foam products (e.g., Alexandersson et al. 1985;

Boeniger 1991; Dharmarajan et al. 1978; Diem et al. 1982; Huang et al. 1991a, 1991b; Jones et al. 1992;

Maitre et al. 1993; Olsen et al. 1989; Omae 1984; Omae et al. 1992; Persson et al. 1993; Peters and

Wegman 1975; Rando et al. 1987; Rosenberg 1984; Rosenberg and Savolainen 1986; Venables et al.

1985; Wegman et al. 1977; Weill et al. 1975). A more limited number of studies of occupational

exposures to HDI and HDI-BT have been reported in the literature, some of which have found

atmospheric HDI concentrations above the NIOSH TWA of 35 μg/m3

 (0.005 ppm) (NIOSH 1992).

These are summarized below.

Hulse (1984) conducted a combined workplace and laboratory study to evaluate the exposure potential of

HDI in vapor and aerosol states from HDI polyurethane spray paint aerosols. Sampling in 2 workplaces

selected to represent worst-case exposures typical of spray painting in light aircraft maintenance facilities

resulted in total personal and area HDI concentrations ranging from 13 to 43 μg/m3

 (0.002-0.006 ppm)

and 39-63 μg/m3

 (0.006-0.009 ppm), respectively, with an average of 47% of the atmospheric HDI in the

vapor phase. Results of the laboratory study suggested that for a given paint and process, a linear

relationship exists between the aerosol mass concentration and the HDI concentration. Although this

relationship was not found in workplace samples, the author concluded that the results of this study

indicated that the NIOSH ceiling limit for HDI (140 μg/m3

; 0.020 ppm) is unlikely to be exceeded at

aerosol mass concentrations below 20 mg/m3

. An average HDI concentration of 0.230 ppm

(1,600 μg/m3

) in curing oven exhaust was calculated from the results of a laboratory study using metal

panels coated with a formulation (Desmodur LS-2800®) containing <0.3% HDI monomer and a blocking

agent (Mobay Corporation 1986). A release profile indicated that once curing oven temperatures capable

of releasing the HDI blocking agent in the formulation are reached (<400°C in this study), most of the

HDI may be released over a relatively short time, which could result in momentary excursions of HDI

concentrations above those calculated by a time average.
HEXAMETHYLENE DIISOCYANATE 119

5. POTENTIAL FOR HUMAN EXPOSURE

In a study of U.S. workers involved in the mixing or application of HDI-BT spray paints, mean ambient

air concentrations of HDI and HDI-BT ranged from <0.08 to 3.8 μg/m3

 (<0.0000l-0.0006 ppm) and

5.3-75 μg/m3

 (0.0008-0.011 ppm), respectively (Grammar et al. 1988). Substantially higher ambient air

concentrations of HDI were found at 4 different spray-painting operations at a U.S. Air Force base in

Biloxi, Mississippi, ranging from 12 to 59 μg/m3

 (0.002-0.009 ppm); HDI-BT levels were not monitored

(Rudzinski et al. 1994). Ambient air concentrations of HDI and HDI-BT in 3 automobile spray paint

shops and one trade school for spray painters in Finland were found to range from 6 to 120 μg/m3

(0.001-0.018 ppm) and 280-3,600 μ/m3 (0.043-0.55 ppm), respectively, with the concentration of

HDI-BT 40 times that of HDI on average (Rosenberg and Tuomi 1984). In this same study, mean

personal exposure concentrations (outside the respirator) of HDI and HDI-BT were 490±220 μg/m3

(0.074±0.033 ppm) and 1,440±1,130 μg/m3 (0.219±0.172 ppm) (±SD, n=l0), respectively. The use of

respirators with combined charcoal and particle filters reduced mean personal exposure concentrations of

HDI and HDI-BT to <1 and <10 μg/m3

 (<0.0002 and <0.002 ppm), respectively. In another study

involving reconstruction of exposures of automobile painters in Sweden using HDI-BT spray paints, the

mean exposure concentration of HDI was reported to be 1 .0 μg/m3

 (0.0002 ppm); whereas, the mean

exposure concentration of HDI-BT was 115 μg/m3

 (0.017 ppm) (range, 10-385 μg/m3;

0.002-0.058 ppm), with short-term peak exposures up to 13,500 μg/m3

 (2.05 ppm) (Alexandersson et al.

1987).

In some industrial processes, exposure to HDI may occur in the presence of its hydrolysis products,

namely 1,6-hexamethyleneaminoisocyanate and HDA, both of which may also cause adverse respiratory

effects (Beard and Noe 1981). In a study to evaluate isocyanate exposures of automobile repair workers

involved in welding activities, which could lead to pyrolysis of HDI polymer-based paint, the

atmospheric concentration ranges of HDI, 1,6-hexamethyleneaminoisocyanate, and HDA were found to

be 0.18-l .3 mg/m3

 (0.027-0.20 ppm), <0.004-0.24 mg/m3 (<0.0006-0.036 ppm), and

<0.004-0.10 mg/m3

 (<0.0006-0.0 15 ppm), respectively, 25 cm from the source (Skarping et al. 1988).

Concentrations of 1,6-hexamethyleneaminoisocyanate and HDA were on the order of 15% of the HDI

concentrations.

Recent studies indicate that the toxic metabolite of HDI, HDA, may be monitored in urine hydrosylates

as a biomarker of short-term exposure to HDI (Berode et al. 1991; Brorson et al. 1990a, 1990b; Dalene et

al. 1990, 1994a; Rosenberg and Savolainen 1986). When exposure to HDI is on the order of the NIOSH

REL of 35 μg/m3

 (NIOSH 1992), urinary HDA analyses are sensitive enough to monitor occupational
HEXAMETHYLENE DIISOCYANATE 120

5. POTENTIAL FOR HUMAN EXPOSURE

HDI exposure (Brorson et al. 1990b; Dalene et al. 1990); however, the correlation between HDI exposure

and urinary HDA levels requires further investigation. Because HDI exposures frequently involve

exposure to prepolymeric HDI (e.g., HDI-BT), the uptake, metabolism, and excretion of HDI-BT must

also be investigated (Brorson et al. 1990b). HDA is rapidly removed from the urine by N-acetylation

(half-life, ≈01.2-l .4 hours; >90% urinary elimination of HDA within ≈ 4 hours of termination of

inhalation exposure; therefore, HDA analysis must be performed on urine samples collected immediately

after exposure (Brorson et al. 1990b; Dalene et al. 1990). Thus, HDA is not a suitable biomarker for

assessing past exposures or other long-term exposures to HDI. No studies were found in the available

literature reporting values of urinary HDA among occupationally exposed groups.

Workers in industries involved in the production or processing of HDI or prepolymeric HDI-BT, or who

use products containing these materials (e.g., spray paints) also have potentially high exposures to HDI,

HDI-BT, and HDI hydrolysis products (i.e., 1,6-hexamethyleneaminoisocyanate and HDA). The

prevalence of respiratory symptoms among workers exposed to diisocyanate monomers has been

estimated to range from 5 to 20%; although, in most studies, the occurrence of occupational asthma has

not been confirmed through inhalation challenge tests (NIOSH 1978). Approximately 5% of workers in

the isocyanate industry suffer from acute or chronic obstructive lung disease (Dewair et al. 1983).

Immunologic screening for IgE and IgG antibodies against HDI and HDI-BT conjugated to human serum

albumin has also been used to identify occupational exposures to HDI and prepolymeric HDI-BT

(Cvitanovic et al. 1989; Grammar et al. 1988, 1990; Karol and Hauth 1982; Malo et al. 1983; Welinder et

al. 1988) (see Sections 2.2.1.3 and 2.6). However, because of the high amount of cross-reactivity

between different isocyanates using HDI-human serum albumin (HSA) antibodies, this test is currently

only reliable to detect exposures to diisocyanates in general, and not specifically to HDI or its

prepolymers.

5.6 POPULATIONS WITH POTENTIALLY HIGH EXPOSURES

Among the general population, subpopulations with potentially high HDI exposures include residents

near industrial facilities releasing HDI into the environment and residents in the vicinity of HDIcontaining hazardous waste sites. No data were found in the available literature, however, to characterize

the extent of such exposures. The prevalence of asthma among residents in the vicinity of a factory in

Finland producing TDI-based polyurethane was not found to be significantly different from controls,
HEXAMETHYLENE DIISOCYANATE 121

5. POTENTIAL FOR HUMAN EXPOSURE

except for the middle-age (46-60 years old) group (Nuorteva et al. 1987). No additional information was

found in the available literature to document exposures of these subpopulations. HDI has also not been

detected in any media at any of the 1,445 current or former hazardous waste sites.

5.7 ADEQUACY OF THE DATABASE

Section 104(I)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with

the Administrator of EPA and agencies and programs of the Public Health Service) to assess whether

adequate information on the health effects of hexamethylene diisocyanate is available. Where adequate

information is not available, ATSDR, in conjunction with the NTP, is required to assure the initiation of

a program of research designed to determine the health effects (and techniques for developing methods to

determine such health effects) of hexamethylene diisocyanate.

The following categories of possible data needs have been identified by a joint team of scientists from

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean

that all data needs discussed in this section must be filled. In the future, the identified data needs will be

evaluated and prioritized, and a substance-specific research agenda will be proposed.

5.7.1 Identification of Data Needs

The following data needs are limited to HDI; however, these also apply to HDI prepolymers, which are

often found in association with the monomer.

Physical and Chemical Properties. Although hydrolysis has been identified as a significant

reaction in determining the fate of HDI (aqueous hydrolysis half-life, <10 minutes) (Chadwick and

Cleveland 1981; Hulse 1984; Kennedy and Brown 1992; Saunders and Frisch 1962; SRC 1994b), this

chemical is not well-defined in terms of its physical and chemical properties (see Table 3-3). Because of

the rapid hydrolysis of the isocyanate functional group, available estimates of Kow and Henry’s law

constant (H) are questionable (SRC 1994a, 1995b). No estimates of Koc were found in the available

literature. Further information on estimates of Kow, Koc, and Henry’s law constant (H) would be helpful

in determining the environmental fate of this chemical and the rate at which it degrades in air, water, soil,

and sediments.
HEXAMETHYLENE DIISOCYANATE 122

5. POTENTIAL FOR HUMAN EXPOSURE

Production, Import/Export, Use, Release, and Disposal. The available production, use, and

release information for HDI is insufficient to determine the amount of HDI that may be present in the

environment. There is a need for data on HDI production and import/export volumes. The potential for

exposure to HDI during production may be substantial; however, no information on past, present, or

projected production volumes was found in the available literature. Descriptive information on the uses

of HDI and HDI-based products is extensive and appears to be adequate; however, quantitative data on

use patterns are needed. In addition, there is a lack of data on the amount of HDI that may be stored at

waste sites. Little or no information was found in the available literature on current disposal methods for

HDI, their efficiencies, the need for improvement, or the amount disposed of by each method. Additional

information in this area is needed to assess the potential for human exposure to HDI from disposal

activities.

According to the Emergency Planning and Community Right-to-Know Act of 1986, 42 U.S.C. Section

11023, industries are required to submit chemical release and off-site transfer information to the EPA.

The Toxics Release Inventory (TRI), which contains this information for 1994, became available in May

of 1996. This database will be updated yearly and should provide a list of industrial production facilities

and emissions.

No information is available in the TRI database, however, because this substance is not included under

SARA, Title III, and therefore, is not among the chemicals that facilities are required to report (EPA

1995). There is a need for such information in order to assess the potential for human exposure to this

substance from their release from industrial production or processing facilities.

Environmental Fate. Extensive information is available on the general reactions of isocyanates that

may pertain to the environmental fate of HDI (Chadwick and Cleveland 1981; Kennedy and Brown

1992). However, investigations of the environmental fate of isocyanates have focused primarily on TDI

and MDI (Duff 1983, 1985; Gilbert 1988; Holdren et al. 1984). Only one laboratory study was located in

the available literature specifically on the chemical reactions of HDI (i.e., bicarbonate buffer-catalyzed

hydrolysis) that may be relevant to the environmental fate of HDI in water (Berode et al. 1991). HDI is

expected to react relatively rapidly with hydroxyl radicals in the atmosphere and to be rapidly hydrolyzed

in water and moist soils and sediment. The significance of atmospheric hydrolysis has not been

evaluated. Additional field and laboratory studies are needed to adequately characterize the

environmental fate of HDI in air, water, soil, and sediment.
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5. POTENTIAL FOR HUMAN EXPOSURE

Bioavailability. HDI may be absorbed following inhalation (Alexandersson et al. 1987; Grammar et

al. 1988; Malo et al. 1983; Tulane Medican 1982a), ingestion (Haskell Laboratory 1946, 1961), and

dermal contact (Alexandersson et al. 1987; Grammar et al. 1988; Malo et al. 1983; Tulane Medican

1982a). The environmental factors that may influence the bioavailability of HDI from various

environmental media have not been studied. The predicted rapid hydrolysis of HDI in water suggests

that human exposure via contaminated drinking water or surface waters is unlikely, and no studies on the

bioavailability of this compound from water are indicated at this time. No data were found in the

available literature on the sorption of HDI to soil, sediments, or airborne particles; however, because of

the rapid hydrolysis of HDI (see Section 5.3.2), sorption may not be a significant determinant in

assessing the bioavailability of HDI from these media. There is a need for data characterizing the factors

that affect the bioavailability of HDI from soils and sediments only if environmental fate or monitoring

studies indicate that HDI persists in these media at significant concentrations.

Food Chain Bioaccumulation. Because of the rapid hydrolysis of HDI in water and the ease with

which this substance is metabolized in higher trophic animals (see Section 2.3), it is not expected that

this substance will bioconcentrate in aquatic organisms, or bioaccumulate in the food chain. Neither TDI

and MDI, nor their diamine hydrolysis products, TDA and MDA, have been found to bioaccumulate in

fish (Cyprinus curpio) in river model studies (International Isocyanate Institute 1990). No information

on BCFs and food chain bioaccumulation could be found for HDI in the available literature; however, a

BCF of approximately 100 was calculated using the method of Veith et al. (1979), which indicates a very

low bioaccumulation potential for HDI. Further studies on the bioaccumulation of HDI do not appear to

be warranted.

Exposure Levels in Environmental Media. No information was found in the available literature

on concentrations of HDI in ambient air; surface, ground, and drinking water; sediment and soil; or other

environmental media such as food. Because of the relatively rapid reaction of HDI with hydroxyl

radicals in the atmosphere (SRC 1995a), and the rapid hydrolysis of HDI in other media (Chadwick and

Cleveland 1981; Hulse 1984; Kennedy and Brown 1992; SRC 1994b), significant concentrations would

not be expected to occur in air, water, or sediment and soil, except near potential emission sources of this

substance (e.g., industrial waste streams, hazardous waste sites, occupational settings, environmental

spills). Small amounts of unreacted HDI may persist in water, or sediment and soil, if encapsulated in

water-insoluble polyurea crusts formed during hydrolysis (Brochagen and Grieveson 1984; Gilbert

1988). Data on concentrations in all environmental media (air, water, sediment and soil) near potential
HEXAMETHYLENE DIISOCYANATE 124

5. POTENTIAL FOR HUMAN EXPOSURE

emission sources and at background sites are necessary to assess the potential for exposure to HDI of

populations living near these potential sources. Only a small number of studies reporting concentrations

of HDI in occupational settings in the United States were found in the available literature; additional data

are needed to more fully characterize occupational exposures in this country. Because of the rapid

hydrolysis of HDI in water and the ease with which this substance is metabolized in higher trophic

animals (see Section 2.3), it is not expected that this substance will bioconcentrate in aquatic organisms,

or bioaccumulate in the food chain. Consequently, concentrations of HDI in food should be insignificant

and the need for data in this area is not pressing.

Reliable monitoring data for the levels of HDI in contaminated media at hazardous waste sites are needed

so that the information obtained on levels of HDI in the environment can be used in combination with the

known body burden of HDI to assess the potential risk of adverse health effects in populations living in

the vicinity of hazardous waste sites.

No information was found in the available literature on estimates of human intake of HDI from the

various environmental media. Because the potential for significant widespread environmental

contamination is expected to be very low, such estimates would appear to be needed primarily for

populations living or working near potential emission sources of HDI.

Exposure Levels in Humans. Recent studies indicate that the toxic metabolite of HDI, I-IDA, may

be monitored in urine hydrosylates as a biomarker of short-term exposure to HDI (Berode et al. 199 1;

Brorson et al. 1990b; Dalene et al. 1990, 1994a; Rosenberg and Savolainen 1986). When exposure to

HDI is on the order of the NIOSH TWA of 35 μg/m3

 (NIOSH 1992), urinary HDA analyses are sensitive

enough to monitor occupational HDI exposure (Brorson et al. 1990b; Dalene et al. 1990); however, the

correlation between HDI exposure and urinary HDA levels requires further investigation. Because HDA

is rapidly removed from urine by N-acetylation (half-life, ≈ 1.2-l .4 hours; >90% urinary elimination of

HDA within -4 hours of termination of inhalation exposure), it is not a suitable biomarker for assessing

past exposures or other long-term exposures to HDI and analyses must be performed on urine samples

collected immediately after exposure (Brorson et al. 1990b; Dalene et al. 1990). The use of urinary HDA

as a biomarker is still in the developmental stages and no data on concentrations of urinary HDA in

occupationally exposed populations, populations living near hazardous waste sites, control groups, or the

general population were found in the available literature. There is a need for further method
HEXAMETHYLENE DIISOCYANATE 125

5. POTENTIAL FOR HUMAN EXPOSURE

development in this area, as well as for biomonitoring data in these populations. This information is

necessary for assessing the need to conduct health studies on these populations.

Immunologic screening for IgE and IgG antibodies against HDI and HDI-BT conjugated to human serum

albumin (HSA) has also been used to identify occupational exposures to HDI (Cvitanovic et al. 1989;

Grammar et al. 1988, 1990; Karol and Hauth 1982; Malo et al. 1983; Welinder et al. 1988) (see Sections

2.2.1.3 and 2.6). However, because of the high amount of cross-reactivity between different isocyanates

using HDI-HSA antibodies, this test is currently only reliable to detect exposures to diisocyanates in

general, and not specifically to HDI or its prepolymers. Additional research is needed to develop

bioassays that are specific for HDI and HDI prepolymers.

Exposure Registries. No exposure registries for hexamethylene diisocyanate were located. This

substance is not currently one of the compounds for which a subregistry has been established in the

National Exposure Registry. The substance will be considered in the future when chemical selection is

made for subregistries to be established. The information that is amassed in the National Exposure

Registry facilitates the epidemiological research needed to assess adverse health outcomes that may be

related to exposure to this substance.

5.7.2 Ongoing Studies

A search of the Federal Research in Progress database (FEDRIP 1995) indicates that no research studies

are in progress to fill the data gaps discussed in Section 5.7.1.

HEXAMETHYLENE DIISOCYANATE 127

6. ANALYTICAL METHODS

The purpose of this chapter is to describe the analytical methods that are available for detecting, and/or

measuring, and/or monitoring hexamethylene diisocyanate, its metabolites, and other biomarkers of

exposure and effect to hexamethylene diisocyanate. The intent is not to provide an exhaustive list of

analytical methods. Rather, the intention is to identify well-established methods that are used as the

standard methods of analysis. Many of the analytical methods used for environmental samples are the

methods approved by federal agencies and organizations such as EPA and the National Institute for

Occupational Safety and Health (NIOSH). Other methods presented in this chapter are those that are

approved by groups such as the Association of Official Analytical Chemists (AOAC) and the American

Public Health Association (APHA). Additionally, analytical methods are included that modify

previously used methods to obtain lower detection limits, and/or to improve accuracy and precision.

6.1 BIOLOGICAL SAMPLES

Some of the methods used for determining HDI in biological media are reported in Table 6-l. Methods

for the biological monitoring of exposures to HDI have been based on determination of the corresponding

amine, hexamethylene diamine. Methods are available for determination of HDI in urine and plasma;

however, no methods were found for mother’s milk, or tissues.

The analysis methods use acid or base hydrolysis of the compound, extraction into solvent (usually

toluene), derivatization, followed by chromatographic analysis. Determination is based on sensitive gas

chromatography/mass spectrometry (GC/MS) techniques. Alternately, GC with specific detectors or high

performance liquid chromatography (HPLC) may be sued. The method based on hydrolysis and basic

extraction of the liberated amine into toluene provides sensitive measurements, but the extraction

recovery is low (Dalene et al. 1990). A method using hydrolysis followed by a two-phase derivatization

procedure showed good recovery (above 90%) but less sensitivity (Dalene et al. 1994a). Very good

sensitivity was reported for determination using HPLC-MS (Dalene et al. 1994b); however, this method

requires state-of-the-art instrumentation.

Analysis of the degradation products of the oligomeric form of HDI, HDI-BT, may be possible by the

above procedures; no experimental data are available.

HEXAMETHYLENE DIISOCYANATE 129

6. ANALYTICAL METHODS

6.2 ENVIRONMENTAL SAMPLES

Methods are available for the determination of HDI in occupational and ambient air. A summary of

representative methods is shown in Table 6-2. Much of the methodology involves drawing air through an

impinger or sorbent-containing derivatizing reagent. The sample is then analyzed by HPLC with

ultraviolet (UV) and/or electrochemical detection. A good review of these techniques is available. Two

NIOSH methods (Methods 5505 and 5521) have been used to analyze for the isocyanates (NIOSH 1984).

Both use HPLC with UV, or UV and electrochemical detection. Collection is carried out by passing

sampled air through toluene containing l-(2-methoxyphenyl)-piperazine to derivatize the HDI. They

were specified for analysis of monomeric and oligomeric forms of isocyanates providing a total

isocyanate concentration in air as well as some speciation. Method 5505 has been removed from the

current NIOSH manual. Problems have been reported for Method 5521. The ratio criterion may not be

suitable for qualitative identification of HDI oligomers (Key-Schwartz 1995).

There are also other techniques which use a similar sampling and analysis scheme. A number of methods

used 9-(N-methylaminomethyl)anthracene as a reagent to derivatize the HDI allowing subsequent

detection of a stable derivative by UV absorbance or fluorescence detection (Andersson et al. 1983;

Gudehn 1984; Kormos et al. 198 1; Sang6 and Zimerson 1980). The “nitro reagent” or N-4-nitrobenzylN-n-propylamine has also been used for derivatization prior to analysis by HPLC/UV or differential pulse

polarography (Corbini et al. 1991; Dunlap et al. 1976; Graham 1980; Hakes et al. 1986). Various column

types and eluent conditions were used in the HPLC separations cited above. The 1-(2-methoxyphenyl)-

piperazine used as a derivatization reagent in the NIOSH methods was also used on a sorbent (Schmidtke

and Seifert 1990) and inert supports (Huynh et al. 1992; Sesana et al. 1991) avoiding the use of toluene

in the collection impinger. Another derivatization agent used was 1-(2-pyridyl)piperazine either in a

toluene solution (Ellwood et al. 1981; Goldberg et al. 198 1) or impregnated on glass fiber filters

(Rudzinski et al. 1994). Samples were analyzed using TLC, capillary zone electrophoresis, or HPLC

techniques. In addition to those listed in Table 6-2, there are methods which may be used for

determination of the oligomeric form of HDI or HDI-BT (biuret trimer) (Bagon et al. 1984; Ellwood et

al. 1981; Goldberg et al. 1981; Levine et al. 1979).

Additional novel analytical techniques include coating a polystyrene strip with cholinesterase, exposing

the strip to an atmosphere (passive sampling), then immersing the strip in a cuvette with reagent for assay


HEXAMETHYLENE DIISOCYANATE 132

6. ANALYTICAL METHODS

by UV absorbance measurement (Brown et al. 1984). The disadvantage is that it does not differentiate

between isocyanate species, but it is reasonably sensitive. Also available is a dedicated instrument, the

Chemcasette® 7005, which uses proprietary paper tape chemistry and was reviewed for total isocyanates

(Dharmarajan and Rando 1980). The only direct measurement technique for HDI using chromatography

involved collection by impinger containing isooctane, then analysis by GC using the nitrogen sensitive

thermionic specific detector (Skarping et al. 1985).

Methods for analysis of HDI in other media were not found. However, since HDI hydrolyzes rapidly in

water, it is unlikely that significant amounts of HDI monomer would be found in water, soils, sediment,

or food, except near sources of release.

6.3 ADEQUACY OF THE DATABASE

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with

the Administrator of EPA and agencies and programs of the Public Health Service) to assess whether

adequate information on the health effects of hexamethylene diisocyanate is available. Where adequate

information is not available, ATSDR, in conjunction with the NTP, is required to assure the initiation of

a program of research designed to determine the health effects (and techniques for developing methods to

determine such health effects) of hexamethylene diisocyanate.

The following categories of possible data needs have been identified by a joint team of scientists from

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean

that all data needs discussed in this section must be filled. In the future, the identified data needs will be

evaluated and prioritized, and a substance-specific research agenda will be proposed.

6.3.1 Identification of Data Needs

Methods for Determining Biomarkers of Exposure and Effect. There is currently only one

biomarker hexamethylene diamine, HDA, which is used for the monitoring exposure of individuals to

HDI. This biomarker is convenient as it is readily excreted in urine and it can be detected at low

concentration levels as indicated in Table 6-1. Additional recovery data are needed for HDA in order to
HEXAMETHYLENE DIISOCYANATE 133

6. ANALYTICAL METHODS

improve exposure estimates, especially since the half-life of HDA in urine is difficult to determine for

persons suspected of being exposed to HDI. No other biomarker has been cited as an alternative.

Methods for Determining Parent Compounds and Degradation Products in

Environmental Media. Currently there is a fairly broad selection of analytical methods for the

analysis of HDI in air. Since it degrades very quickly upon contact in water and soil, and degrades more

slowly by reaction with hydroxyl radicals and water vapor in air, it is important for analyses to be

focused on air as a medium. It is probably most important at this point to improve the speciation

(chromatographic separations) of isocyanate forms including prepolymers which may provide toxic

effects to individuals. This will allow more accurate measurement and control of hazards in the

workplace and general environment. Better data on recovery would be useful for some air sampling

techniques as air sampling is of greatest importance for exposure assessment. It is also important that

more information is determined on chemical and physical properties for the biuret trimer form since little

is currently available in the literature.

6.3.2 Ongoing Studies

No ongoing studies related to methods for detection of HDI were found.

HEXAMETHYLENE DIISOCYANATE 135

7. REGULATIONS AND ADVISORIES

The national and state regulations and guidelines regarding HDI in air, water, and other media are

summarized in Table 7- 1. No international regulations were identified for HDI.

Diisocyanates such as HDI represent a group of chemicals that primarily target the upper and lower

respiratory tract, resulting in chronic pulmonary irritation and asthmatic reaction in humans. HDI is also

known to be a potent respiratory and dermal sensitizing agent. Because of its potential to cause adverse

health effects in exposed people, a number of regulations and advisory values have been established for

HDI.

ATSDR has derived an intermediate-duration inhalation MRL of 3.0x10-5 ppm for HDI, based on an

NOAEL of 0.005 ppm for respiratory effects in rats in a study by Mobay Corporation (1984).

ATSDR has derived a chronic-duration inhalation MRL of 1.0x10-5 ppm for HDI, based on a LOAEL of

0.0001 ppm based on changes in percent closing volume (%CV) in a study of 3 groups of garage workers

by Alexandersson et al. (1987).

HDI has not been evaluated by the EPA for evidence of human carcinogenic potential (IRIS 1997).

However, the EPA has established an inhalation reference concentration (RfC) of 1x10-5 mg/m3

 for HDI

(IRIS 1997).

An Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for HDI

does not exist.

Hexamethylene- 1,6-diisocyanate is listed as a hazardous air pollutant (HAP) under to Section 112 (b) of

the Clean Air Act (CAA) (U.S. Congress 1990). The national emission standards for hazardous air

pollutants (NESHAP) which were established pursuant to Section 112 of the CAA (as amended

November 15, 1990), regulate specific categories of stationary sources that emit or have the potential to

emit one or more HAPS. HDI is listed as a volatile HAP for wood furniture manufacturing operations in

Title 40 of the Code of Federal Regulations (CFR), Part 63, Subpart JJ (EPA 1995).
HEXAMETHYLENE DIISOCYANATE 136

7. REGULATIONS AND ADVISORIES

On November 30, 1994, EPA added HDI and 285 other chemicals to the list of toxic chemicals that are

subject to reporting under Section 313 of the Emergency Planning and Community Right-to-Know Act

of 1986 (EPCRA) and Section 6607 of the Pollution Prevention Act of 1990 (EPA 1994). Section 313

of Title III of EPCRA requires owners and operators of certain facilities that manufacture, import,

process, or otherwise use the chemicals on this list to report annually their release of those chemicals to

any environmental media.

In 1989, the Interagency Testing Committee (ITC) designated HDI for health effects testing for chronic

toxicity, oncogenicity, and reproductive and development effects. This decision was partially based on

the potential for a substantial number of workers to be exposed to the chemical during its production and

use in polyurethane paint systems (EPA 1997). EPA later responded to the ITC designation by issuing a

proposed testing rule which also required testing for mutagenicity, neurotoxicity, pharmacokinetics, and

hydrolysis. As the proposed rule was not finalized and EPA has reviewed new scientific data addressing

chronic toxicity, subchronic toxicity, mutagenicity, and hydrolysis, the final scope of the testing

requirements proposed earlier have changed. On September 30, 1997, EPA issued a testing consent order

that incorporated an enforceable consent agreement with several companies to perform certain health

effects tests on HDI (EPA 1997).


HEXAMETHYLENE DIISOCYANATE 139

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inhalation exposure to diisocyanate. Pneumologie 49(6):373-377.

*Wu WS, Stoyanoff RE, Szklar RS, et al. 1990. Application of Tryptamine as a derivatising agent for

airborne Isocyanate determination. Part 3 evaluation of total isocyanates analysis by high-performance

liquid chromatography with fluorescence and amperometric detection. Analyst 115:801-807.
HEXAMETHYLENE DIISOCYANATE 155

9. GLOSSARY

Acute Exposure-Exposure to a chemical for a duration of 14 days or less, as specified in the

Toxicological Profiles.

Adsorption Coefficient (Koc)-The ratio of the amount of a chemical adsorbed per unit weight of

organic carbon in the soil or sediment to the concentration of the chemical in solution at equilibrium.

Adsorption Ratio (Kd)-The amount of a chemical adsorbed by a sediment or soil (i.e., the solid phase)

divided by the amount of chemical in the solution phase, which is in equilibrium with the solid phase, at

a fixed solid/solution ratio. It is generally expressed in micrograms of chemical sorbed per gram of soil

or sediment.

Bioconcentration Factor (BCF)-The quotient of the concentration of a chemical in aquatic organisms

at a specific time or during a discrete time period of exposure divided by the concentration in the

surrounding water at the same time or during the same period.

Cancer Effect Level (CEL)-The lowest dose of chemical in a study, or group of studies, that produces

significant increases in the incidence of cancer (or tumors) between the exposed population and its

appropriate control.

Carcinogen-A chemical capable of inducing cancer.

Ceiling Value--A concentration of a substance that should not be exceeded, even instantaneously.

Chronic Exposure-Exposure to a chemical for 365 days or more, as specified in the Toxicological

Profiles.

Developmental Toxicity-The occurrence of adverse effects on the developing organism that may result

from exposure to a chemical prior to conception (either parent), during prenatal development, or

postnatally to the time of sexual maturation. Adverse developmental effects may be detected at any point

in the life span of the organism.

Embryotoxicity and Fetotoxicity-Any toxic effect on the conceptus as a result of prenatal exposure to

a chemical; the distinguishing feature between the two terms is the stage of development during which

the insult occurred. The terms, as used here, include malformations and variations, altered growth, and in

utero death.

EPA Health Advisory-An estimate of acceptable drinking water levels for a chemical substance based

on health effects information. A health advisory is not a legally enforceable federal standard, but serves

as technical guidance to assist federal, state, and local officials.

Immediately Dangerous to Life or Health (IDLH)-The maximum environmental concentration of a

contaminant from which one could escape within 30 min without any escape-impairing symptoms or

irreversible health effects.

Intermediate Exposure-Exposure to a chemical for a duration of 15-364 days, as specified in the

Toxicological Profiles.
HEXAMETHYLENE DIISOCYANATE 156

9. GLOSSARY

Immunologic Toxicity-The occurrence of adverse effects on the immune system that may result from

exposure to environmental agents such as chemicals.

In Vitro-Isolated from the living organism and artificially maintained, as in a test tube.

In Vivo-Occurring within the living organism.

Lethal Concentration(LO) (LCLO)-The lowest concentration of a chemical in air which has been

reported to have caused death in humans or animals.

Lethal Concentration(50) (LC50)-Acalculated concentration of a chemical in air to which exposure for

a specific length of time is expected to cause death in 50% of a defined experimental animal population.

Lethal Dose(LO) (LDLO)-The lowest dose of a chemical introduced by a route other than inhalation that

is expected to have caused death in humans or animals.

Lethal Dose(50) (LD50)-The dose of a chemical which has been calculated to cause death in 50% of a

defined experimental animal population.

Lethal Time50 ( LT50)-A calculated period of time within which a specific concentration of a chemical

is expected to cause death in 50% of a defined experimental animal population.

Lowest-Observed-Adverse-Effect Level (LOAEL)-The lowest dose of chemical in a study, or group

of studies, that produces statistically or biologically significant increases in frequency or severity of

adverse effects between the exposed population and its appropriate control.

Malformations-Permanent structural changes that may adversely affect survival, development, or

function.

Minimal Risk Level-An estimate of daily human exposure to a dose of a chemical that is likely to be

without an appreciable risk of adverse noncancerous effects over a specified duration of exposure.

Mutagen-A substance that causes mutations. A mutation is a change in the genetic material in a body

cell. Mutations can lead to birth defects, miscarriages, or cancer.

Neurotoxicity-The occurrence of adverse effects on the nervous system following exposure to

chemical.

No-Observed-Adverse-Effect Level (NOAEL)-The dose of chemical at which there were no

statistically or biologically significant increases in frequency or severity of adverse effects seen between

the exposed population and its appropriate control. Effects may be produced at this dose, hut they are not

considered to be adverse.

Octanol-Water Partition Coefficient (Kow)-The equilibrium ratio of the concentrations of a chemical

in n-octanol and water, in dilute solution.

Permissible Exposure Limit (PEL)-An allowable exposure level in workplace air averaged over an Shour

shift.
HEXAMETHYLENE DIISOCYANATE 157

9. GLOSSARY

q1*-The upper-bound estimate of the low-dose slope of the dose-response curve as determined by the

multistage procedure. The q1* can be used to calculate an estimate of carcinogenic potency, the

incremental excess cancer risk per unit of exposure (usually μg/L for water, mg/kg/day for food, and

μg/m3

 for air).

Reference Dose (RfD)-An estimate (with uncertainty spanning perhaps an order of magnitude) of the

daily exposure of the human population to a potential hazard that is likely to be without risk of

deleterious effects during a lifetime. The RfD is operationally derived from the NOAEL (from animal

and human studies) by a consistent application of uncertainty factors that reflect various types of data

used to estimate RfDs and an additional modifying factor, which is based on a professional judgment of

the entire database on the chemical. The RfDs are not applicable to nonthreshold effects such as cancer.

Reportable Quantity (RQ)-The quantity of a hazardous substance that is considered reportable under

CERCLA. Reportable quantities are (1) 1 pound or greater or (2) for selected substances, an amount

established by regulation either under CERCLA or under Sect. 3 11 of the Clean Water Act. Quantities

are measured over a 24-hour period.

Reproductive Toxicity-The occurrence of adverse effects on the reproductive system that may result

from exposure to a chemical. The toxicity may be directed to the reproductive organs and/or the related

endocrine system. The manifestation of such toxicity may be noted as alterations in sexual behavior,

fertility, pregnancy outcomes, or modifications in other functions that are dependent on the integrity of

this system.

Short-Term Exposure Limit (STEL)-The maximum concentration to which workers can be exposed

for up to 15 min continually. No more than four excursions are allowed per day, and there must be at

least 60 min between exposure periods. The daily TLV-TWA may not be exceeded.

Target Organ Toxicity-This term covers a broad range of adverse effects on target organs or

physiological systems (e.g., renal, cardiovascular) extending from those arising through a single limited

exposure to those assumed over a lifetime of exposure to a chemical.

Teratogen-A chemical that causes structural defects that affect the development of an organism.

Threshold Limit Value (TLV)-A concentration of a substance to which most workers can be exposed

without adverse effect. The TLV may be expressed as a TWA, as a STEL, or as a CL.

Time-Weighted Average (TWA)--An allowable exposure concentration averaged over a normal S-hour

workday or 40-hour workweek.

Toxic Dose (TD50)-A calculated dose of a chemical, introduced by a route other than inhalation, which

is expected to cause a specific toxic effect in 50% of a defined experimental animal population.

Uncertainty Factor (UF)-A factor used in operationally deriving the RfD from experimental data.

UFs are intended to account for (1) the variation in sensitivity among the members of the human

population, (2) the uncertainty in extrapolating animal data to the case of human, (3) the uncertainty in

extrapolating from data obtained in a study that is of less than lifetime exposure, and (4) the uncertainty

in using LOAEL data rather than NOAEL data. Usually each of these factors is set equal to 10.
HEXAMETHYLENE DIISOCYANATE A-l

APPENDIX A

ATSDR MINIMAL RISK LEVEL

The Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) [42 U.S.C.

9601 et seq.], as amended by the Superfund Amendments and Reauthorization Act (SARA) [Pub. L.

99-4991, requires that the Agency for Toxic Substances and Disease Registry (ATSDR) develop jointly

with the U.S. Environmental Protection Agency (EPA), in order of priority, a list of hazardous substances

most commonly found at facilities on the CERCLA National Priorities List (NPL); prepare toxicological

profiles for each substance included on the priority list of hazardous substances; and assure the initiation

of a research program to fill identified data needs associated with the substances.

The toxicological profiles include an examination, summary, and interpretation of available toxicological

information and epidemiologic evaluations of a hazardous substance. During the development of

toxicological profiles, Minimal Risk Levels (MRLs) are derived when reliable and sufficient data exist to

identify the target organ(s) of effect or the most sensitive health effect(s) for a specific duration for a

given route of exposure. An MRL is an estimate of the daily human exposure to a hazardous substance

that is likely to be without appreciable risk of adverse noncancer health effects over a specified duration

of exposure. MRLs are based on noncancer health effects only and are not based on a consideration of

cancer effects. These substance-specific estimates, which are intended to serve as screening levels, are

used by ATSDR health assessors to identify contaminants and potential health effects that may be of

concern at hazardous waste sites. It is important to note that MRLs are not intended to define clean-up or

action levels.

MRLs are derived for hazardous substances using the no-observed-adverse-effect level/uncertainty factor

approach. They are below levels that might cause adverse health effects in the people most sensitive to

such chemical-induced effects. MRLs are derived-for acute (1-14 days), intermediate (15-364 days),

and chronic (365 days and longer) durations and for the oral and inhalation routes of exposure.

Currently, MRLs for the dermal route of exposure are not derived because ATSDR has not yet identified

a method suitable for this route of exposure. MRLs are generally based on the most sensitive chemicalinduced

end point considered to be of relevance to humans. Serious health effects (such as irreparable

damage to the liver or kidneys, or birth defects) are not used as a basis for establishing MRLs. Exposure

to a level above the MRL does not mean that adverse health effects will occur.
HEXAMETHYLENE DIISOCYANATE A-2

APPENDIX A

MRLs are intended only to serve as a screening tool to help public health professionals decide where to

look more closely. They may also be viewed as a mechanism to identify those hazardous waste sites that

are not expected to cause adverse health effects. Most MRLs contain a degree of uncertainty because of

the lack of precise toxicological information on the people who might be most sensitive (e.g., infants,

elderly, nutritionally or immunologically compromised) to the effects of hazardous substances. ATSDR

uses a conservative (i.e., protective) approach to address this uncertainty consistent with the public health

principle of prevention. Although human data are preferred, MRLs often must be based on animal

studies because relevant human studies are lacking. In the absence of evidence to the contrary, ATSDR

assumes that humans are more sensitive to the effects of hazardous substance than animals and that

certain persons may be particularly sensitive. Thus, the resulting MRL may be as much as a hundredfold

below levels that have been shown to be nontoxic in laboratory animals.

Proposed MRLs undergo a rigorous review process: Health Effects/MR.L Workgroup reviews within the

Division of Toxicology, expert panel peer reviews, and agencywide MRL Workgroup reviews, with

participation from other federal agencies and comments from the public. They are subject to change as

new information becomes available concomitant with updating the toxicological profiles. Thus, MRLs in

the most recent toxicological profiles supersede previously published levels. For additional information

regarding MRLs, please contact the Division of Toxicology, Agency for Toxic Substances and Disease

Registry, 1600 Clifton Road, Mailstop E-29, Atlanta, Georgia 30333.






HEXAMETHYLENE DIISOCYANATE A-9

APPENDIX A

DeWilde and Hathaway (1994), again using chronically exposed workers at the plant in Freeport, Texas,

found no statistically significant differences in pulmonary function data among HDI-exposed individuals and

the control group. The dose in that study was estimated to be between 0.5 and 7 ppb (0.0005-0.007 ppm).

Both the Shepperly and Hathaway (1991) and the DeWilde and Hathaway (1994) studies provided estimates

of doses to which the workers were exposed, but could not provide definitive exposure doses to the worker

populations. Both studies also had a number of difficulties (discussed above), which precluded their use in

deriving a chronic-duration MRL based on human exposures, but they do lend some support to the MRL

based on results found using the rat model.

It should be noted that the EPA Reference Concentration (RfC) for HDI was based on the same study as this

chronic-duration inhalation MRL (Mobay 1989) and was also calculated to be 1.0x10-5 ppm. A report by

Foureman et al. (1994) described how this RfC was derived using the 0.005 ppm as the NOAEL dose end

point; for purposes of chronic-duration inhalation MRL derivation, the MRL was based on the same dose end

point, but was classified as a minimal LOAEL. Fouremen et al. (1994) argue that although an effect was

seen at the 0.005 ppm dose (nasal epithelial hyperplasia), this response should be classified as an adaptive

response (as noted with many types of other irritants) and not a true toxic response, and therefore should be

classified as a NOAEL. Fouremen et al. (1994) conclude that the olfactory degenerative response should be

considered the significant effect in these rats, and not the hyperplastic response, supported by the fact that the

degeneration of the olfactory epithelium did follow a concentration-response relationship for both incidence

and severity. In contrast, the hyperplastic and inflammatory responses followed the traditional dose-response

for incidence, but not for severity of the lesions. The ATSDR Minimal Risk Level Workgroup carefully

reviewed this data and the arguments presented by the Fouremen et al. (1994) report and concluded that the

degeneration of the olfactory epithelium was an adverse (toxic) response and warranted a classification as a

minimal LOAEL. After uncertainty factors were applied, the RfC and the MRL concentration values resulted

in the same value, 1.0x10-5 ppm, despite the differences in end point classification. This study involving the

exposure of rats to HDI demonstrates that the line between an adaptive and toxic response in not always

clearly defined, and it may be a matter of opinion as to whether the effects are true adverse toxic responses.

Agency Contact (Chemical Managed): Henry Abadin
HEXAMETHYLENE DIISOCYANATE B-l

APPENDIX B

USER’S GUIDE

Chapter 1

Public Health Statement

This chapter of the profile is a health effects summary written in non-technical language. Its intended

audience is the general public especially people living in the vicinity of a hazardous waste site or

chemical release. If the Public Health Statement were removed from the rest of the document, it would

still communicate to the lay public essential information about the chemical.

The major headings in the Public Health Statement are useful to find specific topics of concern. The

topics are written in a question and answer format. The answer to each question includes a sentence that

will direct the reader to chapters in the profile that will provide more information on the given topic.

Chapter 2

Tables and Figures for Levels of Significant Exposure (LSE)

Tables (2- 1,2-2, and 2-3) and figures (2- 1 and 2-2) are used to summarize health effects and illustrate

graphically levels of exposure associated with those effects. These levels cover health effects observed

at increasing dose concentrations and durations, differences in response by species, minimal risk levels

(MRLs) to humans for noncancer end points, and EPA’s estimated range associated with an upper- bound

individual lifetime cancer risk of 1 in 10,000 to 1 in 10,000,000. Use the LSE tables and figures for a

quick review of the health effects and to locate data for a specific exposure scenario. The LSE tables and

figures should always be used in conjunction with the text. All entries in these tables and figures

represent studies that provide reliable, quantitative estimates of No-Observed-Adverse- Effect Levels

(NOAELs), Lowest-Observed-Adverse-Effect Levels (LOAELs), or Cancer Effect Levels (CELs).

The legends presented below demonstrate the application of these tables and figures. Representative

examples of LSE Table 2-l and Figure 2-l are shown. The numbers in the left column of the legends

correspond to the numbers in the example table and figure.

LEGEND

See LSE Table 2-l

(1) Route of Exposure One of the first considerations when reviewing the toxicity of a substance using

these tables and figures should be the relevant and appropriate route of exposure. When sufficient

data exists, three LSE tables and two LSE figures are presented in the document. The three LSE

tables present data on the three principal routes of exposure, i.e., inhalation, oral, and dermal (LSE

Table 2- 1,2-2, and 2-3, respectively). LSE figures are limited to the inhalation (LSE Figure 2- 1)

and oral (LSE Figure 2-2) routes. Not all substances will have data on each route of exposure and

will not therefore have all five of the tables and figures.
HEXAMETHYLENE DIISOCYANATE B-2

APPENDIX B

(2) Exposure Period Three exposure periods - acute (less than 15 days), intermediate (15-364 days),

and chronic (365 days or more) are presented within each relevant route of exposure. In this

example, an inhalation study of intermediate exposure duration is reported. For quick reference to

health effects occurring from a known length of exposure, locate the applicable exposure period

within the LSE table and figure.

(3) Health Effect The major categories of health effects included in LSE tables and figures are death,

systemic, immunological, neurological, developmental, reproductive, and cancer. NOAELs and

LOAELs can be reported in the tables and figures for all effects but cancer. Systemic effects are

further defined in the “System” column of the LSE table (see key number 18).

(4) Key to Figure Each key number in the LSE table links study information to one or more data

points using the same key number in the corresponding LSE figure. In this example, the study

represented by key number 18 has been used to derive a NOAEL and a Less Serious LOAEL (also

see the 2 “1 Sr” data points in Figure 2- 1).

(5) Species The test species, whether animal or human, are identified in this column. Section 2.5,

“Relevance to Public Health,” covers the relevance of animal data to human toxicity and Section

2.3, “Toxicokinetics,” contains any available information on comparative toxicokinetics. Although

NOAELs and LOAELs are species specific, the levels are extrapolated to equivalent human doses

to derive an MRL.

(6) Exposure Frequency/Duration The duration of the study and the weekly and daily exposure

regimen are provided in this column. This permits comparison of NOAELs and LOAELs from

different studies. In this case (key number 1 S), rats were exposed to 1,1,2,2-tetrachloroethane via

inhalation for 6 hours per day, 5 days per week, for 3 weeks. For a more complete review of the

dosing regimen refer to the appropriate sections of the text or the original reference paper, i.e.,

Nitschke et al. 198 1.

(7) System This column further defines the systemic effects. These systems include: respiratory,

cardiovascular, gastrointestinal, hematological, musculoskeletal, hepatic, renal, and dermal/ocular.

“Other” refers to any systemic effect (e.g., a decrease in body weight) not covered in these systems.

In the example of key number 18, 1 systemic effect (respiratory) was investigated.

(8) NOAEL A No-Observed-Adverse-Effect Level (NOAEL) is the highest exposure level at which no

harmful effects were seen in the organ system studied. Key number 18 reports a NOAEL of 3 ppm

for the respiratory system which was used to derive an intermediate exposure, inhalation MRL of

0.005 ppm (see footnote “b”).

(9) LOAEL A Lowest-Observed-Adverse-Effect Level (LOAEL) is the lowest dose used in the study

that caused a harmful health effect. LOAELs have been classified into “Less Serious” and

“Serious” effects. These distinctions help readers identify the levels of exposure at which adverse

health effects first appear and the gradation of effects with increasing dose. A brief description of

the specific end point used to quantify the adverse effect accompanies the LOAEL. The respiratory

effect reported in key number 18 (hyperplasia) is a Less serious LOAEL of 10 ppm. MRLs are not

derived from Serious LOAELs.

(10) Reference The complete reference citation is given in chapter 8 of the profile.
HEXAMETHYLENE DIISOCYANATE B-3

APPENDIX B

(11) CEL A Cancer Effect Level (CEL) is the lowest exposure level associated with the onset of

carcinogenesis in experimental or epidemiologic studies. CELs are always considered serious

effects. The LSE tables and figures do not contain NOAELs for cancer, but the text may report

doses not causing measurable cancer increases.

(12) Footnotes Explanations of abbreviations or reference notes for data in the LSE tables are found in

the footnotes. Footnote “b” indicates the NOAEL of 3 ppm in key number 18 was used to derive an

MRL of 0.005 ppm.

LEGEND

See Figure 2-l

LSE figures graphically illustrate the data presented in the corresponding LSE tables. Figures help the

reader quickly compare health effects according to exposure concentrations for particular exposure

periods.

(13) Exposure Period The same exposure periods appear as in the LSE table. In this example, health

effects observed within the intermediate and chronic exposure periods are illustrated.

(14) Health Effect These are the categories of health effects for which reliable quantitative data exists.

The same health effects appear in the LSE table.

(15) Levels of Exposure concentrations or doses for each health effect in the LSE tables are graphically

displayed in the LSE figures. Exposure concentration or dose is measured on the log scale “y” axis.

Inhalation exposure is reported in mg/m3

 or ppm and oral exposure is reported in mg/kg/day.

(16) NOAEL In this example, 18r NOAEL is the critical end point for which an intermediate inhalation

exposure MRL is based. As you can see from the LSE figure key, the open-circle symbol indicates

to a NOAEL for the test species-rat. The key number 18 corresponds to the entry in the LSE table.

The dashed descending arrow indicates the extrapolation from the exposure level of 3 ppm (see

entry 18 in the Table) to the MRL of 0.005 ppm (see footnote “b” in the LSE table).

(17) CEL Key number 38r is 1 of 3 studies for which Cancer Effect Levels were derived. The diamond

symbol refers to a Cancer Effect Level for the test species-mouse. The number 38 corresponds to

the entry in the LSE table.

(18) Estimated Upper-Bound Human Cancer Risk Levels This is the range associated with the

upper-bound for lifetime cancer risk of 1 in 10,000 to 1 in 10,000,000. These risk levels are

derived from the EPA’s Human Health Assessment Group’s upper-bound estimates of the slope of

the cancer dose response curve at low dose levels (qi*).

(19) Key to LSE Figure The Key explains the abbreviations and symbols used in the figure.
HEXAMETHYLENE DIISOCYANATE B-4

APPENDIX B

Chapter 2 (Section 2.5)

Relevance to Public Health

The Relevance to Public Health section provides a health effects summary based on evaluations of

existing toxicologic, epidemiologic, and toxicokinetic information. This summary is designed to present

interpretive, weight-of-evidence discussions for human health end points by addressing the following

questions.

1. What effects are known to occur in humans?

2 . What effects observed in animals are likely to be of concern to humans?

3 . What exposure conditions are likely to be of concern to humans, especially around hazardous

waste sites?

The section covers end points in the same order they appear within the Discussion of Health Effects by

Route of Exposure section, by route (inhalation, oral, dermal) and within route by effect. Human data are

presented first, then animal data. Both are organized by duration (acute, intermediate, chronic). In vitro

data and data from parenteral routes (intramuscular, intravenous, subcutaneous, etc.) are also considered

in this section. If data are located in the scientific literature, a table of genotoxicity information is

included.

The carcinogenic potential of the profiled substance is qualitatively evaluated, when appropriate, using

existing toxicokinetic, genotoxic, and carcinogenic data. ATSDR does not currently assess cancer

potency or perform cancer risk assessments. Minimal risk levels (MRLs) for noncancer end points (if

derived) and the end points from which they were derived are indicated and discussed.

Limitations to existing scientific literature that prevent a satisfactory evaluation of the relevance to public

health are identified in the Data Needs section.

Interpretation of Minimal Risk Levels

Where sufficient toxicologic information is available, we have derived minimal risk levels (MRLs) for

inhalation and oral routes of entry at each duration of exposure (acute, intermediate, and chronic). These

MRLs are not meant to support regulatory action; but to acquaint health professionals with exposure

levels at which adverse health effects are not expected to occur in humans. They should help physicians

and public health officials determine the safety of a community living near a chemical emission, given

the concentration of a contaminant in air or the estimated daily dose in water. MRLs are based largely on

toxicological studies in animals and on reports of human occupational exposure.

MRL users should be familiar with the toxicologic information on which the number is based. Chapter

2.5, “Relevance to Public Health,” contains basic information known about the substance. Other sections

such as 2.7, “Interactions with Other Substances,” and 2.8, “Populations that are Unusually Susceptible”

provide important supplemental information.

MRL users should also understand the MRL derivation methodology. MRLs are derived using a

modified version of the risk assessment methodology the Environmental Protection Agency (EPA)

provides (Barnes and Dourson 1988) to determine reference doses for lifetime exposure (RfDs).


HEXAMETHYLENE DIISOCYANATE B-7

APPENDIX B

To derive an MRL, ATSDR generally selects the most sensitive end point which, in its best judgement,

represents the most sensitive human health effect for a given exposure route and duration. ATSDR

cannot make this judgement or derive an MRL unless information (quantitative or qualitative) is

available for all potential systemic, neurological, and developmental effects. If this information and

reliable quantitative data on the chosen end point are available, ATSDR derives an MRL using the most

sensitive species (when information from multiple species is available) with the highest NOAEL that

does not exceed any adverse effect levels. When a NOAEL is not available, a lowest-observed-adverseeffect

level (LOAEL) can be used to derive an MRL, and an uncertainty factor (UF) of 10 must be

employed. Additional uncertainty factors of 10 must be used both for human variability to protect

sensitive subpopulations (people who are most susceptible to the health effects caused by the substance)

and for interspecies variability (extrapolation from animals to humans). In deriving an MRL, these

individual uncertainty factors are multiplied together. The product is then divided into the inhalation

concentration or oral dosage selected from the study. Uncertainty factors used in developing a

substance-specific MRL are provided in the footnotes of the LSE Tables.