The Current MDI Industrial Hygiene Data on Spray Foam

MARK SPENCE

Spence Consulting, LLC

 1232 Holyrood St.

 Midland, MI 48640

ABSTRACT

Spray application of polyurethane foam for thermal insulation of buildings represents the polyurethane application with the 

highest potential worker exposure to MDI. This paper reviews the existing work done to characterize the MDI exposures of 

the sprayed polyurethane foam workers as well as identifying several elements of an effective industrial hygiene program 

suggested by the existing work. In addition, areas for additional possible work are suggested. 

The existing work shows clearly that MDI exposures for both the sprayed foam applicator and helper can exceed both task 

and shift occupational exposure limits. Additional feasible engineering controls are not obvious, so control schemes based 

on administrative controls (work practices, establishment of “work zones” and re-entry clearance times) and personal 

protective equipment (respirators, gloves, and protective clothing) are necessary. Suggestions for more work in the areas of 

establishing re-entry clearance times and work zones as well as characterizing amine catalyst exposures are made.

INTRODUCTION

The use of sprayed polyurethane foam based on 4,4’-methylene diphenyl diisocyanate (methyl di-p-phenylene isocyanate, 

or MDI) for thermal insulation in residential and commercial buildings has been growing steadily over the last two decades. 

With the current emphasis on energy conservation, this trend in sprayed polyurethane thermal insulation use is likely to 

continue. Sprayed polyurethane foam offers advantages of high thermal insulation value, good sealing and water-resistance 

characteristics, and avoidance of indoor air quality problems introduced by some other insulation choices. From an 

industrial hygiene perspective, sprayed polyurethane foam application is the operation presenting the highest airborne MDI 

exposure potential for workers of all MDI uses [1], and is second only to vehicle spray painting in presenting the highest 

airborne exposure potential for diisocyanates of all types [2]. For these reasons, an effective industrial hygiene program 

based on a good understanding of the details of the MDI exposure patterns is important for protecting workers involved in 

applying sprayed polyurethane foam. This paper is intended to review the MDI worker exposure data that exists for sprayed 

polyurethane foam, summarize practical conclusions that can be drawn from it, and suggest areas where further investigation

could be considered. 

BACKGROUND

MDI in Sprayed Foam Formulations

Sprayed polyurethane foam systems consist of two liquid parts which when mixed, react to form a polyurethane polymer: a 

diisocyanate (referred to commonly as the “A side”), and a polyhydroxy alcohol (polyol; referred to commonly as the “B 

side”). The B side also contains a blowing agent for foam cell formation (typically a low-boiling hydro-flourocarbon) and 

small amounts of amine catalysts and other additives. During application, the A and B sides are heated and fed separately to 

the spray gun, mixing in the spray gun just prior to application. The resulting spray consists of droplets of the warm, rapidlyreacting mixture, as well as vapor from the more volatile components of the system (the blowing agent, and, to some extent, 

the diisocyanate and the amine catalyst). 

The properties of the foam produced are governed primarily by the composition of the B side of the formulation; systems 

differ in the polyol, catalysts and blowing agents used as well as the relative amounts of each. The A side used in sprayed 

polyurethane foam insulation is almost always “polymeric MDI” (PMDI) -- an undistilled mixture of MDI and higher 

molecular weight MDI oligomers. Like MDI, MDI oligomers have active isocyanate functional groups (-N=C=O) that react 

to form polyurethane polymers. In spray foam formulations, the PMDI used consists of 45-55% of each component (MDI 

and MDI oligomers) [3].
MDI is a skin, eye and respiratory tract irritant [4]. Exposure to excessive airborne concentrations of MDI can lead to 

respiratory sensitization, which may result in occupational asthma. MDI may cause skin sensitization in rare cases [4].

There is limited data in animals suggesting that dermal exposure to diisocyanates may also contribute to respiratory 

sensitization, but this has not been confirmed in humans [5].

Application of Sprayed Foam Insulation

Sprayed polyurethane foam insulation may be applied to residential and commercial buildings during initial construction 

or as a retrofit insulation improvement. It may be applied to the building exterior (roof tops and walls before sheathing is

added), or to interior building spaces (walls, attics, crawl spaces). Because of the need for customized application, sprayed 

polyurethane foam insulation is usually applied by hand rather than by automated equipment. Application crews typically 

consist of an applicator, who operates the spray gun, and one or more helpers who work in close proximity to the applicator 

conducting support activities (guiding hoses, holding windscreens, etc.). Individual workers may switch between the 

applicator and helper role throughout the workday [3].

EXPOSURE MONITORING CONSIDERATIONS

The complex nature of the exposure environment in sprayed polyurethane foam application (i.e., the presence of both 

aerosol and vapor forms of air contaminants and the multiple isocyanate-containing species in PMDI) presents several 

complicating factors in conducting and interpreting MDI exposure monitoring. 

Sampler Type

Since isocyanates are reactive species, exposure monitoring methods used for them employ a derivatizing agent, usually a 

primary amine which serves to stabilize the reactive isocyanate group by forming a urea derivative with them. In addition, 

the derivatizing agents often contain molecular features which enhance the analytical sensitivity of the resulting derivative

[6]. The MDI exposure monitoring done during sprayed foam application reviewed for this paper employed a variety of 

derivatizing agents, but only two methods of sampling were used: (1) air was drawn through a glass fiber filter which was 

coated with the derivatizing agent or (2) air was drawn through a glass impinger or bubbler containing an organic solvent 

solution of the derivatizing agent. In one case [10], a combination of the two techniques was used: the sample train consisted

of an impinger followed by an inline coated glass fiber filter.

The coated filter sampling technique has the clear practical advantage of ease of use in the field since the apparatus is less 

bulky and prone to breakage, and liquid organic solvents, which may evaporate or spill while an impinger is worn by a

worker are not used. However, when sampling reactive aerosols such as those present during sprayed foam application, the 

impinger sampler offers a more accurate assessment of the MDI level present since the reacting droplets that enter the 

impinger are dissolved in solution with the derivatizing agent, facilitating rapid and complete reaction. In the case of coated 

filters, the aerosol droplet which is trapped on the filter must spread out and wet the filter in order to come into complete

contact with the derivatizing agent. Lesage [7] compared the results of MDI monitoring in sprayed foam environments using 

both coated filter and impinger-based sampling methodologies, finding that filter sampling results were 6-40% lower than 

side-by-side samples obtained using impingers. In light of this finding, a practical solution to the dilemma of sampler type 

versus ease of use for the field industrial hygienist might be to use filter sampling and multiply results by an appropriate 

compensating factor (e.g., two).

Exposure Guidelines

Exposure monitoring methods are generally designed to yield results that can be compared to established occupational 

exposure guidelines for interpretation. Such exposure guidelines are set by governmental or professional bodies based on a 

review of the toxicological, medical and human experience data. One of the most widely-recognized set of occupational 

exposure guidelines are the Threshold Limit Values (TLVs®), set by the American Conference of Governmental Hygienists 

(ACGIH®). The ACGIH TLV for MDI is 0.005 ppm (5 ppb or 51 g/m3) as an 8-hour time weighted average. The 

Permissible Exposure Limit (PEL) set by the United States Occupational Safety and Health Administration (OSHA) for MDI 

is 0.020 ppm (20 ppb or 200 g/m3) as a ceiling (never to be exceeded) value. 

These exposure limits are representative of the majority of those followed globally. A notable exception is the United 

Kingdom Health and Safety Executive, which has set an occupational exposure limit based on the determination of “total 

reactive isocyanate group” (TRIG). The HSE TRIG exposure guideline is 70 g/m3as a ceiling (never to be exceeded) 

value, and 20 g/m3as an 8-hour time weighted average [2]. While the TRIG method has the advantage of accounting for 

exposure to the isocyanate groups present in the MDI oligomers in addition to those in MDI alone, commentators in the U.S. 
and elsewhere have noted that neither it nor any other current analytical method is capable of accurately quantitating all 

reactive isocyanate groups [7].

Chemical Species Monitored

Exposure monitoring methods designed to produce data for comparison with the TRIG exposure guideline quantitate both 

the levels of MDI and the MDI oligomers. As such, they require a PMDI calibration standard, which is often difficult to 

obtain. The mass of MDI found in the sample is added to the mass of MDI oligomers found and the total is converted to a 

TRIG number based on the ratio of the monomer molecular mass to the isocyanate group mass; for MDI, this ratio is 

(250/89=) 0.34 [5].

For exposure monitoring methods whose data is designed to be compared with MDI-based values, only MDI monomer 

need be determined. In actual practice, a number of investigators determine MDI oligomers as well although comparison

with occupational exposure limits is not clear [3,7,10]. 

The results from studies where both MDI and MDI oligomer data was obtained in sprayed polyurethane foam 

environments was examined for trends. A total of 53 personal breathing zone results where both MDI and oligomers were 

detected from four studies [3,5,7,10] were compared. The results of the comparison showed that the mean ratio of total 

PMDI found to MDI was 1.82 (SD = 0.67). Since the composition of the PMDI is 45-55% MDI, this ratio in the bulk PMDI 

would be 1.8 – 2.2. This result indicates that the most typical situation found is that the spray sample retained a 

composition similar to the bulk PMDI material being sprayed. However, there was variation in this relationship: in ten of the 

samples (20%), the MDI oligomer concentration was higher than that of MDI (i.e., PMDI / MDI ratio greater than 2.0) while 

in 28 cases, the PMDI / MDI ratio was less than 1.7 (i.e., MDI made up more than 60% of the total PMDI found). 

Since analysis of samples for MDI oligomers is considerably more difficult (and therefore more expensive) than analyzing 

for MDI alone, a practical application of this observation for the practicing field industrial hygienist taking samples in 

sprayed polyurethane foam environments might be to determine MDI and multiply the results by an appropriate adjustment 

factor (e.g., two). 

SPRAYED FOAM WORKER EXPOSURES

Worker exposure data during sprayed polyurethane foam application from several studies conducted over the last 30 years

[2,3,5,7, 10-15] were examined. The data represented over 240 task and shift personal breathing zone samples taken on 

applicators and helpers during foam application. 

The MDI exposure concentrations of spray foam applicators in the studies ranged from 5-2100 g/m3. Where 8-hour time 

weighted averages were calculated, they ranged from 10-460 g/m3. (The 8-hour time-weighted average takes into account 

the portion of the 8-hour workday for which the measured concentration applies. For instance, if an applicator’s exposure 

concentration during spraying was 100 g/m3and they sprayed four hours out of a workday and had no exposure for the rest 

of the day, their 8-hour time-weighted average exposure would be (100*4/8=) 50 g/m3.) The MDI exposure concentrations 

of workers in the spray foam helper role in the studies ranged from 0.1-408 g/m3. Where 8-hour time weighted averages 

were calculated, they ranged from 1-308 g/m3. 

Both the spray applicator and helpers clearly have the potential for exposures over both the 8-hour TLV and the OSHA 

PEL (51 g/m3and 200 g/m3ceiling, respectively). The exposures for the spray applicator were consistently 2-4 times 

higher than those of the helper, but a high degree of variability was noted for both job roles. The high variability in MDI 

exposures for both the spray applicator and helper was affected by several factors. Indoor spraying resulted in higher 

exposures [11] while outdoor spraying resulting in more variability in exposure since keeping upwind of the point of 

application became more important [12]. Natural ventilation, atmospheric and seasonal conditions, building characteristics, 

surface area and thickness of foam sprayed, applicator’s experience level, were also noted as factors affecting exposure 

levels [3, 11].

Two studies [7, 10] also measured spray foam workers’ exposures to the foam blowing agent and one [10] measured 

exposures to the amine catalyst. In both studies, the exposure of both the applicator and helper to the blowing agent were 

below the relevant exposure guideline. In the one case where amine catalyst exposure levels were measured, concentrations 

ranging from less than detection limit to over 900 ppb were found. No exposure guideline for the amine catalyst was given, 

so evaluating the significance of these results is difficult.

EXPOSURE CONTROL

Given the clear potential for over-exposure to MDI during spray application of polyurethane foam, it is clear that exposure 

controls are important for workers involved. Ideally, engineering controls are the first tool to be used in exposure reduction 

efforts. Exploring the use of air curtains, reactive mist barriers, and remote control spraying have been suggested [14], but 
their application seems difficult given the variability of the construction environment. Since additional engineering controls 

appear to be impractical, consistent, disciplined use of administrative controls and personal protective equipment must be 

relied upon.

Administrative Controls

WORK PRACTICES

One of the fundamental actions that workers can take to reduce exposures to MDI during spraying is positioning 

themselves upwind of the spray stream as much as possible. This concept should be included in spray applicator and helper 

training, and rigorously used in the preparation for each spraying session. While attention to upwind positioning has more 

importance for outside applications because of the variations in wind speed and direction that must be noted and adjusted to, 

even for indoor applications, there is usually a natural air flow direction that should be noted and taken into consideration. 

THE “WORK ZONE” CONCEPT

Several authors [3, 7, 10, 12, 13, 15] report data on the reduction of MDI concentration with distance from the point of 

spray application. Some refer to the concept of a “work zone” – a distance around the spray gun where MDI concentrations 

are highest and outside of which significant exposure is not likely. For indoor application, work zone recommendations in 

the literature range from 6 m (20 ft) [10] to 8 m (25 ft) [12, 15]. For outdoor application, work zone recommendations range 

from 2m (7 ft) [13] to 3 m (10 ft) [12, 15]. One publication [16] suggests that, since outside spray application is highly 

influenced by wind speed, an “exclusion zone” be established for 6m (20 ft) on either side of the applicator in still air 

conditions, or 3 m (10ft) upwind and 8m (25 ft) downwind of the applicator.

Setting a work zone outside of which overexposure to MDI or other components of spray foam system is unlikely to occur 

is useful as a simple, practical workplace exposure management technique. It establishes a discipline for allowing other 

construction trades to complete their work safely in areas of the structure reasonably remote from the area to which foam is 

being applied. Additional data measuring the concentrations of MDI and other spray foam components in areas of the 

structure other than those being sprayed support this concept. While migration of MDI and other spray foam components to 

floors other than the one on which foam was being applied was noted, levels found were consistently well below exposure 

guidelines [3, 10].

CLEARANCE TIME

Another aspect of exposure management is consideration of when to allow re-entry of unprotected workers to the sprayed 

area. Several of the papers reviewed reported pertinent data.

Lesage et al. [7] report MDI levels in the room where sprayed foam was applied as high as 19 g/m3in the first 15 minutes 

after spraying ceased. However, by the time 60 minutes had passed, all samples taken were below the analytical detection 

limit (0.036 g). 

Karlovich [10] studied the variation with time of concentrations of MDI, blowing agent, and amine catalyst following 

spray application in the interior of five residential structures. Of 64 samples taken to measure MDI concentrations up to 2 

hours post-application, 62 were below the detection limits while two detected MDI at a level of 5 g/m3. All 64 samples 

were below detection limits for MDI oligomers. Area levels of blowing agent were determined for one of the structures over 

the period of 40-140 minutes post-application. These levels were well below applicable exposure guidelines and were 

reduced by a factor of approximately six in the period from 40 to 140 minutes after spraying. Similarly, amine catalyst postspraying levels were followed in one of the structures. The amine catalyst levels were detectable throughout the period 

measured and were reduced by a factor of approximately two in the period from 40 to 140 minutes after spraying. Since 

exposure guidelines for the amine catalysts were not given, interpretation of the exposure significance of the levels found is 

difficult.

Other authors [3,5] report MDI and MDI oligomer measurements 2 hours after spraying. In all cases, MDI results were 

below detection limits or detected at levels of 0.03 g/m3or lower. It is interesting to note that in one case [3] where MDI 

levels were measured 30 minutes, 60 minutes and 120 minutes post-spraying, on both the level sprayed and another level of 

the structure to which spray foam had not been applied, MDI levels were observed to peak at 60 minutes post-spraying on 

the level which was not sprayed while they decayed steadily with time on the level sprayed. This is probably an indication of 

the time it took suspended aerosol spray droplets to disperse through the structure as a result of diffusion and natural air 

currents. Achuan et al. [3] report collecting two samples 2 hours after application of sprayed foam on a rooftop in which 

detectable levels of MDI oligomers were found but MDI was below detection limits. The authors conclude that this was a 

reflection of the higher reactivity of the isocyanate groups on MDI compared to those on MDI oligomers.
The MDI and blowing agent clearance results point out the importance of ventilating the structure after spraying, and 

restricting entry for an appropriate period after spraying ceases. It is unclear what clearance time is sufficient to reduce 

exposure to the amine catalysts to safe levels as exposure evaluation criteria for the amine catalysts were not given. Since 

there are many factors that could affect this clearance time at a specific job, more work to develop a simple procedure for 

establishing reentry clearance would be useful.

Personal Protective Equipment

In situations were feasible engineering controls and administrative controls do not adequately lower the potential for 

overexposure, personal protective equipment is appropriate as a last line of defense. Spray polyurethane foam workers 

require protection from MDI exposure through both the inhalation and dermal route.

RESPIRATORY PROTECTION

The studies reviewed for this paper are unanimous in their opinion that respirators need to be used by spray applicators 

and helpers, although the type of respirators observed in use and the observed effectiveness of the employer respiratory 

protection programs and worker attention to appropriate respirator use procedures varies widely. 

Supplied-air respirators (SARs) offer the highest levels of protection, supplying clean compressed breathing air from tanks 

worn by the worker (self-contained breathing apparatus, or SCBA) or through a hose (airline respirators). While offering the 

highest levels of protection, this type of respirator has the disadvantages of being relatively expensive and requiring ongoing 

attention to the maintenance of the equipment and breathing air source.

Air purifying respirators (APRs) offer protection by passing workplace air through cartridges which contain filters and / or 

adsorbents that remove contaminants from the air. APRs are generally less expensive and simpler to maintain, but since they 

are “negative-pressure respirators”, relying on the wearer’s breathing to draw air through the cartridges, APRs are heavily 

dependent on the wearer’s ability to put the face piece on and tighten it correctly to prevent leakage around the face-to-face 

piece seal. APRs typically use elastomeric half- or full-face pieces; the full-face pieces include an integrated polycarbonate 

lens providing face and eye protection. 

One type of APR minimizes the problem of face seal leakage by using a battery-powered air mover to draw air through the 

cartridges; these are referred to as powered APRs (PAPRs). PAPRs are available in hood or helmet styles that have an 

integrated face shield. 

If APRs are selected for use by polyurethane foam spraying workers, APR cartridges that are NIOSH approved for both 

organic vapor protection and particulate (preferably P100 rated) are appropriate. Half-face APR face pieces are not 

preferred, both because the full-face piece provides a higher level of protection and because eye exposure to many of the 

amine catalysts used can produce a “halo vision” effect [10].

Whatever types of respirators are selected for use by sprayed foam workers, their use in the workplace should be governed 

by a respiratory protection program established by the employer (in the United States, this is an OSHA requirement). A 

respiratory protection program addresses such topics as medical approval, fit testing and training of respirator users; 

approved respirators and their maintenance; and responsibilities for program oversight. Assistance in establishing such a 

respiratory protection program that is designed for employers using MDI and PMDI is available [17]. 

PROTECTIVE CLOTHING

Spray foam workers may experience dermal exposure both by direct contact with the fresh foam surface as well as from 

spray drift or overspray; both gloves and body protective clothing are required. Lesage et al [7] monitored the foam surface 

with isocyanate-indicating colorimetric wipes at various times after application. Their results showed the presence of 

reactive isocyanate groups on the foam surface immediately after spraying, but in all cases (20 samples) no reactive 

isocyanate was detectable on the foam surface 15 minutes after application. However, some tasks (e.g., trimming excess 

foam) may result in dermal contact with the interior of the foam before final cure has occurred, so for these tasks wearing 

chemical resistant gloves will help avoid dermal contact with uncured foam. 

Information is available on the resistance of many glove and protective clothing materials to MDI and PMDI [18]. Nearly 

any gloves designed and constructed to be liquid proof will keep the MDI from sprayed foam material from penetrating to 

the skin. The most highly permeation-resistant materials are needed only when workers are using solvents (as when cleaning 

spray guns and equipment); in these cases, the gloves should be selected based on permeation resistance to the solvent being 

used. Likewise, protective clothing used should be designed to keep the sprayed foam material off street clothes and shoes 

and be readily disposable. Many of the limited-use coverall and boot cover materials available today (e.g., Tyvek TM and 

Kleenguard TM) do an excellent job of this.

Disposable or limited-use clothing and gloves offer the added advantage of reducing dermal contamination from contact 

with outside surfaces during storage or re-use. 
CONCLUSIONS AND SUGGESTIONS

The literature reviewed in this paper suggest that many aspects of worker exposure and its control during sprayed 

polyurethane foam application have been well studied and the elements of an effective industrial hygiene program for this 

important energy conservation product can be reliably described.

Exposure Monitoring

An impinger-based sampling method with an analysis that quantitates MDI oligomers as well as MDI will produce the 

most accurate characterization of PMDI exposures in sprayed polyurethane foam environments. If a filter-based sampling 

technique is used, an appropriate multiplier (e.g., four) can help correct for filter under-sampling of reactive spray aerosols 

and MDI oligomer concentrations. 

Exposure Control

Both the spray foam applicators and helpers can be exposed to MDI concentrations in excess of the relevant task and shift 

occupational exposure guidelines. Administrative controls such as keeping upwind of the spray drift, establishing a work 

zone around the point of spray application inside of which respiratory and dermal protection is required, and establishing a 

clearance time before which unprotected re-entry to the sprayed area is prevented are useful tools in controlling exposure. A 

set work zone distance in all directions from the point of spray application can be effective. Setting an appropriate re-entry 

clearance time after spraying is also advisable, although it should be noted that the types and levels of amine catalysts in the 

sprayed foam formulation could make the set distances and times variable. The work zone distance or re-entry clearance 

time could potentially be shortened or lengthened by considering conditions on a case-by-case basis. 

Respiratory protection to prevent over-exposure to MDI is needed for both sprayed foam applicators and helpers while 

working in the work zone. Either supplied-air respirators or air purifying respirators with cartridges having both an organic 

vapor sorbent and a particulate filter can provide adequate protection. Where face and eye protection is needed, respirator 

styles with full-face pieces, hoods, or helmets with an integral face shield are appropriate for use. No matter which respirator 

is chosen, a respiratory protection program must be established and diligently adhered to if the expected respiratory 

protection is to be realized.

Protective clothing and gloves are essential to reduce MDI exposure by the dermal route. Clothing items (coveralls, boot 

covers, etc.) should be selected to keep the sprayed foam material off street clothes and any exposed skin surfaces. Except 

for the times when solvents are used, any liquid-proof glove should provide adequate protection. Disposal of contaminated 

gloves and clothing whenever they are removed is an important practice to reduce dermal exposure to MDI. 

Further Work for Consideration

This review has suggested several areas where further work would be of benefit in clarifying the understanding or 

reduction of sprayed foam worker exposures.

ESTABLISHMENT OF RE-ENTRY CLEARANCE TIMES 

The indication that some sprayed foam components (i.e., MDI oligomers, blowing agents, and amine catalysts) may 

remain present in the air following sprayed foam application for longer periods than indicated by MDI concentrations 

warrants further study in order to help provide additional guidance in setting clearance times for re-entry.

ESTABLISHMENT OF WORK ZONE LIMITS

The author suggests that an appropriate work zone be established around the point of application. Developing some sort 

of procedure based on reliable field measurements of MDI concentrations, as well as a better understanding of blowing agent 

and amine catalyst exposures, would help provide the information needed to understand the site conditions and variables that 

affect the size of the work zone.

EXPOSURES TO BLOWING AGENTS AND AMINE CATALYSTS

The data on exposure to blowing agents and amine catalysts during sprayed foam application is limited. While measures 

taken to protect workers from MDI exposure during foam spraying would likely provide protection from blowing agent and 

amine catalyst exposure, having a more complete database would support this assertion.
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BIOGRAPHY

Mark W. Spence

Mark Spence is currently a consultant specializing in health & safety programs for manufacturing 

operations and products in the chemical & plastics industry following a 30-year career with the Dow 

Chemical Company. Following assignments as industrial hygiene chemist and then Laboratory Director 

of Dow's AIHA-accredited Industrial Hygiene Laboratory, Mark served as a Senior Industrial Hygienist 
with Dow's Global Industrial Hygiene Expertise Center. While at Dow, Mark supported Dow’s Polyurethanes business on 

industrial hygiene issues for 20 years. Mark has a Bachelor of Science degree in chemistry from Clarkson University, a 

Master of Science degree in chemistry from Michigan State University and is a Certified Industrial Hygienist.

The author would like to acknowledge funding for this project from the American Chemistry Council’s Center for the 

Polyurethanes Industry.