Crystalline Silica and 

Isocyanate Exposures 

during Parking Garage 

Repair 

Chandran Achutan, PhD

Ayodele Adebayo, MD, MPH

Fariba Nourian, BS

Health Hazard Evaluation Report

HETA 2008-0058-3108

Aduddell Restoration and Waterproofing, Inc.

Arlington, Virginia

April 2010

Department of Health and Human Services

Centers for Disease Control and Prevention

Workplace

Safety and Health

 National Institute for Occupational 

Safety and Health

This Health Hazard Evaluation (HHE) report and any recommendations made herein are for the specific 

facility evaluated and may not be universally applicable. Any recommendations made are not to be 

considered as final statements of NIOSH policy or of any agency or individual involved. Additional HHE 

reports are available at http://www.cdc.gov/niosh/hhe/
The employer shall post a copy of this report 

for a period of 30 calendar days at or near 

the workplace(s) of affected employees. The 

employer shall take steps to insure that the 

posted determinations are not altered, defaced, 

or covered by other material during such 

period. [37 FR 23640, November 7, 1972, as 

amended at 45 FR 2653, January 14, 1980].
Health Hazard Evaluation Report 2008-0058-3108 Page i

Appendix A

Report Abbreviations.......................................................................ii

Highlights of the NIOSH Health Hazard Evaluation............ iii

Summary .............................................................................v

Introduction..........................................................................1

Assessment.........................................................................1

Results.................................................................................2

Discussion ...........................................................................5

Conclusions.........................................................................6

Recommendations...............................................................6

References ..........................................................................9

Contents

Occupational Exposure Limits and Health Effects.............11

Acknowledgments Acknowledgments and Availability of Report.....................19
Page ii Health Hazard Evaluation Report 2008-0058-3108

μg/m3 Micrograms per cubic meter

μm Micrometer 

ACGIH® American Conference of Governmental Industrial Hygienists

CFR Code of Federal Regulations

HHE Health hazard evaluation

IARC International Agency for Research on Cancer

Lpm Liters per minute

mg/m3 Milligrams per cubic meter

mm Millimeter

mppcf Million particles per cubic foot

MDI Methylenebis(phenyl isocyanate)

MSDS Material safety data sheet

NAICS North American Industry Classification System

NIOSH National Institute for Occupational Safety and Health

OEL Occupational exposure limit

OSHA Occupational Safety and Health Administration

PBZ Personal breathing zone

PPE Personal protective equipment

PEL Permissible exposure limit

REL Recommended exposure limit

STEL Short-term exposure limit

TLV® Threshold limit value

TRIG Total reactive isocyanate group

TWA Time-weighted average

UK HSE United Kingdom Health and Safety Executive

WEEL Workplace environmental exposure limit

Abbreviations
Health Hazard Evaluation Report 2008-0058-3108 Page iii

Highlights of the

NIOSH Health

Hazard Evaluation

The National Institute for 

Occupational Safety and 

Health (NIOSH) received a 

management request for 

a health hazard evaluation 

at Aduddell Restoration 

and Waterproofing, Inc. 

in Arlington, Virginia. The 

request was submitted 

because management 

wanted to ensure 

that employees were 

adequately protected 

against silica and 

methylenebis(phenyl 

isocyanate) exposure 

(MDI).

What NIOSH Did

● We evaluated the worksite in January and February 2008.

● We talked to all employees at the site about their health 

concerns. 

● We measured airborne dust and crystalline silica.

● We analyzed samples of filler Part A and Part B for MDI 

monomer.

● We tested how long it takes for MDI-containing Part A and 

castor oil-containing Part B to react with each other.

What NIOSH Found

● No employees at the site reported work-related health 

concerns.

● Employees were overexposed to crystalline silica while 

jackhammering and sandblasting.

● Filler Part A contained 52% MDI monomer. Part B 

contained no MDI monomer.

● Unreacted MDI monomer was present for at least 40 minutes 

after employees mixed Parts A and B mixed together, causing 

potential dermal exposure for employees. 

● Employees wore respirators when jackhammering, 

sandblasting, and mixing Parts A and B, but not when 

applying filler material.

● Employees were not respirator fit-tested.

● Employees did not clean or maintain their respirators 

properly.

● Employees who used the jack hammer wore leather gloves 

instead of antivibration gloves.

● Some employees did not wear eye protection while 

sandblasting, jackhammering, or mixing and applying filler 

material.

● Noise levels during jackhammering and sandblasting 

need evaluation. A hearing conservation program may be 

warranted.

What Managers Can Do

● Require employees to wear respirators while jackhammering 

and sandblasting.
Page iv Health Hazard Evaluation Report 2008-0058-3108

Highlights of the

NIOSH Health

Hazard Evaluation

(continued)

● Require employees to wear eye protection while sandblasting, 

jackhammering, or mixing and applying filler material.

● Comply with the Occupational Safety and Health 

Administration (OSHA) Respiratory Protection Standard. 

This standard includes elements of training, correct use and 

maintenance, and fit testing of respirators.

● Explore possible engineering controls to reduce dust levels 

while jackhammering and sandblasting.

● Comply with the OSHA Hazard Communication Standard. 

Provide training on all hazards such as silica, isocyanates, and 

vibration.

● Evaluate employees’ noise exposure during jackhammering 

and sandblasting.

● Provide employees who use vibrating tools, such as 

jackhammers, with antivibration gloves.

● Provide employees who work with Part A and the mixed 

compound with butyl rubber gloves.

● Establish a smoking cessation program and encourage 

employees to use it to quit smoking.

What Employees Can Do

● Continue to wear respirators while jackhammering and 

sandblasting.

● Clean and store your respirator at the end of the work day.

● Be clean shaven so the respirator fits properly.

● Wear antivibration gloves when jackhammering.

● Wear butyl rubber gloves when handling Part A and the Part 

A and B mixture.

● Wear eye protection while sandblasting, jackhammering, or 

mixing and applying filler material.

● Stop smoking.
Health Hazard Evaluation Report 2008-0058-3108 Page v

Summary

Employees were 

overexposed to 

crystalline silica when 

jackhammering and 

sandblasting. Employees 

may be exposed to MDI 

through skin contact 

and should wear butyl 

rubber gloves to minimize 

exposure. Although the 

appropriate respirator was 

available for crystalline 

silica exposure, not all job 

tasks requiring respirators 

were clearly identified in 

the company’s respirator 

program. 

On November 30, 2007, NIOSH received a request from managers 

at Aduddell Restoration and Waterproofing, Inc. for an HHE 

at the Ballston Mall Parking Garage in Arlington, Virginia. The 

managers wanted to know if the employees were adequately 

protected against silica and MDI during parking garage repair. 

Full-shift PBZ air samples for respirable particulates and silica 

were collected on four employees over 2 days. The amount of MDI 

monomer in a bulk sample of Part A and Part B was measured. We 

also evaluated the curing time after mixing MDI-containing Part A 

and the inert Part B. 

Employees were exposed to hazardous levels of respirable crystalline 

silica during jackhammering and sandblasting. Of the eight PBZ 

air samples for respirable dust and silica, seven reached or exceeded 

the silica (as quartz) ACGIH TLV of 0.025 mg/m3, and six 

reached or exceeded the NIOSH REL of 0.05 mg/m3. None of the 

samples exceeded the OSHA Construction PEL for respirable dust 

containing silica (quartz). Approximately 52% of the bulk sample 

of Part A was MDI monomer. Part B contained no MDI monomer. 

A quantitative analysis of the reaction between Part A and Part B 

showed that approximately 80% of the MDI monomer reacted in 

the first 10 minutes. At 60 minutes, the mixture was hardened. We 

considered inhalation exposure to MDI unlikely because of the low 

vapor pressure of MDI, the relatively short curing time between the 

MDI-containing Part A and the inert Part B, and the method used 

to pour and apply the MDI-containing slurry. However, we believed 

there was a potential for dermal exposure to MDI that could result 

in sensitization, asthma, and contact dermatitis.

We interviewed all 10 employees who were working during our site 

visit; none reported work-related health concerns. The company 

provided the appropriate type of respirator for crystalline silica and 

required employees to wear it. However, not all job tasks requiring 

respirators were clearly defined. Additionally, employees were 

neither respirator fit-tested nor did they clean or maintain their 

respirators properly.

We recommend informing employees that MDI monomer may 

still exist after Parts A and B are mixed together and requiring 

them to wear butyl rubber gloves when mixing these compounds. 

We also recommend requiring employees to wear respirators 

during jackhammering and sandblasting. The company’s 

respirator program should comply with the OSHA Respiratory 
Page vi Health Hazard Evaluation Report 2008-0058-3108

Summary (continued)

Protection Standard. Additional recommendations included 

exploring possible engineering controls to reduce dust levels while 

jackhammering and sandblasting; complying with the OSHA 

Hazard Communication Standard; evaluating employees’ exposure 

to noise during jackhammering and sandblasting activities; 

providing antivibration gloves to employees who use vibrating tools 

such as jackhammers; wearing eye protection while sandblasting, 

jackhammering, or mixing and applying filler material; and 

establishing a smoking cessation program. 

Keywords: NAICS 238390 (Other Building Finishing Contractors),

respirable dust, silica, quartz, isocyanate, MDI, curing time
Health Hazard Evaluation Report 2008-0058-3108 Page 1

Introduction

On November 30, 2007, NIOSH received a request from the 

management of Aduddell Restoration and Waterproofing, Inc. 

(Aduddell) for an HHE at the Ballston Mall Parking Garage in 

Arlington, Virginia. Aduddell is based in Minneapolis, Minnesota, 

but carries out contract work across the country. Managers at 

Aduddell wanted to know if their employees were adequately 

protected against silica and MDI during parking garage repair. 

On January 12–14, 2008, and February 13–14, 2008, NIOSH 

investigators visited the Ballston Mall Parking Garage in Arlington, 

Virginia, to evaluate employee exposure to respirable silica and 

MDI. During both visits, employees were repairing the first and 

second floors of the parking garage. Areas needing minor repairs 

are first prepped by applying MDI-containing slurry as a filler 

material. Employees remove damaged concrete by jackhammering 

and sandblasting continuously for a few hours or sporadically 

throughout the day. Employees prepare the filler material by 

mixing one part of Part A, an MDI-containing product, and two 

parts of Part B, containing primarily castor oil, in a bucket. Sand is 

added to the mixture to make it easier to spread. The entire mixing 

process takes about 30 seconds.

To determine the presence of potential work-related health 

effects, employees were asked to describe any health problems that 

they were experiencing including those they attributed to work 

exposures. We interviewed all 10 employees present in a private 

setting.

We collected full-shift PBZ air samples for respirable particulates 

from four employees jackhammering and sandblasting over 2 days. 

The samples were collected and analyzed according to NIOSH 

Method 0600 [NIOSH 2009]. Samples were collected on 37-mm, 

5-μm polyvinyl chloride filters, at a flow rate of 1.7 Lpm using a 

10-mm nylon cyclone preselector for respirable particulate. The 

respirable particulate samples were also analyzed for silica content 

by x-ray diffraction with NIOSH Method 7500 [NIOSH 2009]. 

According to the MSDS, the active ingredient in Part A is 25%–

70% MDI monomer, and the active ingredient in Part B is 60%–

100% castor oil. Bulk samples of Parts A and B were collected and 

analyzed for MDI monomer in accordance with NIOSH Method 

5525 [NIOSH 2009]. Managers assumed that once Parts A and B 

Assessment
Page 2 Health Hazard Evaluation Report 2008-0058-3108

Assessment (continued) were mixed in the bucket the reaction was complete and that MDI 

monomer, the active ingredient that can potentially cause adverse 

health effects, no longer existed. To test this assumption, we 

studied the reaction that occurs between Parts A and B over time. 

To accomplish this, we mixed one part of Part A and two parts of 

Part B in six vials and allowed them to react for 5, 10, 15, 20, 40, 

or 60 minutes. The reactions were halted at these time intervals by 

diluting the reaction solutions and analyzing for MDI monomer 

according to NIOSH Method 5525 [NIOSH 2009]. 

For information on the OELs and health effects of silica and 

isocyanates (including MDI), please see Appendix A.

Employee Interviews 

All interviewed employees were men who had worked for Aduddell 

from 2 months to 17 years. Only three of the employees had 

worked at the company for more than a year. Four employees 

reported ever working with Part A. None of the 10 employees 

reported adverse health symptoms or work-related health problems. 

All the employees reported that hard hats and safety shoes were 

mandatory during work, while respirators, earplugs, safety glasses, 

and leather or rubber gloves were used when deemed necessary by 

the employer. Employees reported that they had not been fit-tested 

for respirator use. 

Respirable Dust and Silica 

All of the crystalline silica in the respirable dust samples was 

present as quartz. As summarized in Table 1, of the eight respirable 

dust and quartz PBZ air samples, seven reached or exceeded the 

quartz ACGIH TLV of 0.025 mg/m3 [ACGIH 2009], and six 

reached or exceeded the NIOSH REL of 0.05 mg/m3 [NIOSH 

2005]. None of the samples exceeded the OSHA Construction PEL 

for respirable dust containing silica (quartz) (refer to Appendix A 

for a description on how to calculate the PEL) [29 CFR 1910.1000]. 

Results
Health Hazard Evaluation Report 2008-0058-3108 Page 3

Results (continued)

Methylenebis(phenyl isocyanate) 

Analysis of a Part A bulk sample showed that it contained 

approximately 52% MDI monomer, while the analysis of Part B 

did not detect MDI monomer. Quantitative analysis of the reaction 

between Part A and Part B showed that approximately 80% of the 

MDI monomer reacted in the first 10 minutes. The mixture was 

completely hardened at 60 minutes and therefore could not be 

analyzed for MDI. Figure 1 shows the curing time following mixing 

of Parts A and B.

Table 1. Full-shift PBZ respirable dust and respirable silica (as quartz) exposures during jack hammering and sandblasting

Employee

Sampling 

Time 

(min)

Respirable 

Dust 

(mg/m3)

Respirable 

Dust 8-hr 

(mg/m3)

Respirable 

Dust 

(mppcf)

Respirable 

Dust 8-hr 

(mppcf)

Respirable 

Quartz 

(mg/m3)

Respirable 

Quartz 8-hr 

(mg/m3)

Quartz 

(%)

OSHA 

PEL* 

(mppcf)

2/13/08

1 323 0.67 0.45 6.7 4.5 0.15 0.10 21.6 9.4

2 305 0.65 0.41 6.5 4.1 0.13 0.08 19.4 10

3 309 0.25 0.16 2.5 1.6 0.05 0.03 21.5 9.4

4 297 1.3 0.78 13 7.8 0.22 0.13 17.2 11

2/14/08

1 349 0.23 0.17 2.3 1.7 0.04 0.03 16.4 12

2 346 0.56 0.40 5.6 4.0 0.08 0.06 14.9 13

3 348 0.42 0.31 4.2 4.2 0.07 0.05 17.2 11

4 347 0.25 0.18 2.5 2.5 0.02 0.01 7.3 20

NIOSH REL 0.05 0.05

ACGIH TLV 0.025 0.025

*OSHA Construction PEL for respirable dust containing silica (quartz)
Page 4 Health Hazard Evaluation Report 2008-0058-3108

Results (continued)

Other Observations 

Employees were wearing respirators during jackhammering, 

sandblasting, and the mixing of Parts A and B, but not during 

the filler material application. Although Aduddell had a written 

respiratory protection program, the job tasks requiring respirators 

were not clearly identified. Employees were not respirator fit-tested 

and did not clean or maintain their respirators properly.

All the employees performed their tasks in street clothes without 

a set of clean clothes for changing after work. Some employees 

wore no eye protection while sandblasting, jackhammering, or 

mixing and applying filler material. We noted that employees used 

leather gloves for jackhammering instead of antivibration gloves. 

Employees were smoking in the workplace without first washing 

their hands. Although noise exposures were not part of this HHE 

and were not evaluated, our opinion is that the noise generated by 

jackhammering and sandblasting activities may exceed OELs and 

warrant future evaluation. 
Health Hazard Evaluation Report 2008-0058-3108 Page 5

The results from the particulate sampling showed that employees 

were overexposed to crystalline silica. However, employees were 

wearing NIOSH-certified, air-purifying elastomeric half-mask 

respirators equipped with P100 particulate filters. These respirators 

can protect them from crystalline silica exposure if worn properly 

within the context of a complete respirator program. A complete 

program requires that respirator wearers are clean-shaven, fit-tested, 

and medically cleared to wear a respirator and that respirators are 

cleaned and maintained properly. Additionally, to avoid confusion, 

Aduddell’s respirator program should clearly identify the activities 

where respirator use is required. Implementing engineering or 

administrative controls to reduce employee exposures to silicacontaining dust generated by jackhammering and sandblasting 

could reduce or eliminate the need for respirator use. The OSHA 

website at http://www.osha.gov/SLTC/silicacrystalline/index.html 

and NIOSH website at http://www.cdc.gov/niosh/topics/silica/ 

provide information on engineering and administrative controls 

that may be useful to Aduddell in reducing employee exposures.

The low vapor pressure of MDI, the relatively short curing time 

between the MDI-containing Part A and the inert Part B, and 

the pour-application method of the MDI-containing slurry (as 

opposed to spray application), makes it unlikely that employees 

applying the slurry were overexposed to airborne MDI. However, 

there is a potential for dermal exposure among these employees. 

Skin sensitization to MDI is possible, and if skin sensitization 

were to occur, employees would not be able to work around MDIcontaining substances without serious medical problems, such as 

dermatitis or asthma. Employees were under the impression that 

after they mixed Parts A and B, the resulting mixture was inert. 

Although 80% of the MDI monomer reacted in the first 

10 minutes, our reaction time study shows that the reaction goes 

on for at least 40 minutes. Additionally, employees smoking in 

the workplace without first washing their hands can increase MDI 

exposure by ingesting chemicals from hand to mouth.

Jackhammering, sandblasting, and preparing and applying the 

filler material all have the potential to create an eye hazard. Using 

goggles or safety glasses while conducting these tasks can reduce 

the possibility of eye injuries. Additional information is available at 

http://www.cdc.gov/niosh/topics/eye/.

Discussion
Page 6 Health Hazard Evaluation Report 2008-0058-3108

Although employees reported no health concerns, they were 

overexposed to respirable crystalline silica during jackhammering 

and sandblasting. Inhalation exposure to MDI appears unlikely; 

however, there was a potential for dermal exposure to MDI, which 

can cause sensitization to isocyanates and put employees at risk for 

dermatitis and asthma.

Based on our findings, we recommend the actions listed below 

to create a more healthful workplace. We encourage Aduddell to 

use a labor-management health and safety committee or working 

group to discuss the recommendations in this report and develop 

an action plan. Those involved in the work can best set priorities 

and assess the feasibility of our recommendations for the specific 

situation at Aduddell. Our recommendations are based on the 

hierarchy of controls approach (refer to Appendix A: Occupational 

Exposure Limits and Health Effects). This approach groups actions 

by their likely effectiveness in reducing or removing hazards. 

In most cases, the preferred approach is to eliminate hazardous 

materials or processes and install engineering controls to reduce 

exposure or shield employees. Until such controls are in place, or 

if they are not effective or feasible, administrative measures and/or 

personal protective equipment may be needed. 

Engineering Controls

Engineering controls reduce exposures to employees by removing 

the hazard from the process or placing a barrier between the 

hazard and the employee. Engineering controls are very effective 

at protecting employees without placing primary responsibility of 

implementation on the employee. 

1. Reduce dust levels while jackhammering and sandblasting 

by exhausting the dust close to its point of production. 

Links for information on engineering controls are available 

at http://www.cdc.gov/niosh/topics/silica/.

Administrative Controls 

Administrative controls are management-dictated work practices 

and policies to reduce or prevent exposures to workplace hazards. 

Conclusions

Recommendations
Health Hazard Evaluation Report 2008-0058-3108 Page 7

Recommendations

(continued) The effectiveness of administrative changes in work practices 

for controlling workplace hazards is dependent on management 

commitment and employee acceptance. Regular monitoring and 

reinforcement are necessary to ensure that control policies and 

procedures are not circumvented in the name of convenience or 

production.

1. Ensure that requirements listed in the OSHA Respiratory 

Protection Standard [29 CFR 1910.134] are established in 

Aduddell’s written program and are followed. Ensure that 

employees are medically cleared, fit-tested, clean-shaven, and 

adequately trained on respirator use and care before they 

use respirators. Additional respirator use information is 

available at http://www.osha.gov/SLTC/etools/silica/index.

html and http://www.osha.gov/Publications/SECG_RPS/

secg_rps.html.

2. Update the current respiratory protection program to reflect 

the job tasks requiring the use of a respirator and the type of 

respirator to be worn.

3. Educate employees on the health effects and proper 

work practices when working with crystalline silica, 

isocyanates, and vibration as required by the OSHA Hazard 

Communication Standard [29 CFR 1910.1200]. Make 

them aware that MDI monomer may still be present even 

after Parts A and B are mixed together. Additional hazard 

communication information is available at http://www.

osha.gov/dsg/hazcom/index.html.

4. Evaluate employee noise exposures and, if needed, establish 

a hearing conservation program. The basic elements of the 

program should meet, at a minimum, the requirements 

of the OSHA hearing conservation amendment [29 CFR 

1910.95]. Other sources for defining effective hearing 

conservation programs are also available [Royster and 

Royster 1990; NIOSH 1996; Suter 2002].

5. Establish a smoking cessation program and encourage 

employees to use it to quit smoking. MDI from their hands 

could be transferred onto the cigarettes, and this could lead 

to respiratory exposure. In addition, tobacco products have 

been shown to cause many adverse health effects including 

respiratory diseases and cancer. Further information 

regarding workplace smoking policies and smoking cessation 

programs can be found in (1) NIOSH Current Intelligence 

Bulletin 54: Environmental Tobacco Smoke in the Workplace, 
Page 8 Health Hazard Evaluation Report 2008-0058-3108

Recommendations

(continued) Lung Cancer and Other Health Effects, (2) The Health 

Consequences of Smoking: a Report of the Surgeon General, and

(3) Environmental Tobacco Smoke [NIOSH 1991; DHHS 2004; 

ASHRAE 2005].

Personal Protective Equipment 

PPE is the least effective means for controlling employee exposures. 

Proper use of PPE requires a comprehensive program and calls 

for a high level of employee involvement and commitment to be 

effective. The use of PPE requires the choice of the appropriate 

equipment to reduce the hazard and the development of 

supporting programs such as training, change-out schedules, and 

medical assessment if needed. PPE should not be relied upon as 

the sole method for limiting employee exposures. Rather, PPE 

should be used until engineering and administrative controls can 

be demonstrated to be effective in limiting exposures to acceptable 

levels.

1. Continue to use the respirators currently provided to 

employees until engineering or administrative controls can 

be implemented to reduce crystalline silica exposures to 

below the OELs.

2. Employees should have separate work clothes or clean sets 

of clothes at the worksite. This will allow them to change to 

clean clothes if they spill MDI-containing products on their 

clothing.

3. Employees should wear goggles and butyl rubber gloves 

when mixing filler material Parts A and B.

4. Ensure that employees jackhammering and sandblasting 

wear eye protection, and provide antivibration gloves to 

employees who work with jackhammers. Hearing protection 

should be used if noise levels exceed OELs.
Health Hazard Evaluation Report 2008-0058-3108 Page 9

References

ACGIH [2009]. 2009 TLVs® and BEIs®: threshold limit values for 

chemical substances and physical agents and biological exposure 

indices. Cincinnati, OH: American Conference of Governmental 

Industrial Hygienists.

ASHRAE [2005]. Environmental tobacco smoke: position 

document [http://www.ashrae.org/content/ASHRAE/ASHRAE/

ArticleAltFormat/20058211239_347.pdf]. Date accessed: 

December 2009.

CFR. Code of Federal Regulations. Washington, DC: U.S. 

Government Printing Office, Office of the Federal Register.

DHHS [2004]. The health consequences of smoking: a report of 

the Surgeon General. Atlanta, GA: U.S. Department of Health 

and Human Services, Centers for Disease Control and Prevention, 

National Center for Chronic Disease Prevention and Health 

Promotion, Office on Smoking and Health.

NIOSH [1991]. Current intelligence bulletin 54: environmental 

tobacco smoke in the workplace, lung cancer and other health 

effects. By Votaw DM, Steenland K. Cincinnati, OH: U.S. 

Department of Health and Human Services. Centers for Disease 

Control, National Institute for Occupational Safety and Health, 

DHHS (NIOSH) Publication No. 91-108.

NIOSH [1996]. Preventing occupational hearing loss–a practical 

guide. By Franks JR, Stephenson MR, Merry CJ. Cincinnati, 

OH: U.S. Department of Health and Human Services, Centers 

for Disease Control and Prevention, National Institute for 

Occupational Safety and Health, DHHS (NIOSH) Publication 

No. 96-110.

NIOSH [2005]. NIOSH pocket guide to chemical hazards. 

Cincinnati, OH: U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute 

for Occupational Safety and Health, DHHS (NIOSH) Publication 

No. 2005-149. [http://www.cdc.gov/niosh/npg/]. Date accessed: 

December 2009.

NIOSH [2009]. NIOSH manual of analytical methods (NMAM®). 

4th ed. Schlecht PC, O’Connor PF, eds. Cincinnati, OH: U.S. 

Department of Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety 
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References (continued) and Health, DHHS (NIOSH) Publication 94-113 (August, 1994); 

1st Supplement Publication 96-135, 2nd Supplement Publication 

98-119; 3rd Supplement 2003-154. [http://www.cdc.gov/niosh/

docs/2003-154/].

Royster JD, Royster LH [1990]. Hearing conservation programs: 

practical guidelines for success. Chelsea, MI: Lewis Publishers.

Suter AH [2002]. Hearing conservation manual. 4th ed. 

Milwaukee, WI: Council for Accreditation in Occupational 

Hearing Conservation.
Health Hazard Evaluation Report 2008-0058-3108 Page 11

In evaluating the hazards posed by workplace exposures, NIOSH investigators use both mandatory (legally 

enforceable) and recommended OELs for chemical, physical, and biological agents as a guide for making 

recommendations. OELs have been developed by Federal agencies and safety and health organizations to 

prevent the occurrence of adverse health effects from workplace exposures. Generally, OELs suggest levels 

of exposure that most employees may be exposed up to 10 hours per day, 40 hours per week for a working 

lifetime without experiencing adverse health effects. However, not all employees will be protected from 

adverse health effects even if their exposures are maintained below these levels. A small percentage may 

experience adverse health effects because of individual susceptibility, a preexisting medical condition, 

and/or a hypersensitivity (allergy). In addition, some hazardous substances may act in combination with 

other workplace exposures, the general environment, or with medications or personal habits of the 

employee to produce health effects even if the occupational exposures are controlled at the level set by 

the exposure limit. Also, some substances can be absorbed by direct contact with the skin and mucous 

membranes in addition to being inhaled, which contributes to the individual’s overall exposure. 

Most OELs are expressed as a TWA exposure. A TWA refers to the average exposure during a normal 8- 

to 10-hour workday. Some chemical substances and physical agents have recommended STEL or ceiling 

values where health effects are caused by exposures over a short period. Unless otherwise noted, the STEL 

is a 15-minute TWA exposure that should not be exceeded at any time during a workday, and the ceiling 

limit is an exposure that should not be exceeded at any time.

In the United States, OELs have been established by Federal agencies, professional organizations, state 

and local governments, and other entities. Some OELs are legally enforceable limits, while others are 

recommendations. The U.S. Department of Labor OSHA PELs (29 CFR 1910 [general industry]; 29 

CFR 1926 [construction industry]; and 29 CFR 1917 [maritime industry]) are legal limits enforceable in 

workplaces covered under the Occupational Safety and Health Act. NIOSH RELs are recommendations 

based on a critical review of the scientific and technical information available on a given hazard and the 

adequacy of methods to identify and control the hazard. NIOSH RELs can be found in the NIOSH Pocket 

Guide to Chemical Hazards [NIOSH 2005]. NIOSH also recommends different types of risk management 

practices (e.g., engineering controls, safe work practices, employee education/training, personal protective 

equipment, and exposure and medical monitoring) to minimize the risk of exposure and adverse health 

effects from these hazards. Other OELs that are commonly used and cited in the United States include 

the TLVs recommended by ACGIH, a professional organization, and the WEELs recommended by the 

American Industrial Hygiene Association, another professional organization. The TLVs and WEELs are 

developed by committee members of these associations from a review of the published, peer-reviewed 

literature. They are not consensus standards. ACGIH TLVs are considered voluntary exposure guidelines 

for use by industrial hygienists and others trained in this discipline “to assist in the control of health 

hazards” [ACGIH 2009]. WEELs have been established for some chemicals “when no other legal or 

authoritative limits exist” [AIHA 2009].

Outside the United States, OELs have been established by various agencies and organizations and include 

both legal and recommended limits. Since 2006, the Berufsgenossenschaftliches Institut für Arbeitsschutz 

(German Institute for Occupational Safety and Health) has maintained a database of international 

Appendix A: Occupational Exposure Limits and Health Effects
Page 12 Health Hazard Evaluation Report 2008-0058-3108

Appendix A: Occupational Exposure Limits and Health Effects (continued)

OELs from European Union member states, Canada (Québec), Japan, Switzerland, and the United 

States available at http://www.dguv.de/bgia/en/gestis/limit_values/index.jsp. The database contains 

international limits for over 1250 hazardous substances and is updated annually. 

Employers should understand that not all hazardous chemicals have specific OSHA PELs, and for some 

agents the legally enforceable and recommended limits may not reflect current health-based information. 

However, an employer is still required by OSHA to protect its employees from hazards even in the absence 

of a specific OSHA PEL. OSHA requires an employer to furnish employees a place of employment free 

from recognized hazards that cause or are likely to cause death or serious physical harm [Occupational 

Safety and Health Act of 1970 (Public Law 91–596, sec. 5(a)(1))]. Thus, NIOSH investigators encourage 

employers to make use of other OELs when making risk assessment and risk management decisions to 

best protect the health of their employees. NIOSH investigators also encourage the use of the traditional 

hierarchy of controls approach to eliminate or minimize identified workplace hazards. This includes, in 

order of preference, the use of: (1) substitution or elimination of the hazardous agent, (2) engineering 

controls (e.g., local exhaust ventilation, process enclosure, dilution ventilation), (3) administrative controls 

(e.g., limiting time of exposure, employee training, work practice changes, medical surveillance), and (4) 

personal protective equipment (e.g., respiratory protection, gloves, eye protection, hearing protection). 

Control banding, a qualitative risk assessment and risk management tool, is a complementary approach 

to protecting employee health that focuses resources on exposure controls by describing how a risk 

needs to be managed. Information on control banding is available at http://www.cdc.gov/niosh/topics/

ctrlbanding/. This approach can be applied in situations where OELs have not been established or can be 

used to supplement the OELs, when available. 

Silica (Quartz and Cristobalite) 

Silica or silicon dioxide occurs in a crystalline or noncrystalline (amorphous) form. In crystalline silica, the 

silicon dioxide molecules are oriented in a fixed pattern versus the random arrangement of the amorphous 

form. The more common crystalline forms in workplace environments are quartz and cristobalite, and to a 

lesser extent, tridymite. Occupational exposures to respirable crystalline silica (quartz and cristobalite) have 

been associated with silicosis, lung cancer, pulmonary tuberculosis, and airway diseases. 

Silicosis is a fibrotic disease of the lung caused by the deposition of fine crystalline silica particles in 

the lungs. It is the disease most often associated with exposure to respirable crystalline silica. This lung 

disease is caused by the inhalation and deposition of crystalline silica particles that are 10 μm or less 

in diameter. Particles 10 μm or below are considered respirable particles and classified as having the 

potential to reach the lower portions of the human lung (alveolar region). Although particle sizes 10 

μm and below are considered respirable, some of these particles can be deposited before they reach the 

alveolar region [Hinds 1999]. Symptoms of silicosis usually develop insidiously, with cough, shortness of 

breath, chest pain, weakness, wheezing, and nonspecific chest illnesses. Silicosis usually occurs after years 

of exposure (chronic), but may appear in a shorter period of time (acute) if exposure concentrations are 

very high. Acute silicosis is typically associated with a history of high exposures from tasks that produce 
Health Hazard Evaluation Report 2008-0058-3108 Page 13

Appendix A: Occupational Exposure Limits and Health Effects (continued)

small particles of airborne dust with a high silica content [NIOSH 1986]. Even though the carcinogenicity 

of crystalline silica in humans has been strongly debated in the scientific community, IARC in 1996 

concluded that there was “sufficient evidence in humans for the carcinogenicity of inhaled crystalline 

silica in the form of quartz or cristobalite from occupational sources [IARC 1997].” A NIOSH publication 

also lists several other serious diseases from occupational exposure to crystalline silica. These include 

lung cancer and noncarcinogenic disorders including immunologic disorders and autoimmune diseases, 

rheumatoid arthritis, renal diseases, and an increased risk of developing tuberculosis after exposure to the 

infectious agent [NIOSH 2002]. 

When proper practices are not followed or controls are not maintained, respirable crystalline silica 

exposures can exceed the NIOSH REL, the ACGIH TLV, or the OSHA PEL. NIOSH recommends an 

exposure limit of 0.05 mg/m3, TWA for up to a 10-hour work day to reduce the risk of developing silicosis, 

lung cancer, and other adverse health effects [NIOSH 2005]. The ACGIH TLV for quartz is 0.025 mg/m3, 

TWA for up to an 8-hour work day [ACGIH 2009].

The current OSHA PEL for respirable dust containing crystalline silica (quartz) for the construction 

industry is measured by impinger sampling. The PEL is expressed in mppcf and is calculated using the 

following formula [29 CFR 1926.55]:

 250 mppcf 

Respirable PEL = % Silica + 5

Since the PELs were adopted, the impinger sampling method has been rendered obsolete by gravimetric 

sampling [OSHA 1996]. OSHA is not aware of any government agencies or employers in this country 

that are currently using impinger sampling to assess worker exposure to dust containing crystalline silica, 

and impinger samples are generally recognized as less reliable than gravimetric samples. OSHA currently 

instructs its compliance officers to apply a conversion factor of 0.1 mg/m3 per mppcf when converting 

between gravimetric sampling and the particle count standard when characterizing construction operation 

exposures [OSHA 2001]. Virginia OSHA uses the conversion factor to calculate mppcf and ultimately 

compare to the calculated construction PEL [VDLI 2010]. Therefore, in this report, respirable dust 

concentrations are presented in mppcf.

Isocyanates (including MDI) 

Diisocyanates are a group of highly reactive, low-molecular-weight aromatic and aliphatic compounds, 

characterized by two isocyanate functional groups (N=C=O). The most common diisocyanates include 

the aliphatic compounds, hexamethylene diisocyanate and isophorone diisocyanate, and the aromatic 

compounds, toluene diisocyanate and MDI. Monomeric and polymeric diisocyanates are widely used 

in the production of polyurethane materials such as foams, adhesives, resins, elastomers, binders, and 

coatings. In industry, polyurethane is synthesized via a polymer chemistry reaction between polyisocyanates 

and polyols.
Page 14 Health Hazard Evaluation Report 2008-0058-3108

Appendix A: Occupational Exposure Limits and Health Effects (continued)

Exposure to isocyanates can be irritating to the skin, mucous membranes, eyes, and respiratory tract 

[NIOSH 1978, 2005]. The most frequent respiratory effect associated with isocyanate exposure is asthma 

due to sensitization [Markowitz 2005; NIOSH 2005]. Contact dermatitis (both irritant and allergic forms) 

is less common and can result in symptoms such as rash, erythema, and itching [Goossens et al. 2002]. An 

employee with isocyanate-induced asthma exhibits the traditional symptoms of acute airway obstruction 

such as coughing, wheezing, shortness of breath, tightness in the chest, and nocturnal awakening 

[NIOSH 1978, 1986]. Isocyanate-induced asthma occurs with variable latency following the initial 

exposure, although characteristically the asthma develops within 2 years of exposure [Markowitz 2005]. 

The asthmatic reaction may occur minutes after exposure (immediate onset) and/or several hours after 

exposure (delayed onset) [Chan-Yeung and Lam 1986; NIOSH 1986]. After sensitization, any exposure, 

even to levels below OELs, can produce an asthmatic response that may be life threatening [NIOSH 1978, 

1996, 2006]. 

Monomeric, polymeric, and prepolymeric isocyanates appear to be capable of producing respiratory 

sensitization in exposed employees [Harries et al. 1979; Berlin et al. 1981; Woolrich 1982; Mobay 

Corporation 1983, 1991; Zammit-Tabona et al. 1983; Chang and Karol 1984; Nielsen et al. 1985; 

Alexandersson et al. 1986; Seguin et al. 1987; Mapp et al. 1988; Liss et al. 1988; Keskinen et al. 1988; 

Cartier et al. 1989; Vandenplas et al. 1992a,b; Baur et al. 1994]. Several animal studies have shown that 

dermal exposure to diisocyanates may also produce respiratory sensitization [Karol et al. 1981; Erjefalt and 

Persson 1992; Bickis 1994; Rattray et al. 1994; Herrick 2002]. Employees exposed to isocyanates primarily 

through the dermal route have developed respiratory sensitization and occupational asthma in addition to 

skin sensitization (allergic contact dermatitis) [Bello 2007]. 

Diagnosis of isocyanate-induced asthma requires a thorough occupational history. As with other asthmatic 

conditions, pulmonary function tests may be within normal limits between asthmatic episodes. The 

prevalence of diisocyanate-induced asthma in exposed workers is believed to be 5%–10% [Chan-Yeung and 

Malo 1995; Bernstein 1996]. The only effective intervention for employees with isocyanate-induced asthma 

is cessation of all isocyanate exposure. This can be accomplished by removing the employee from the work 

environment where isocyanate exposure occurs. 

The current OELs for the different isocyanates are provided in Table 1. Most of the OELs apply to specific 

diisocyanates. However, the UK HSE has developed a standard based on the concentration of TRIG in a 

volume of air [Silk and Hardy 1983]. TRIG is a measurement of the concentration of isocyanate functional 

groups (N=C=O) in a sample of air. Airborne TRIG concentrations can be determined using NIOSH 

Method 5525 [NIOSH 2009].

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Table A1. Current OELs (μg/m3) for isocyanates

OSHA PEL

[29 CFR 1910.1000]

NIOSH REL

[NIOSH 2005]

ACGIH TLV

[ACGIH 2009]

UK HSE

[HSE 2005]

8-hr TWA Ceiling 10-hr 

TWA

10-min 

ceiling 8-hr TWA 15-min STEL 8-hr TWA 10-min ceiling

TDI* 140 36 140

MDI 240 50 200 51

HDI† 35 140 34

IPDI‡ 45 180 45

TRIG 20 70

 * toluene diisocyanate

 † hexamethylene diisocyanate

 ‡ isophorone diisocyanate
Page 16 Health Hazard Evaluation Report 2008-0058-3108

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Acknowledgments and

Availability of Report

The Hazard Evaluations and Technical Assistance Branch 

(HETAB) of the National Institute for Occupational Safety and 

Health (NIOSH) conducts field investigations of possible health 

hazards in the workplace. These investigations are conducted 

under the authority of Section 20(a)(6) of the Occupational Safety 

and Health Act of 1970, 29 U.S.C. 669(a)(6) which authorizes 

the Secretary of Health and Human Services, following a written 

request from any employer or authorized representative of 

employees, to determine whether any substance normally found 

in the place of employment has potentially toxic effects in such 

concentrations as used or found. HETAB also provides, upon 

request, technical and consultative assistance to federal, state, and 

local agencies; labor; industry; and other groups or individuals to 

control occupational health hazards and to prevent related trauma 

and disease.

The findings and conclusions in this report are those of the 

authors and do not necessarily represent the views of NIOSH. 

Mention of any company or product does not constitute 

endorsement by NIOSH. In addition, citations to websites 

external to NIOSH do not constitute NIOSH endorsement 

of the sponsoring organization or their programs or products. 

Furthermore, NIOSH is not responsible for the content of these 

websites. All Web addresses referenced in this document were 

accessible as of the publication date.

This report was prepared by Chandran Achutan and Ayodele 

Adebayo of HETAB, Division of Surveillance, Hazard Evaluations 

and Field Studies and Fariba Nourian of the Division of Applied 

Research and Technology (DART). Industrial hygiene field 

assistance was provided by SeungHee Jang. Analytical support 

was provided by Kathleen Ernst and Robert Streicher of DART, 

and Bureau Veritas, Michigan. Health communication assistance 

was provided by Stefanie Evans. Editorial assistance was provided 

by Ellen Galloway. Desktop publishing was performed by Robin 

Smith.

Copies of this report have been sent to employee and management 

representatives at Aduddell Restoration and Waterproofing 

Company and the Occupational Safety and Health Administration 

Regional Office. This report is not copyrighted and may be freely 

reproduced. The report may be viewed and printed from the 

following internet address: http://www.cdc.gov/niosh/hhe. Copies 

may be purchased from the National Technical Information Service 

at 5825 Port Royal Road, Springfield, Virginia 22161.
Below is a recommended citation for this report:

NIOSH [2010]. Health hazard evaluation report: crystalline silica and isocyanate 

exposures during parking garage repair, Arlington, Virginia. By Achutan C, Adebayo 

A, Nourian F. Cincinnati, OH: U.S. Department of Health and Human Services, 

Centers for Disease Control and Prevention, National Institute for Occupational 

Safety and Health, NIOSH HETA No. 2008-0058-3108.

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