Evaluation of Silica Exposures 

During Dowel Drilling

HHE Report No. 2019-0178-3368

January 2020 
Author: Michael P. Grant, ScD

Analytical Support: Bureau Veritas North America, Inc.

Desktop Publisher: Shawna Watts

Editor: Cheryl Hamilton

Industrial Hygiene Field Assistance: Donald Booher

Logistics: Donald Booher, Kevin Moore

Keywords: North American Industry Classification System (NAICS) 237310 (Highway, Street, and

Bridge Construction), Illinois, Dowel Drilling, Road Repair, Silica, Construction, Respirable

Crystalline Silica, Respirable Dust

Disclaimer 

The Health Hazard Evaluation Program investigates possible health hazards in the workplace under

the authority of the Occupational Safety and Health Act of 1970 [29 USC 669a(6)]. The Health

Hazard Evaluation Program also provides, upon request, technical assistance to federal, state, and

local agencies to investigate occupational health hazards and to prevent occupational disease or

injury. Regulations guiding the Program can be found in Title 42, Code of Federal Regulations, 

Part 85; Requests for Health Hazard Evaluations [42 CFR Part 85]. 

Availability of Report 

Copies of this report have been sent to the employer and employee representatives. The state and 

local health departments and the Occupational Safety and Health Administration Regional Office 

have also received a copy. This report is not copyrighted and may be freely reproduced.

Recommended Citation 

NIOSH [2020]. Evaluation of silica exposures during dowel drilling. By Grant MP. Cincinnati, OH: 

U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 

National Institute for Occupational Safety and Health, Health Hazard Evaluation Report 2019-

0178-3368, https://www.cdc.gov/niosh/hhe/reports/pdfs/2019-0178-3368.pdf. 
Table of Contents

Main Report

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

Our Approach ................................................................................................................................................... 1

Our Key Findings.............................................................................................................................................. 1

Our Recommendations.................................................................................................................................... 2

Supporting Technical Information 

Section A: Workplace Information............................................................................................................ A-1

Workplace.................................................................................................................................................. A-1

Employee Information ............................................................................................................................ A-1

Process Description ................................................................................................................................. A-1

Section B: Methods, Results, and Discussion .......................................................................................... B-1

Methods: Observations of Work Processes, Practices, and Conditions .......................................... B-1

Results: Observations of Work Processes, Practices, and Conditions ............................................. B-1

Methods: Exposure Assessment ............................................................................................................ B-2

Results: Exposure Assessment ............................................................................................................... B-3

Discussion ................................................................................................................................................. B-4

Limitations................................................................................................................................................. B-4

Conclusions............................................................................................................................................... B-5

Section C: Tables.......................................................................................................................................... C-1

Section D: Occupational Exposure Limits............................................................................................... D-1

Respirable Crystalline Silica.................................................................................................................... D-2

Section E: References.................................................................................................................................. E-1
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1

Introduction

Request

Management from a dowel drilling company requested a health hazard evaluation concerning employee 

exposure to respirable crystalline silica during dowel drilling activities.

Workplace 

Dowel drilling is one step in the process of repairing a road, with the work taking place on a roadway 

construction site. Two employees who performed this task were evaluated in this project—a backhoe 

operator who operated the backhoe and drill rig operator who used the drill rig to drill the dowel holes. 

A water spray was used to reduce the amount of dust in the air during these activities. During our visits, 

the crew worked a single shift about 5 hours long. The length of time spent drilling could change 

depending on the project. For this project, the drilling took from 4 to 6 hours. 

To learn more about the workplace, go to Section A in the Supporting Technical Information

Our Approach

We visited the work location over 3 days in September 2019. During our visits, we did the following 

activities: 

• Observed work processes and work practices. 

• Estimated the amount of water used to control dust during each day. 

• Collected air samples for respirable crystalline silica and respirable dust.

• Collected bulk samples of the slurry produced from drilling to determine its silica content.

• Used a portable weather station to record information about temperature, relative humidity, and 

wind speed.

To learn more about our methods, go to Section B in the Supporting Technical Information

Our Key Findings

Silica concentrations for some employees exceeded occupational exposure limits

• Concentrations of respirable crystalline silica were above the Occupational Safety and Health 

Administration (OSHA) regulatory limit on Day 2 of 3 of sampling for the drill rig operator. 

• Concentrations of respirable crystalline silica for the drill rig operator were above the American 

Conference of Governmental Industrial Hygienists (ACGIH®) recommended limit and the 
2

OSHA action level on Day 1 of 3 of sampling. The action level is when employers must take 

certain steps to further protect their employees. 

• Concentrations of respirable crystalline silica were below all relevant occupational exposure 

limits for the backhoe operator on all days.

Samples of respirable dust did not exceed any exposure limits 

• Concentrations of respirable dust were below relevant occupational exposure limits for all 

samples. 

Improvements to the respiratory protection program could be made

• The drill rig operator was required to wear a respirator while working, but there was no written 

respiratory protection program, medical clearance, or annual respiratory training. 

• The respirator worn by the drill rig operator was appropriate for controlling exposures to 

respirable crystalline silica.

• The respirator was put on and taken off correctly, cleaned after each shift, and stored 

appropriately.

To learn more about our results, go to Section B in the Supporting Technical Information

Our Recommendations

The Occupational Safety and Health Act requires employers to provide a safe workplace.

Benefits of Improving Workplace Health and Safety:

 Improved employee health and well-being  Enhanced image and reputation 

 Better workplace morale  Superior products, processes, and services

 Easier employee recruiting and retention  May increase overall cost savings

The recommendations below are based on the findings of our evaluation. For each recommendation, 

we list a series of actions you can take to address the issue at your workplace. The actions at the 

beginning of each list are preferable to the ones listed later. The list order is based on a well-accepted 

approach called the “hierarchy of controls.” The hierarchy of controls 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 practical, administrative 

measures and personal protective equipment might be needed. Read more about the hierarchy of 

controls at https://www.cdc.gov/niosh/topics/hierarchy/. 
3

We encourage the company to use a health and safety committee to discuss our 

recommendations and develop an action plan. Both employee representatives and 

management representatives should be included on the committee. Helpful guidance can be 

found in “Recommended Practices for Safety and Health Programs” at

https://www.osha.gov/shpguidelines/index.html. 

Recommendation 1: Improve the respiratory protection program

Why? The drill rig operator is required to wear a respirator while operating the drill rig. The drill rig 

operator was wearing an appropriate, protective respirator; also, the operator was comfortable 

wearing the respirator and knowledgeable about its care and storage.

However, there is no written respiratory protection program as is required by OSHA when respirator 

use is mandatory. Opportunities existed to create a strong respiratory protection program that 

ensures employees are protected from respirable crystalline silica while they are working. 

Concentrations of respirable crystalline silica were found to be above occupational exposure limits. 

Occupational exposures to respirable crystalline silica have been associated with silicosis, lung cancer, 

pulmonary tuberculosis disease, and other airway diseases. Exposure can happen through breathing 

dust in the air that contains silica.

How? At your workplace, we recommend these specific actions:

Create a written respiratory protection program. 

• Because the company performed dowel drilling more than 30 calendar days per year and 

required the drill rig operator to wear a respirator, a written program should be 

developed. 

• Respiratory protection programs should comply with the OSHA respiratory protection 

standard for construction 29 CFR 1926.103, which is identical to the general industry 

standard 29 CFR 1910.134. 

• Because roadways contain varying levels of crystalline silica, other projects could expose 

employees to similar, lower, or higher respirable crystalline silica levels. A mandatory 

respiratory protection program would ensure that all employees are appropriately 

protected from exposures to respirable crystalline silica, regardless of the project 

conditions.

• More information about respirable crystalline silica requirements can be found here: 

https://www.osha.gov/Publications/OSHA3681.pdf. The local OSHA consultation 

office may be able to help with complying with the respiratory protection and respirable 

crystalline silica standards. The local consultation program can be found here: 

https://www.osha.gov/html/RAmap.html. 
4

Require drill rig operators wearing respirators to receive medical 

clearance at least every 3 years. 

• Employees who are required to wear respirators must receive medical clearance to wear 

them. The OSHA respirable crystalline silica standard requires medical examinations for 

respiratory protection at least every 3 years (29 CFR 1926.1153). Fit testing, in addition 

to the medical clearance, is only required when the respirator is tight fitting. Loose 

fitting powered air purifying respirators, or PAPRs, worn by the drill rig operators, do 

not require fit testing.

• Medical examinations should be performed by a licensed healthcare professional familiar 

with occupational medicine. At a minimum, medical evaluations should cover 

information found in Appendix C of the OSHA respiratory protection standard: 

https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.134Appc. 

Train drill rig operators on proper respirator use. 

• Training must be comprehensive and offered at least annually to all employees who 

wear respirators. 

• Training should include putting on and removing the respirator, limitations on its use, 

and care and maintenance.

Improve respirator cleaning and storage practices. 

• Use alcohol-free respirator cleaning wipes to clean respirators before and after each 

shift. Cleaning and storage of PAPRs seemed appropriate, but alcohol-based wipes can 

degrade silicone and plastic components of respirators, reducing their service life. 

• Store clean and dry respirators in plastic bags inside the totes in the work truck. 

Explore alternative respirators.

• Suitable alternatives exist if the respirator an employee currently wears is uncomfortable 

or difficult to work with. For example, there are different types of loose fitting PAPRs 

that can incorporate a hard hat and an American National Standards Institute (ANSI) 

Z87.1-2015 compliant face shield. 

Recommendation 2: Address other health and safety issues we identified during 

our evaluation 

Why? A workplace can have multiple health hazards that cause employee illness or injury. Similar to 

the ones identified above, these hazards can potentially cause serious health symptoms, lower morale 

and quality of life for your employees, and possibly increase costs to your business. We saw the 

following potential issues at your workplace: 
5

• The drill rig got caked with slurry and dust during the shift and then was left to dry overnight.

• Employees were observed to be in the construction zone without high visibility apparel

because they failed to put on high visibility vests before leaving their equipment cabs. 

• Some employees were observed stepping into the live traffic lane while filling and covering 

patches.

• Vehicle spacing and logistical difficulties resulted in safety issues. Instead of waiting at a 

distance for work to finish in front of them, some employees were observed crowding the 

crew working in front of them—one incident resulted in a vehicle sustaining damage.

Although they were not the focus of our evaluation, these hazards could cause harm to your 

employees’ health and safety and should be addressed.

How? At your workplace, we recommend these specific actions:

Spray the drill rig with water at the end of each shift. 

• In the morning, dried dust can be resuspended in the air. Spraying the drill rig at the end 

of the shift will prevent this, decreasing dust exposure. 

Require high visibility safety apparel for all equipment operators. 

• Although wearing high visibility safety apparel is not required for the operation of 

equipment, always wearing it will ensure that employees do not forget to put it on when 

exiting the equipment. 

Explore alternatives to stepping into the live traffic lane.

• Consider having a separate flagger or spotter move with the crew for intermittent traffic 

control when filling the patches with cement and applying the tarps.

Improve vehicle spacing and safety within the work zone.

• Vehicle spacing and safety within the work zone can be improved. The following 

options (or a combination of them) would result in a safer work zone for all employees:

o Enforce appropriate spacing between the different tasks. Provide training and 

reminders that employees should wait for the crew ahead of them to finish 

working on the patch before moving forward. The following crew cannot begin 

their tasks until the crew in front of them is finished. Moving forward while a 
6

crew is still working puts unnecessary vehicles near the patch and can result in 

harm to employees and/or damage to equipment. 

o Stagger the start times for each crew. Instruct the crews to wait an additional 

amount of time before starting their work tasks to allow the crews ahead of them 

time to complete their work without interference.

o Identify the slow part of the process and add a second crew to help speed up the 

overall work. During our visit, the crew digging out the patches seemed to be the 

slow part of the process. Adding a second digging crew could speed up that 

process and keep the timing of the crews more even. 
Supporting Technical Information

Evaluation of Silica Exposures During Dowel Drilling 

HHE Report No. 2019-0178-3368

January 2020
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A-1

Section A: Workplace Information 

Workplace 

The work took place on a state road construction site. Weather permitting, work occurred during week 

days. The total length of the project varies from project to project and depends on the scope of the 

work.

Employee Information

Two employees participated in the evaluation and belonged to a union. During our visit, the crew 

worked a single shift that was about 5 hours long. The length of the shift could change depending on 

the scope and stage of the project. Employees did not take a lunch break and were observed eating 

between drilling activities. 

Process Description 

Roadway repair occurs in multiple steps. First, sections of the road were identified for repair. Each 

section was referred to as a “patch” in the road. After patch locations were identified, the edges of the 

patch were cut by a drivable concrete cutter, the asphalt was broken up with a jackhammer, and the 

patch was dug out by a backhoe. Once the patch was clear of debris, holes were drilled for the dowels. 

Dowels and rebar were inserted into the patch, and the patch was filled in with concrete. 

This evaluation focused on the dowel-drilling step. For each patch, the drill rig (Figure A1) would drill a 

maximum of 10 holes on each side (total of 20) of the patch. Some patches received less holes 

depending on the condition of the walls within each patch. There were two employees who performed 

the dowel-drilling step. One operated the backhoe, and one operated the drill rig. The drill rig operator 

stood in or on the edge of the patch when the drilling was happening (Figure A2). The backhoe 

operator remained in the cab of the backhoe during drilling.

When the drilling was complete for a patch, the team moved to the next patch to repeat the drilling 

process. If the next patch was not ready for the drilling team, they waited between the two patches until 

the patch was cleared. 
A-2

Figure A1. The entire backhoe and drill rig setup. 

The drills are mounted on the arm of the 

backhoe and the water control system is 

mounted on the front. The water control system 

is connected to the drills with tubing. Photo by 

the National Institute for Occupational Safety 

and Health (NIOSH).

Figure A2. Employee operating the drill rig. 

Photo by NIOSH.
B-1

Section B: Methods, Results, and Discussion

Methods: Observations of Work Processes, Practices, and Conditions 

We evaluated the following during our site visit: 

• Work processes

• Personal protective equipment (PPE) use 

• Workplace conditions

Results: Observations of Work Processes, Practices, and Conditions

• The crew drilled 301 holes on Day 1, 252 holes on Day 2, and 204 holes on Day 3. This work 

site required holes to be drilled 9 inches deep. The drill bits were Brunner and Lay, Inc., Model 

T1607LO heavy duty 1-5/8′′ diameter H-thread carbide rock bits. The bits were 3–5 days old, 

and we were informed by management that the bits lasted 7–10 days, depending on the 

pavement. The drill steels were Brunner and Lay, Inc., Model E11018H 18′′ H-thread steels. The 

shank size was 7/8′′ × 3/4′′. The steels were approximately 2 weeks old, and we were informed 

by management that the steels lasted about 30 days, depending on the pavement.

• The drill rig and water control were mounted on a John Deere® backhoe, Model 410E. The air 

compressor and water pump were mounted on the front of the backhoe, and the drill rig was 

mounted on the back of the backhoe. The drill rig was an EZ Drill Model 201-5 multi-gang 

concrete drill. The air compressor was a LeRoi, Model Q375 DCU. There were TeeJet® Model 

XR8002VS flat spray nozzles fed by a Fimco Model HFP-45060-113 High-Flo 4.5 gallons per 

minute water pump. The water was drawn from a 120-gallon drum of water, which was 

mounted on the backhoe. 

• We learned that the water drum was topped off approximately once per shift. The drill rig used 

55–60 gallons of water each day. 

• The drill rig operator wore a loose-fitting PAPR with P100 filters, a high visibility shirt, long 

pants, and boots. The backhoe operator wore long pants and boots. There was a high visibility 

shirt or jacket in the backhoe cab.

• Employees did not report that their PAPR was uncomfortable. We had discussions with 

management and employees while on site about how the belt and hose might get in the way of 

employees while they are working. 

• Drill rig operators were required to wear PAPRs while working, but we were told by 

management that there was no written respiratory protection program and that employees had 

not received medical clearance to wear respirators.
B-2

• The drill rig operator drove a work truck between patches and removed the PAPR prior to 

entering the truck. We observed that the employee was comfortable and did not show obvious 

signs of difficulty when putting on and taking off (donning and doffing) the PAPR. 

• The PAPR was stored inside a plastic tote in the work truck cab. We observed that the tote 

contained extra PAPR supplies (e.g., filters, air hoses, extra batteries). Employees reported that 

the respirator was cleaned at the end of each shift with an alcohol-based cleaning wipe before 

being stored inside the tote. 

• Dust that caked onto the drill rig at the end of the day appeared to cause more visible dust at the 

beginning of the next shift. We observed that the first use of the drill had a larger visible dust 

cloud than the other times of use. Suggestions that the visible cloud was caused by water vapor 

and cooler morning temperatures were reported, but the second use of the drill happened 

minutes after the first, and the cloud was not as large as the first use of the drill. Water vapor 

may have contributed to the visible cloud, but it was not consistent through the colder morning 

hours.

• Some machine operators were observed exiting the cab of the machine without wearing their 

high visibility safety apparel. Even if they were in the cab for most of the shift, they still needed

to be wearing the high visibility safety apparel when they exited the cab in the work zone. 

• The edges of the patches were close to the live traffic lane, and employees stepped into the live 

traffic lane when they were filling the patch with cement and when putting tarps over the 

patches. 

• Vehicle spacing and logistics difficulties resulted in safety issues. Instead of waiting at a distance 

for work to finish in front of them, some employees and equipment were observed crowding 

the crew working in front of it—one incident resulted in a vehicle sustaining damage.

Methods: Exposure Assessment

Air Sampling 

We collected air samples for respirable crystalline silica (RCS) from the breathing zone of two

employees on 3 days. We used three-piece, 37-millimeter diameter cassettes with 5-micrometer (μm) 

pore size polyvinyl chloride filters. We also used a Mesa Labs Model GK 2.69 high-flow personal 

sampling cyclone at a flow rate of 4.2 liters of air per minute. 

We analyzed each sample for respirable dust and RCS using NIOSH Methods 0600 and 7500 with a 

tetrahydrofuran preparation [NIOSH 2019]. 

Weather 

To capture weather parameters, we set up a Kestrel Instruments Model 4500 portable weather station, 

with data logging capabilities. The weather station logged information once per minute on a variety of 

parameters including wind speed, temperature, and relative humidity. We moved the weather station as 

the crew moved along the road during the workday.
B-3

Bulk Samples 

We collected bulk samples of slurry produced by drilling on each day. Our objective was to determine 

the silica concentration in the substrate that was being drilled and identify anything in those samples 

that would interfere with the analyses of the air samples. 

Results: Exposure Assessment

Air Sampling 

• The air sampling results are presented in Table C1. Air samples for RCS ranged 12 to 

93 micrograms per cubic meter of air (μg/m3) for the drill rig operator (Figure B1). On Day 1, 

the drill rig operator exceeded the ACGIH limit and the OSHA action level of 25 μg/m3. On 

Day 2, the drill rig operator exceeded the OSHA, NIOSH, and ACGIH limits of 50 μg/m3, 

50 μg/m3, and 25 μg/m3, respectively. On Day 3, air samples for RCS were lower than the 

OSHA, NIOSH, and ACGIH limits.

Figure B1. A bar graph of RCS concentrations compared to occupational exposure limits (OELs) for 

the drill rig operator. 

• We did not detect RCS in any of the air samples for the backhoe operator. The minimum 

detectable concentrations for RCS ranged 4 to 6 μg/m3. 

• Air samples for respirable dust ranged from not detected to 1,000 μg/m3. All air samples for 

respirable dust were lower than the OSHA and ACGIH limits of 5,000 μg/m3 and 3,000 μg/m3, 

respectively. 
B-4

Weather 

For the 3 days we sampled, average wind speed was 1.7 meters per second, average temperature was

67°F, and average relative humidity was 59%. 

Bulk Samples 

We collected three bulk dust samples of the slurry produced by the drill. The bulk samples contained 

10%–13% quartz (Table C2). 

Discussion 

RCS has been recognized as a carcinogen and steps should be taken to protect employees from 

potential exposures [IARC 1997, NIOSH 2002]. Some personal air samples we collected for RCS were

above the OSHA regulatory limit, OSHA action level, or ACGIH recommended limit. No respirable 

dust samples were higher than the OSHA or ACGIH limits. Based on our personal air sampling results

for RCS (Table C1) and using NIOSH respiratory selection criteria, the drill rig operator was wearing an 

appropriate respirator [NIOSH 2004]. The NIOSH assigned protection factor (APF) for a hood PAPR 

is 25. Multiplying the NIOSH APF of 25 by the OEL for RCS of 50 μg/m3 gives a maximum use 

concentration of 1,250 μg/m3. 

The respirator that the drill rig operator was required to wear is protective for the exposures that we 

measured during this evaluation. Additionally, the employee wearing the respirator appeared

comfortable wearing it and knowledgeable about its care and storage. Because the airborne exposures to 

RCS did not exceed the maximum use concentration of 1,250 μg/m3, a hood PAPR would also be 

sufficiently protective. Periodic training for employees required to wear respirators is important for 

continued proper use and maintenance of respirators.

Employees and management reported concerns that the belt and air hose of the PAPR could interfere 

with drill rig operation. We discussed the potential for alternative PAPR configurations and told them 

that one PAPR is not necessarily better than another. Employee comfort is an important factor with any 

PPE decision. Providing a more comfortable model of respirator (or other PPE) may increase

compliance with respiratory protection requirements. In other words, employees are more likely to wear 

correctly and care for PPE if they are comfortable. Appropriate wear and care of PPE is vital for

protection against contaminants like RCS. 

Different work sites from where this evaluation took place may have different conditions that could 

affect exposure levels. Furthermore, the amount of silica in the substrate (e.g., asphalt or concrete) is 

variable and likely will change from work site to work site, and potentially from patch to patch on the 

same work site. The existing practices around respiratory protection used by the drill rig operators are 

adequate to protect employees from RCS exposures. Some modifications to existing practices will allow 

the company to become compliant with regulatory requirements of the OSHA respiratory protection 

and RCS standards as well as best practices for cleaning and storage. 

Limitations 

This evaluation is subject to several limitations. Industrial hygiene sampling and an engineering control 

evaluation can only document exposures and conditions in the locations evaluated and on the days the 
B-5

evaluation occurred. These results may not be representative of conditions during other days or on 

other work sites. Additionally, the small size and homogenous nature of the population sampled limit 

the generalizability of our evaluation results.

Conclusions

Our air sampling showed that the drill rig operation led to concentrations of RCS above relevant 

occupational exposure limits; however, the operator was wearing appropriate respiratory protection. We 

recommended specific improvements that could reduce potential exposures and improve respirator use. 

These recommended improvements included developing a written respiratory protection program,

sending drill rig operators for medical respirator clearance, and providing employees training about 

respirator use and maintenance. We also recommended improved safety around other nonrelated 

hazards on the work site. 
C-1

Section C: Tables

Table C1. Personal air sample results for respirable dust and RCS exposures (μg/m3) 

Job title Sample duration 

(minutes)

Respirable dust Respirable 

crystalline silica*

Day 1 

Backhoe operator 302 59 ND

Drill rig operator 209† 780 44

Day 2 

Backhoe operator 277 86 ND

Drill rig operator 259 1,000 92

Day 3‡ 

Drill rig operator 306 200 13

OSHA permissible exposure limit 5,000 50

NIOSH recommended exposure limit — 50

ACGIH threshold limit value 3,000 25

ND = Not detected

* The minimum detectable concentrations for RCS for the backhoe operator ranged 4 to 6 μg/m3. 

† This sample lost approximately 90 minutes of sampling time throughout the shift. We suspect this 

occurred when the operator was sitting in the cab between drilling patches.

‡ The backhoe operator sample from Day 3 is not presented because there was a problem with the 

sampling pump.

Table C2. Percent silica detected in bulk samples of slurry 

collected from drill spoils

Day % Silica* 

1 13

2 10

3 12

* The limit of quantification was 0.83%.
D-1

Section D: Occupational Exposure Limits

NIOSH investigators refer to mandatory (legally enforceable) and recommended OELs for chemical, 

physical, and biological agents when evaluating workplace hazards. OELs have been developed by 

federal agencies and safety and health organizations to prevent adverse health effects from workplace 

exposures. Generally, OELs suggest levels of exposure that most employees may be exposed to for 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 if their exposures are maintained below these levels. Some 

may have adverse health effects because of individual susceptibility, a preexisting medical condition, or 

a hypersensitivity (allergy). In addition, some hazardous substances act in combination with other 

exposures, with the general environment, or with medications or personal habits of the employee to 

produce adverse health effects. Most OELs address airborne exposures, but some substances can be 

absorbed directly through the skin and mucous membranes.

Most OELs are expressed as a time-weighted average (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 short-term exposure limits (STEL) or ceiling values. Unless otherwise noted, the STEL is 

a 15-minute TWA exposure. It should not be exceeded at any time during a workday. The ceiling limit 

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; others are 

recommendations. 

• OSHA, an agency of the U.S. Department of Labor, publishes permissible exposure limits 

[29 CFR 1910 for general industry; 29 CFR 1926 for construction industry; and 29 CFR 1917 for 

maritime industry] called PELs. These legal limits are enforceable in workplaces covered under 

the Occupational Safety and Health Act of 1970. 

• NIOSH recommended exposure limits (RELs) are recommendations based on a critical review 

of the scientific and technical information and the adequacy of methods to identify and control 

the hazard. NIOSH RELs are published in the NIOSH Pocket Guide to Chemical Hazards [NIOSH 

2007]. NIOSH also recommends risk management practices (e.g., engineering controls, safe 

work practices, employee education/training, PPE, and exposure and medical monitoring) to 

minimize the risk of exposure and adverse health effects.

• Another set of OELs commonly used and cited in the United States include the threshold limit 

values or TLVs, which are recommended by the ACGIH. The ACGIH TLVs are developed by 

committee members of this professional organization from a review of the published, peerreviewed literature. TLVs are not consensus standards. They 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 2019].
D-2

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

include legal and recommended limits. The Institut für Arbeitsschutz der Deutschen Gesetzlichen 

Unfallversicherung (Institute for Occupational Safety and Health of the German Social Accident 

Insurance) maintains a database of international OELs from European Union member states, Canada 

(Québec), Japan, Switzerland, and the United States. The database, available at 

https://www.dguv.de/ifa/gestis/gestis-stoffdatenbank/index-2.jsp, contains international limits for 

more than 2,000 hazardous substances and is updated periodically. 

OSHA (Public Law 91-596) 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. This is true in 

the absence of a specific OEL. It also is important to keep in mind that OELs may not reflect current 

health-based information.

When multiple OELs exist for a substance or agent, NIOSH investigators generally encourage 

employers to use the lowest OEL when making risk assessment and risk management decisions.

Respirable Crystalline Silica

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 RCS (quartz and cristobalite) 

have been associated with silicosis, lung cancer, pulmonary tuberculosis disease and other airway 

diseases, kidney disease, and autoimmune disorders. 

Silicosis is an irreversible but preventable fibrotic disease of the lung caused by the deposition of fine 

crystalline silica particles in the lungs. Silicosis is caused by the inhalation and deposition of crystalline 

silica particles that are 10 μm or less in diameter. Particles 10 μm and smaller are considered respirable 

particles and have the potential to reach the lower portions of the human lung (alveolar region). 

Although particle sizes 10 μm and smaller 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 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, the International Agency for Research 

on Cancer (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]. Several other serious diseases from occupational exposure to crystalline silica 

include lung cancer and noncarcinogenic disorders such as immunologic disorders and autoimmune 

diseases, rheumatoid arthritis, renal diseases, and an increased risk of developing tuberculosis disease 

after exposure to the infectious agent [NIOSH 2002]. 
D-3

When proper practices are not followed or controls are not maintained, RCS exposures can exceed the 

OSHA PEL, NIOSH REL, or the ACGIH TLV. The OSHA PEL and NIOSH REL for RCS are both

50 μg/m3 [NIOSH 2007, OSHA 2016]. The ACGIH TLV for quartz is 25 μg/m3 as an 8-hour TWA 

[ACGIH 2019]. The OSHA action level for RCS is 25 μg/m3 [OSHA 2016]. An action level is the level 

at which the employer must provide periodic medical surveillance and air monitoring.
E-1

Section E: References 

Discussion

IARC [1997]. IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans. 

Vol. 68. Lyons, France: World Health Organization, International Agency for Research on Cancer, 

https://monographs.iarc.fr/wp-content/uploads/2018/06/mono68.pdf. 

NIOSH [2002]. NIOSH hazard review: health effects of occupational exposure to respirable crystalline 

silica. 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. 2002-129, https://www.cdc.gov/niosh/docs/2002-129/default.html. 

NIOSH [2004]. NIOSH respirator selection logic. Bollinger N. 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. 2005-100, 

https://doi.org/10.26616/nioshpub2005100. 

Methods 

NIOSH [2019]. NIOSH manual of analytical methods (NMAM). 5th ed. O’Connor PF, Ashley K, eds. 

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. 

2014-151, https://www.cdc.gov/niosh/nmam. 

Occupational Exposure Limits 

ACGIH [2019]. 2019 TLVs® and BEIs®: threshold limit values for chemical substances and physical 

agents and biological exposure indices. Cincinnati, OH: American Conference of Governmental 

Industrial Hygienists. 

CFR [2019]. Code of Federal Regulations. Washington, DC: U.S. Government Printing Office, Office 

of the Federal Register, https://www.ecfr.gov/cgi-bin/ECFR?page=browse. 

NIOSH [2007]. 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, https://www.cdc.gov/niosh/npg/. 

Respirable Crystalline Silica 

Hinds WC [1999]. Aerosol technology: properties, behavior, and measurement of airborne particles. 

2nd ed. New York: John Wiley & Sons, Inc. 

IARC [1997]. IARC monographs on the evaluation of the carcinogenic risk of chemicals to humans. 

Vol. 68. Lyons, France: World Health Organization, International Agency for Research on Cancer, 

https://monographs.iarc.fr/wp-content/uploads/2018/06/mono68.pdf. 
E-2

NIOSH [1986]. Occupational respiratory diseases. Merchant JA, Boehlecke BA, Taylor G, PicketHarner M, eds. 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. 

1986-102, http://www.cdc.gov/niosh/docs/86-102/86-102.pdf. 

NIOSH [2002]. NIOSH hazard review: health effects of occupational exposure to respirable crystalline 

silica. 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. 2002-129, https://www.cdc.gov/niosh/docs/2002-129/default.html. 

OSHA [2016]. Silica 29 CFR 1926.1153. Washington, DC: U.S. Department of Labor, Occupational 

Safety and Health Administration, https://www.osha.gov/lawsregs/regulations/standardnumber/1926/1926.1153. 
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