Evaluation of the Approach to Respirable Quartz Exposure

Control in U.S. Coal Mines

Gerald J. Joy

National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research,

Pittsburgh, Pennsylvania

Correspondence to: Gerald J. Joy, National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research, 

P.O. Box 18070, NIOSH – OMSHR, Pittsburgh, PA 15236; e-mail: 

gjoy@cdc.gov. 

The findings and conclusions in this report are those of the author, 

and do not necessarily represent the views of the National Institute 

for Occupational Safety and Health. 

Occupational exposure to high levels of respirable quartz 

can result in respiratory and other diseases in humans. The 

Mine Safety and Health Adminstration (MSHA) regulates exposure to respirable quartz in coal mines indirectly through 

reductions in the respirable coal mine dust exposure limit based 

on the content of quartz in the airborne respirable dust. This 

reduction is implemented when the quartz content of airborne 

respirable dust exceeds 5% by weight. The intent of this dust 

standard reduction is to restrict miners’ exposure to respirable 

quartz to a time-weighted average concentration of 100 μg/m3

. 

The effectiveness of this indirect approach to control quartz 

exposure was evaluated by analyzing respirable dust samples 

collected by MSHA inspectors from 1995 through 2008. The 

performance of the current regulatory approach was found to 

be lacking due to the use of a variable property—quartz content 

in airborne dust—to establish a standard for subsequent exposures. In one situation, 11.7% (4370/37,346) of samples that 

were below the applicable respirable coal mine dust exposure 

limit exceeded 100 μg/m3 quartz. In a second situation, 4.4% 

(895/20,560) of samples with 5% or less quartz content in the 

airborne respirable dust exceeded 100 μg/m3 quartz. In these 

two situations, the samples exceeding 100 μg/m3 quartz were 

not subject to any potential compliance action. Therefore, the 

current respirable quartz exposure control approach does not 

reliably maintain miner exposure below 100 μg/m3 quartz. 

A separate and specific respirable quartz exposure standard 

may improve control of coal miners’ occupational exposure to 

respirable quartz. 

Keywords coal mining, exposure, resipirable quartz 

INTRODUCTION

I

nhalation of respirable quartz can cause disease in humans. Silicosis, a pulmonary fibrotic condition, is the most 

commonly recognized result of overexposure to respirable 

quartz.(1,2) Chronic obstructive pulmonary disease (COPD) 

and lung cancer are also elevated among quartz-exposed workers.(2–4) Non-respiratory disease has also been associated with 

respirable quartz exposure, including scleroderma, rheumatoid 

arthritis, and renal disease.(5–7) 

Miners, including coal miners, are occupationally exposed 

to respirable quartz during the extraction and processing of 

ore.(8–10) In coal miners, exposure to respirable quartz occurs 

when rock above, below, or within the coal seam is disturbed 

as the coal is extracted. Exposure can also occur when ground 

control methods are implemented in underground coal mines 

(e.g., installation of roof bolts) and when drill holes for exploration or placement of blasting agents are installed. 

Coal miners’ occupational exposure to respirable coal mine 

dust (RCMD) with or without respirable quartz is regulated by 

the Mine Safety and Health Administration (MSHA). MSHA 

inspectors collect air samples to evaluate exposure in relation to the respirable dust standard (RDS), to assess the adequacy of the operator’s dust control practices, and for other 

purposes. 

The RDS for RCMD is an 8-hr time-weighted average 

(TWA) concentration of 2.0 mg/m3

. Although not explicitly 

stated, the exposure limit for respirable quartz is essentially 

a TWA concentration of 100 μg/m3

. MSHA controls quartz 

exposure indirectly by reducing the 2.0 mg/m3 RCMD RDS 

when the content of quartz in airborne RCMD exceeds 5% 

by weight. The reduction of the RDS is made by the following formula: applicable RCMD RDS (mg/m3

) = 10/percent 

quartz.(11) This relationship results in RCMD respirable dust 

standards that range from 2.0 mg/m3 when 5% or less quartz 

is present, to 0.1 mg/m3 at 100% quartz concentration. 

When RCMD samples collected by MSHA inspectors exceed the minimum weight gain of 100 μg specified in the 

MSHA quartz analytical procedure, the sample is analyzed 

for quartz content using a Fourier-transform infrared method 

(MSHA Method P7).(12) 

The reduced RCMD RDS becomes the respirable dust 

exposure limit for the section(s) of the mine represented by 

the sample data. Different sections within a single mine may 
have differing RCMD RDSs at a given time, based on the 

samples collected within those sections. 

Mine operators may optionally submit their own samples 

for evaluation of quartz content if they believe the inspector’s 

samples are not representative. Further details of the coal mine 

dust inspection process are available from MSHA.(13) 

MSHA’s respirable dust sampling convention is based on 

methods established by the British Medical Research Council (BMRC) using a horizontal elutriator.(14) To match the 

sampling characteristics of the elutriator, MSHA uses a DorrOliver 10-mm nylon cyclone (Zefon International, Ocala, Fla.) 

at a flow rate of 2.0 L/min; the resulting dust concentration is 

multiplied by a constant factor of 1.38.(15) Results from this 

method differ from International Organization for Standardization (ISO)-based approaches, and conversion factors must 

be applied if data from the differing sampling methods are 

to be compared.(16) This article uses MSHA sampling data 

within MSHA’s regulatory framework; therefore, no conversion of the MSHA dust concentration to the ISO basis is 

necessary. 

For the current RCMD RDS reduction approach to control 

quartz to a 100 μg/m3 level, two conditions must be assumed: 

(1) the quartz content of the airborne respirable dust will not 

increase as mining progresses, and (2) the RCMD concentration will not exceed the applicable RDS. This article examines 

the validity of the two stated assumptions using data collected 

by MSHA inspectors. 

METHODS

MSHA inspector-collected respirable dust samples of underground coal occupations from 1995 through 2008 

were retrieved from the MSHA Standardized Information System (MSIS) Samples database.(17) Mine operators also collect 

RCMD samples, although very few of these samples are evaluated by MSHA for quartz content except when in association 

with establishment of a reduced RCMD RDS as described 

above. Consequently, these operator samples are not actually 

used to evaluate miner exposure and are not included in this 

analysis. Samples that had been voided for any reason or that 

lacked a value for dust concentration, dust standard, quartz 

content, sample type, sample date, or occupation code were 

excluded from the data set. 

Samples in the data set were classified by type of MSHAassigned entity classification into two groups. The first group 

includes samples of designated occupations (DO) and nondesignated occupations (NDO); this subset, labeled the Occupations group, included 38,038 samples. The second group 

comprises roof bolter-designated area (RBDA) samples; this 

group included 8220 samples. 

A DO is the occupation on a mechanized mining unit 

(MMU) that is expected to have the highest respirable dust 

exposure.(18) An MMU is the organizational work group that 

extracts coal. NDOs are any occupations on an MMU except 

the DO. A designated area is an area identified by the mine 

operator where RCMD exposure samples must be collected 

bimonthly. The RBDA is known to have an elevated potential 

for exposure to respirable quartz and is included in this analysis 

for that reason.(19) 

Samples were further classified by whether the mine section 

was on a 2.0 mg/m3 RDS or a reduced standard and by the 

quartz content of the sample. 

The grouped samples were examined to identify how many 

exceeded 100 μg/m3 respirable quartz concentration. 

RESULTS

Analysis of the MSHA inspector-collected RCMD sample 

data demonstrates that the existing indirect approach of 

controlling coal miners’ exposure to airborne respirable quartz 

through regulation of respirable coal mine dust may be less 

protective than intended. 

Table I presents data on samples with a dust concentration 

less than or equal to the applicable RCMD RDS. If the current 

quartz exposure control approach was effective, few or no 

samples in Table I would be expected to exceed 100 μg/m3 

quartz. However, it can be seen that 21.6% of the samples 

with a quartz content of >5% exceeded 100 μg/m3 quartz 

TABLE I. Samples with RCMD Concentration ≤ Applicable Respirable Dust Standard

≤5% Quartz Content >5% Quartz Content

RCMD RDS Total N

N > 100

μg/m3

% >100

μg/m3 Total N

N > 100

μg/m3

% >100

μg/m3

Occupations 

Group 

Roof Bolter DAA 

Group 

2 mg/m3 

<2 mg/m3 

All 

2 mg/m3 

<2 mg/m3 

All 

13,956 

1336 

15,292 

1344 

517 

1861 

0 

0 

0 

0 

0 

0 

0% 

0% 

0% 

0% 

0% 

0% 

10,135 

5211 

15,346 

1376 

3471 

4847 

2,751 

942 

3693 

243 

434 

677 

27.1% 

18.1% 

24.1% 

17.7% 

12.5% 

14.0% 

Total 17,153 0 0% 20,193 4370 21.6% 

ADA = designated area. 
concentration. For these samples, the RCMD RDS is not low 

enough to control respirable quartz exposure to 100 μg/m3 or 

less. The basis of this problem is that the applicable RCMD 

RDS, which if not the default unreduced RCMD standard of 

2.0 mg/m3

, was established based on earlier sampling results 

that do not reflect the quartz concentration encountered during 

subsequent mining. This can occur, for example, because the 

geology changed as mining progressed and more (or higher 

content) quartz-bearing rock is being extracted. 

Samples exceeding 100 μg/m3 are mathematically prohibited when the RCMD concentration is less than or equal to the 

RDS (a maximum of 2.0 mg/m3

) and the quartz content does 

not exceed 5%. 

All samples in Table II exceed the applicable RCMD RDS 

and are subject to potential compliance actions on that basis. 

However, Table II samples that contain 5% or less quartz are 

excluded from the regulatory framework intended to control 

respirable quartz exposure due to their low quartz content, but 

26.3% of these samples exceed 100 μg/m3

. 

Of the Table II samples that contain more than 5% quartz, 

85% exceed the 100 μg/m3 respirable quartz level. For the 

15% of samples that do not exceed 100 μg/m3

, the reduced 

RDS was low enough to control the quartz exposure, possibly 

due to the quartz content of the extracted material falling after 

the reduced RDS was established. 

TABLE II. Samples with RCMD Concentration > Applicable Respirable Dust Standard

≤5% Quartz Content >5% Quartz Content

RCMD RDS Total N

N > 100 % >100

μg/m3 μg/m3 (%) Total N

N > 100 % >100

μg/m3 μg/m3 (%)

Occupations 2 mg/m3 2350 691 29.4 1654 1654 100.0 

Group < 3 2 mg/m 705 134 19.0 2691 2154 80.0 

All 3055 825 27.0 4345 3808 87.6 

Roof Bolter DA 2 mg/m3 152 41 27.0 105 105 100.0 

Group < 3 2 mg/m 200 29 14.5 1055 767 72.7 

All 352 70 19.9 1160 872 75.2 

Total 3407 895 26.3 5505 4680 85.0 

DISCUSSION

The data presented show that the current MSHA approach 

to regulating miner exposure to respirable quartz does not 

protect miners from excessive exposure to respirable quartz in 

all cases. Specific situations where this occurs include when the 

quartz content of the airborne dust increases due to changes in 

geologic conditions, i.e., more rock, or rock with higher quartz 

content must be extracted, and when the RCMD concentration 

exceeds the applicable RDS. In addition, if the mine operator 

submits optional samples for quartz analysis, the process for 

reducing the RDS may be extended by several weeks. During 

this period, RCMD exposure is regulated at the pre-existing 

dust standard, which may not be protective. 

To assess the effectiveness of an occupational health exposure standard, the most appropriate metric is the extent of 

reduction of the adverse effect that it guards against. With 

occupational illnesses in general and pneumoconioses in particular, the long disease latency makes it difficult to make 

this assessment. However, if compliance with an occupational 

exposure limit (OEL) is expected to be protective against 

a specified health outcome, then evaluating the degree of 

compliance with the OEL may be a reasonable surrogate for 

effectiveness in the short term. When, as in this case, the 

exposure standard is only partially protective under conditions 

that can feasibly be encountered, the effectiveness of the OEL 

becomes very difficult to evaluate until enough time has passed 

to observe cases of disease. 

Adoption of an independent and specific standard for airborne respirable quartz exposure in coal mining has been 

recommended by NIOSH, and by the Secretary of Labor’s 

Advisory Committee on the Elimination of Pneumoconiosis 

among Coal Mine Workers.(20,21) A separate respirable quartz 

standard, as described by NIOSH,(20) could reduce miners’ 

risk of overexposure to respirable quartz and, by extension, 

their risk of developing silicosis. Such a standard may include 

the collection of personal, full-shift samples as frequently as 

necessary to ensure that miners’ exposure is maintained below 

the exposure limit. Also, assessment of each sample against 

a defined, fixed criterion would encompass more exposure 

conditions without reliance on prior sample results and would 

permit MSHA to focus additional resources on a currently 

unregulated exposure condition. 

This analysis has not addressed the adequacy of the 100 

μg/m3 quartz exposure standard but only whether the exposure 

assessment component of the existing respirable quartz control 

strategy was competent to consistently define quartz exposure 

in relation to the permissible exposure level. The protection 

afforded by the current 100 μg/m3 respirable quartz exposure 

standard is a fundamental consideration of any program to 

control pneumoconiosis and other adverse health effects in 

coal miners but is beyond the scope of this report. 
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