Evaluation of Exposure 

to Radon Progeny 

During Closure of 

Inactive Uranium Mines 

– Colorado

Robert D. Daniels, PhD, CHP

David C. Sylvain, MS, CIH

Health Hazard Evaluation Report

HETA 2011-0090-3161

July 2012 

DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

Workplace

Safety and Health

 National Institute for Occupational 

Safety and Health
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 2011-0090-3161 Page i

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

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

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

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

Assessment.....................................................................................................2

Results..............................................................................................................4

Discussion.................................................................................................... 10

Conclusions................................................................................................. 14

Recommendations.................................................................................... 14

References.................................................................................................... 18

Contents

Acknowledgments Acknowledgments and Availability of Report................................ 35

Appendix Occupational Exposure Limits and Health Effects......................... 19
Page ii Health Hazard Evaluation Report 2011-0090-3161

Abbreviations

ACGIH® American Conference of Governmental Industrial Hygienists

ALARA As low as reasonably achievable

ALI Annual limit on intake

Bq Becquerel

CFR Code of Federal Regulations

CI Confidence interval

Ci Curie

CWLM Continuous working level monitor

DCF Dose conversion factor

DOE Department of Energy

EEC Equilibrium equivalent concentration

EPA Environmental Protection Agency

Feq Equilibrium factor

Gy Gray

h–1 Per hour

HHE Health hazard evaluation

ICRP International Commission on Radiological Protection

J Joules

L–1 Per liter

m–3 Per cubic meter

MeV Mega electron volt (106 eV)

MSHA Mine Safety and Health Administration

NAICS North American Industry Classification System

NCRP National Council on Radiation Protection and Measurements

NIOSH National Institute for Occupational Safety and Health

NRC Nuclear Regulatory Commission

OEL Occupational Exposure Limit

OSHA Occupational Safety and Health Administration

PAEC Potential alpha energy concentration

PEL Permissible exposure limit

PPE Personal protective equipment

REL Recommended exposure limit

Sv Sievert

TLV® Threshold limit value

UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation

WL Working level

WLM–1 Per working level month

WLM Working level month

y–1 Per year
Health Hazard Evaluation Report 2011-0090-3161 Page iii

The National Institute 

for Occupational Safety 

and Health (NIOSH) 

received a request for a 

health hazard evaluation 

at abandoned uranium 

mines in several western 

states. Managers of a 

federal agency requested 

assistance in evaluating 

employee exposures to 

radon while constructing 

mine closures.

Highlights of the

NIOSH Health

Hazard Evaluation

What NIOSH Did

● We evaluated mines in Colorado and Utah in September 2011.

● We measured radon levels at several mine openings.

● We measured gamma radiation dose rates on the surface of 

an abandoned ore and waste rock pile.

● We observed the construction of a stone and mortar closure.

● We reviewed past exposure data collected by the state inactive 

mine reclamation program in Colorado.

What NIOSH Found

● Workplace radon concentrations were affected by changing 

environmental conditions.

● The potential for workplace exposures to radon was low. 

Controls were needed in some instances to keep exposures as 

low as reasonably achievable. 

● Gamma radiation was measureable at the surface of the 

waste rock pile.

What Managers Can Do

● Inform employees about radiation hazards in the workplace.

● Adopt reference levels that would prompt the use of control 

measures.

● Improve environmental monitoring that is done pre-bid. This 

will allow for more accurate estimates of potential exposures. 

● Use fans to provide dilution ventilation at mine openings.

● Construct temporary radon barriers at mine openings.

● Plan work before arriving at the worksite. This will minimize 

the amount of time spent working near mine openings. 

● Provide respiratory protection to employees. Respirators 

can be used to control exposures when engineering and 

administrative controls are not sufficient.

What Employees Can Do

● Learn about potential exposures and hazards at mine sites.

● Use temporary barriers, dilution ventilation, and good work 

practices to minimize your exposure to radiation hazards.
Page iv Health Hazard Evaluation Report 2011-0090-3161

Highlights of the

NIOSH Health

Hazard Evaluation

 (continued)

 

● Plan work before arriving at the worksite so you spend less 

time working near mine openings.

● Participate in training offered by your employer.

● Use the appropriate personal protective equipment when 

recommended.
Health Hazard Evaluation Report 2011-0090-3161 Page v

NIOSH evaluated 

employee exposures 

to radon progeny and 

gamma radiation during 

the construction of mine 

closures. We found that 

exposures were generally 

low, but were influenced 

by environmental 

conditions. NIOSH 

measurements were 

similar to those obtained 

by the state inactive mine 

reclamation program. 

Recommendations for the 

use of simple engineering 

controls and respiratory 

protection are made in the 

report to keep exposures 

ALARA.

Summary

In June 2011, NIOSH received an HHE request from managers 

of a federal agency in Colorado. NIOSH was asked to evaluate 

employees’ exposure to ionizing radiation hazards during 

construction of various types of closures at abandoned uranium 

mines. The primary health concern at these sites involved 

inhalation of naturally occurring short-lived radon progeny (i.e., 

polonium-218, lead-214, bismuth-214, and polonium-214) at mine 

entrances (adits). Also of concern, but to a lesser extent, was 

exposure to gamma radiation emitted from mine waste and nearby 

geological formations.

On September 12–15, 2011, we visited several abandoned mines 

on Wedding Bell Mountain in southwest Colorado and the 

Vanadium Queen mine in Utah. We observed the construction of 

a native stone and mortar closure on Wedding Bell Mountain. We 

also conducted continuous monitoring of radon progeny at several 

mine openings at Wedding Bell Mountain and at the Vanadium 

Queen mine. We reviewed the state inactive mine reclamation 

program’s pre-bid radon monitoring protocol.

Monitoring results and onsite observations suggest that employee 

exposures to radon during mine closure activities are generally 

low. However, radon concentrations at mine openings are greatly 

affected by changing environmental conditions such as wind velocity, 

moisture, and barometric pressure. Results of NIOSH exposure 

monitoring did not exceed the average pre-bid PAEC values obtained 

during previous monitoring by state inactive mine reclamation 

program staff. Nevertheless, PAEC results from the CWLMs varied 

widely over the sampling period because of constant fluctuations in 

ventilation patterns. Given this variability, it is unlikely that shortterm sampling, as conducted by state inactive mine reclamation 

program staff, is sufficient to derive long-term average concentrations 

that form the basis of protective actions.

Control measures are needed in some instances to keep exposures 

ALARA. Gamma radiation is likely to be measureable at the 

surface of waste rock piles near mine adits. Occupancy to these 

areas should be limited to minimize exposures to radon. The use of 

simple engineering controls (e.g., barriers, ventilation), along with 

the use of respiratory protection when needed, are recommended 

to keep radon exposures ALARA.

Keywords: NAICS 924120 (Administration of Conservation 

Programs), radon, radon progeny, gamma radiation, inactive mine 

reclamation, uranium mines 
Page vi Health Hazard Evaluation Report 2011-0090-3161

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Health Hazard Evaluation Report 2011-0090-3161 Page 1

Introduction

In June 2011, NIOSH received an HHE request from the managers 

of a federal agency. NIOSH was asked to evaluate employee 

exposure to ionizing radiation hazards during construction 

of various types of closures at abandoned uranium mines on 

federal land. The primary health concern at these sites involves 

inhalation of naturally occurring short-lived radon progeny (i.e., 

polonium-218, lead-214, bismuth-214, and polonium-214) at mine 

entrances (adits), and, to a lesser extent, exposure to gamma 

radiation emitted from mine waste and geological formations. 

NIOSH visited several mines on Wedding Bell Mountain in 

southwest Colorado on September 12–13, 2011, and the Vanadium 

Queen mine in Utah on September 14, 2011. This report 

summarizes the findings of the site visit and our assessment of the 

radiological hazards at the mines.

Background

Uranium exploration, mining, and ore processing in the United 

States have resulted in a number of abandoned sites that present 

significant occupational and environmental hazards. Efforts are 

ongoing to mitigate these hazards. One such effort is the safe 

closure of abandoned underground uranium mines, which entails 

installation of permanent barriers at mine openings to prevent the 

public from entering the mines. The mines are principally located 

in remote areas of the Four Corners region of Colorado, Arizona, 

New Mexico, and Utah. Mine closure activities present numerous 

physical hazards that are exacerbated by the rugged terrain and 

isolation at mine locations. Primary hazards include (1) construction 

hazards associated with trenching, excavation, and heavy equipment 

operations; (2) environmental hazards such as inclement weather, 

mountainous terrain (e.g., falling rock, elevated work), and wildlife; 

and (3) hazards associated with isolation from communications, 

utilities, emergency response, and medical services.

Additionally, employees are exposed to ionizing radiation from 

technologically enhanced naturally occurring radioactive material, 

comprised of primordial naturally-occurring radioactive elements, 

such as radium, uranium, and thorium and their radioactive decay 

products that have been concentrated or made environmentally 

accessible during mining activities. In general, radiation-related 

occupational hazards at mine closure sites are due to inhalation 

of radon gas and its short-lived decay products that emanate from 

mine openings and, to a lesser extent, exposure to elevated levels of 

gamma radiation where radium is present.
Page 2 Health Hazard Evaluation Report 2011-0090-3161

Introduction

 (continued) Abandoned mines that are subject to closure no longer have 

operational ventilation systems; therefore, mine atmospheres may 

contain radon progeny in much higher concentrations than are 

found in operating mines. Although airborne radioactivity is likely 

to be less at mine openings because of fresh air mixing, employees 

working near the adits and other openings could be exposed to 

significant levels of contaminated air from outcasting (exhaling) 

mine atmospheres. For this reason, radon hazards to employees 

supporting mine reclamation were evaluated in this HHE. These 

employees do not make mine entries; therefore, this evaluation did 

not consider the risk to personnel (employees or the public) who 

enter and occupy mine spaces.

.

Assessment Review of Prior State Radiation 

Monitoring 

As an integral part of the Wedding Bell Mine Safety Closure 

Project, staff from the state inactive mine reclamation program 

measured PAEC at least twice at each mine opening that was 

identified on the bid schedule. The first measurement (“pre-bid” 

sample) was performed to gather information necessary for the 

development of construction specifications that are included in 

the contract solicitation. A second PAEC measurement was made 

in the presence of the contractor after the contract was awarded, 

but during pre-construction activities. The selected contractor is 

instructed to use the higher of the two (or more) samples in the 

planning of work activities. All measurements are compiled from 

data collected during 15-minute short-term samples using the 

Bladewerx SabreAlert2TM (Bladewerx, Rio Rancho, New Mexico).

State inactive mine reclamation program staff also conducted 

gamma radiation surveys using a Delta Epsilon SC-133 (Delta 

Epsilon Instruments, Inc., Grand Junction, Colorado) handheld 

portable scintillometer (gamma dose rate meter) at the Wedding 

Bell Mountain sites. Data on 45 survey points at various work 

locations at Wedding Bell Mountain were compiled for the active 

work under observation.

NIOSH Radiation Monitoring

We conducted continuous radon progeny monitoring at several 

mine openings at Wedding Bell Mountain on September 12–13, 
Health Hazard Evaluation Report 2011-0090-3161 Page 3

Assessment

 (cointinued) 2011, and at the Vanadium Queen mine in southeast Utah on 

September 14, 2011. Although not a part of the Wedding Bell 

Mine Safety Closure Project, the Vanadium Queen mine was 

selected because of its proximity to the Fire Fly mine, which was 

referenced in a draft report that measured high airborne alpha 

radiation concentrations inside and near abandoned uranium 

mines [Duraski 2010]. The primary instrument used by NIOSH 

was a model 597-PX3 Continuous Working Level Monitor 

(alphaNUCLEAR, Saskatchewan, Canada), which was specifically 

manufactured for the Canadian uranium mining industry. 

Measurements were also obtained with the Bladewerx SabreAlert2, 

which is the current instrument used by the state inactive mine 

reclamation program. Each monitor was factory calibrated within 

6 months of use; however, only the 597-PX3 was tested in a radon 

chamber. Both instruments are fully-contained, battery-powered, 

microprocessor-based, continuous-area monitors that incorporate 

an air pump, sample filter, and radioactivity counting system 

to calculate individual radon progeny concentrations via alpha 

spectral analysis. Both monitors have data logging capability 

and provide for short-term (acute), and long-term (chronic) WL 

measurements. Short-term measurements are less precise than 

long-term; however, shorter sampling intervals allow for quicker 

responses to transient airborne concentrations that are common 

to mine atmospheres. The monitors were placed in the work areas 

as close to the mine opening as possible. Smoke tubes were used to 

visualize air movement at the monitoring site and other workplace 

locations. On the basis of the observed work practices, occupancy, 

and ventilation patterns, continuous area monitoring was likely to 

overestimate worker exposures.

Gamma radiation surveys were conducted by NIOSH and 

state inactive mine reclamation program staff using the SC-133 

handheld portable scintillometer manufactured by Delta Epsilon 

Instruments, Inc.
Page 4 Health Hazard Evaluation Report 2011-0090-3161

Results

Review of Prior State Radiation 

Monitoring

Potential Alpha Energy Concentration

Forty-six pre-bid radon monitoring results were reported for the 

closure activities identified in the current bid schedule. Sample 

results were available for 40 adits, 4 shafts, and 2 vents. The sample 

distribution was highly right-skewed, with arithmetic mean, median, 

and 95th percentile values of 0.31 WL, 0.04 WL, and 1.5 WL. The 

maximum recorded PAEC value was 4.2 WL at a small vent (897 

V1) that was scheduled for a polyurethane foam closure. The adit 

PAEC ranged from 0.005 to 1.55 WL with mean and median values 

of 0.24 and 0.05 WL. Twelve (30%) short-term samples exceeded 

0.1 WL, of which all but one were assigned to an adit.

Gamma Radiation 

Prior monitoring at the Wedding Bell Mountain sites with 

a handheld portable scintillometer (gamma dose rate meter) 

indicated dose rates ranging from 0.13 to 4.8 μSv∙h–1 with a 

mean dose rate of 1.0 μSv∙h–1 (95% CI: 0.8–1.3) from all sources 

including background.

NIOSH Radiation Monitoring 

Potential Alpha Energy Concentration

Average PAEC values at Wedding Bell Mountain mine locations 

were within the range of expected values on the basis of data from 

the state inactive mine reclamation program pre-bid short-term 

samples (Table 1). The highest Wedding Bell Mountain mine 

progeny concentrations were observed at Adit 908A3, where 

transient (i.e., short-term) WL concentrations exceeded 1.0 WL 

during a period of rapidly changing weather conditions. During 

this time, long-term concentrations did not exceed 0.5 WL. In 

contrast, transient PAEC concentrations in excess of 10 WL were 

found at one of two Utah Vanadium Queen mine adits (VQ1). 

The highest concentrations were obtained approximately 0.5 

meter above the mine floor, which was completely submerged 

with groundwater at the time of sampling. The other opening 

(VQ2) was dry, although both mine entrances were believed to be 

interconnected.
Health Hazard Evaluation Report 2011-0090-3161 Page 5

Results

 (continued)

Table 1. PAECs from continuous monitoring

Date Location Pre-bid WL

597-PX3 SabreAlert2 WL

Avg. WL 

(range)* Run hours WL (range)* Run hours

9-12-2011 908A5 0.007 NM† NM 0.0003 (0.0–0.0008) 0.33

9-12-2011 907S1 NM 0.01 (0.0–0.03) 1.1 0.004 (0.0–0.012) 1.1

9-13-2011 896A12 0.0084 NM NM 0.003 (0.0–0.010) 0.75

9-13-2011 908A3 0.989 0.25 (0.0–1.1) 5.7 0.136 (0.04–0.31) 1.0

9-14-2011 VQ1 NM 6.98 (0–14.38) 1.0 0.431 (0.003–1.265) 1.0

9-14-2011 VQ2 NM 0.91 (0.1–3.25) 2.7 0.078 (0.0–0.984) 2.7

*Values are from “short-term” or “acute” WL measurement algorithms.

†NM=not measured

Although radon gas measurements were not taken, both CWLMs 

calculated Feq values from progeny results. These fractions typically 

ranged between 0.2 and 0.7; average values for the SabreAlert2 

were 0.3 and for the 597-PX3 were 0.5. Although Feq values were 

in reasonable agreement with recommended values of 0.4 for 

indoor air and 0.7 for outdoors [ICRP 1993], the actual algorithms 

used by the CWLMs to determine Feq were not made available, 

thus the validity of these estimates could not be determined. 

Differences in algorithms may partly explain measurement 

inconsistencies between monitor types.

Wedding Bell Mountain Adit 908A3

Continuous PAEC monitoring using both monitors was conducted 

at Adit 908A3 on September 13, 2011 (Table 1). This adit closure 

had not been initiated at the time of sampling, and the planned 

closure method requires the installation of a corrugated steel pipe 

and bat gate. The final pre-bid WL for this opening was 0.989 WL, 

which was taken during observed outflow.

The 597-PX3 was operated from 11:18 to 16:58 hours (5.7 hours). 

The SabreAlert2 was operated from 11:34 to 12:32 (about 1.0 

hour). As shown in Figure 1, the 597-PX3 was placed near the side 

of the adit at a height of about one meter, while the SabreAlert2 

monitor was placed at ground level at the center of the opening.

The weather conditions at the beginning of the sampling period 

were sunny and clear with a mild swirling wind. Intermittent 

outflow from the adit was evident by smoke tube visualization and 

Figure 1. Wedding Bell 

Mountain Adit 908A3 

monitoring on September 13, 

2011.
Page 6 Health Hazard Evaluation Report 2011-0090-3161

Results

 (continued)

by a pronounced variability in air temperature at the adit during the monitoring period. During the time 

period of approximately one hour when both monitors were operated simultaneously, the average shortterm PAEC values were 0.15 WL (range: 0.05–0.24) for the 597-PX3 and 0.136 WL (range: 0.04–0.31) for 

the SabreAlert2.

The short-term results from both monitors were in reasonable agreement; however, results from the longterm algorithm of the 597-PX3 were consistently higher than those reported by the SabreAlert2 (Figure 2). 

During this period, neither monitor indicated results that exceeded pre-bid levels.

Figure 2. PAEC monitoring data for Wedding Bell Mountain Adit 908A3 on September 13, 2011.

SabreAlert2 monitoring was discontinued at approximately 12:30 hours, and the 597-PX3 was relocated 

to the vacated position. The change in position resulted in a slight reduction in the short-term PAEC 

measurements. A plastic tarp was installed at the adit as a temporary barrier at approximately 14:00 

hours. During this time, the 597-PX3 was positioned about 0.5 meters from the ground (Figure 3). 

The barrier was hastily constructed from available materials resulting in a poor seal; nevertheless, a 

noticeable reduction in the PAEC was evident immediately following installation (Figure 4). The barrier 

remained in place throughout the rest of the monitoring period.
Health Hazard Evaluation Report 2011-0090-3161 Page 7

Results

 (continued)

Figure 3. Temporary barrier installed at 

Wedding Bell Mountain Adit 908A3.

Figure 4. PAEC monitoring data for Wedding Bell Mountain Adit 908A3 on September 13, 2011, using the 597-

PX3.

A weather front was observed during the afternoon (approximately 

15:30 hours) with the onset of cloudy conditions, increased wind, 

mild rain, and a drop in barometric pressure. A marked increase in 

outflow frequency and level coincided with the changing weather 

conditions, resulting in steadily rising PAEC levels that peaked 

above 1.0 WL (Figure 4). Monitoring was discontinued at about 

17:00 hours.

Vanadium Queen Mine Adits

Two adits were sampled on September 14, 2011. Both adits were 

part of the Vanadium Queen mine in the La Sal Creek Canyon 

located in southeast Utah. Neither of these adits is included in the 

Colorado reclamation contract; therefore, no pre-bid samples had 

been collected by state inactive mine reclamation program staff. 

Adit VQ1 was approximately 1.5 meters by 1.5 meters without a 

closure device. The floor of the mine was covered with standing 

water. This groundwater continuously drained from the adit 

to form a small stream down the side the mountain. Based on 
Page 8 Health Hazard Evaluation Report 2011-0090-3161

Results

 (continued)

vegetation growth, the mine drainage pattern appeared to be long term. Adit VQ2 was approximately 100 

meters from VQ1, situated along the same ridgeline of the mountain. Standing water was not present at 

the second adit and there was no evidence of drainage from the mine. Weather conditions were cool and 

cloudy with light rain throughout the entire monitoring period.

Sampling at VQ1 with the 597-PX3 started at approximately 12:00 hours on September 14, 2011. Because 

of the poor weather conditions and rising water levels in and around the adit, sampling was discontinued 

at 13:12 hours. During this short monitoring period, the average PAEC at VQ1 was nearly 7.0 WL (range 

0.0–14.38), although levels steadily declined after the first 20 minutes of monitoring (Figure 5). In contrast, 

the average PAEC at the dry location (VQ2) was 0.91 WL (range: 0.1–3.25) (Table 1 and Figure 6).

Figure 5. Continuous PAEC monitoring at VQ1 on September 14, 2011.
Health Hazard Evaluation Report 2011-0090-3161 Page 9

Results

 (continued)

Figure 6. Continuous PAEC monitoring at VQ2 on September 14, 2011.

Gamma radiation monitoring by NIOSH investigators at the Vanadium Queen mine indicated that 

dose rates were typically near background levels; however, peak dose rates in excess of 10 μSv∙h–1 were 

measured on the surface of an abandoned ore and waste rock pile. The general area dose rate at about 30 

centimeters from the pile surface was approximately 3.0 μSv∙h–1.
Page 10 Health Hazard Evaluation Report 2011-0090-3161

Discussion

Review of monitoring data from the state inactive mine 

reclamation program showed that the distribution of short-term 

sampling radon measurements was highly right skewed and 

suggested that few closure areas required intervention for achieving 

recommended exposure levels. If one assumes that the average 

concentration of 0.3 WL is a reasonable approximation of work 

area PAEC levels (using a DCF value of 10 mSv∙WLM–1) and that 

all activities require roughly the same amount of time to complete, 

then working occupancy of these areas over a 9-month period 

results in a potential effective dose of just less than 30 mSv for 

the work year and <10 mSv in any calendar quarter. If it is further 

assumed that occupancy in areas with concentrations >1WL is 

limited to 4 hours per shift, the annual effective dose is reduced 

to ~16 mSv. Additional information concerning the assumptions 

in these calculations is included in the Appendix. In this example, 

regulatory limits were met without intervention, and reasonable 

risk mitigation was achievable with simple administrative controls. 

Of course, this assertion is weakened by uncertain results from 

short-term samples; however, it is also true that actual closure work 

may consume far less time than that assumed.

PAEC results from CWLMs confirmed that work area 

concentrations in excess of 1.0 WL are likely to be encountered 

at some closure sites where standing water is present and/or 

outcasting mine atmospheres is common. Considerable outcasting 

from adits was easily identifiable by a sharp temperature difference 

when approaching the opening, with cooler air signifying escaping 

mine air. Careful attention to weather conditions, mine ventilation 

patterns, and standing water should alert employees to excessive 

radon environments and trigger protective measures.

There were differences in results observed between the 597-PX3 

and SabreAlert2 CWLMs. Some differences are expected because 

of dissimilar design characteristics and computational algorithms. 

However, relatively large disparities between monitors at high 

concentrations emphasize the need for formal intercomparisons 

under controlled conditions.

Average PAEC values from NIOSH exposure monitoring did not 

exceed pre-bid levels from previous monitoring by state inactive 

mine reclamation program staff. Nevertheless, PAEC results from 

the CWLMs varied widely over the sampling period because of 

constant fluctuations in ventilation patterns caused by changing 

environmental conditions. Given this variability, it is unlikely 

that short-term sampling, as conducted by state inactive mine 
Health Hazard Evaluation Report 2011-0090-3161 Page 11

reclamation program staff, is sufficient to derive long-term 

average concentrations that form the basis of protective actions. 

If CWLMs are used for work area characterizations, then 

sampling periods should be maximized (preferably >3 hours) 

or multiple short-term measurements be conducted. As an 

alternative to short-term sampling, long-term radon monitoring 

using inexpensive passive monitors (e.g., CR-39 alpha-track 

detectors or electrets) may provide a better characterization of 

potential radon hazards at mine reclamation sites. For example, 

pre-bid radon characterization could be accomplished with CR39 based alpha-track radon gas detectors designed for outdoor 

area monitoring (e.g., Landauer® Radtrak®, Landauer, Inc., 

Glenwood, Illinois). The typical detection capability of these 

monitors is about 1.1 kBq∙m–3∙d (30 pCi∙L–1∙d); therefore, an 

average concentration of 160 Bq∙m–3 (4.3 pCi∙L–1) is detectable 

in a 7-day sample period. Assuming 40% daughter equilibrium, 

continuous exposure at 160 Bq∙m–3 over a working year (2,000 

hours) results in about 0.2 WLM or about 2 mSv annual 

effective dose. Longer sample periods will increase sensitivity 

and further reduce uncertainties from seasonal and diurnal 

variations. Results from weekly or biweekly sampling would 

better support decisions on radiological controls included in 

the bid specifications for reclamation work. Moreover, as more 

information becomes available from long-term sampling, patterns 

may arise in the data that may be used in predictive models that 

reduce the need for additional characterization by sampling.

Gamma monitoring by state inactive mine reclamation program 

staff and NIOSH indicated that hazards from external exposures 

were small relative to radon exposures. In most instances, 

radiation levels were indistinguishable from background; 

however, elevated dose rates were found near an abandoned 

ore pile at one mine location. Assuming continuous occupancy 

(2,000 hours) in areas with dose rates ranging from 1 μSv∙h–1

to 5 μSv∙h–1 (i.e., mean and maximum levels measured by state 

inactive mine reclamation program staff) results in annual 

doses between 2 mSv and 10 mSv. Therefore, the potential dose 

equivalent contribution from external irradiation is generally 

far less than regulatory limits and appears negligible when 

compared to the effective dose from radon.

As a potential exception, the dose equivalent to an embryo 

or fetus should be kept ALARA and below 5 mSv over the 

gestation period. Therefore, additional controls (e.g., work 

Discussion

 (continued)
Page 12 Health Hazard Evaluation Report 2011-0090-3161

Discussion

 (continued) restrictions) may be necessary to limit external exposures to the 

declared pregnant worker. In the absence of uptakes of long-lived 

radionuclides associated with technologically enhanced naturally 

occurring radioactive material, the radiation hazard to the fetus is 

limited to the dose equivalent from external exposures to gamma 

radiation. The fetal dose-equivalent from radon and short-lived 

radon progeny exposures is very small (about 0.1 mSv∙WLM-1) 

and is not considered a limiting factor in decisions regarding 

radon exposures [Richardson et al. 1991; Kendall and Smith 2002; 

Kendall et al. 2009].

Radiological Controls 

The fundamental principles of justification and optimization are 

recommended for establishing appropriate radiological controls 

to reduce the hazard of radon exposure [ICRP 2007]. Justification 

explicitly refers to weighing the benefit of activity against the 

consequences of the exposure and judging that the net benefit is 

positive. Optimization of protection is achieved when radiation 

doses to the individual or individuals are ALARA, taking into 

account economic and societal factors. The optimization process 

involves four basic steps: 

1. Evaluation of the exposure situation

2. Selection of an appropriate constraint or reference level

3. Identification of control options

4. Selection and implementation of the preferred option 

In this case, the reference level (constraint) is considered as a basic 

level of protection and is always established (for planned exposures 

or existing controllable exposure situations) as some value below 

the applicable dose limits, which in U.S. protection systems are 

generally equivalent to 4 WLM∙y–1 for radon and 50 mSv∙y–1 for 

effective dose.

Selection of a Reference Level

The ICRP recommends an annual reference level of 10 mSv 

effective dose from radon, above which doses should be considered 

“occupational” and require radiological control intervention 

[ICRP 2007]. Assuming continuous occupancy in a work year 

(2,000 hours), the reference level is equivalent to the NIOSH REL 

of 1 WLM∙y–1. Additionally, the reference level can be equated to 
Health Hazard Evaluation Report 2011-0090-3161 Page 13

a continuous workshift-averaged PAEC value of 1.7 × 10–3 mJ∙m–3 (0.083 WL) and a radon concentration 

of about 800 Bq∙m–3 (22 pCi∙L–1) assuming 40% equilibrium (Feq=0.4). This appears to be in reasonable 

agreement with thresholds for posting and controls specified by the major U.S. regulatory agencies [10 

CFR 835; 10 CFR 20; 29 CFR 1926.53] as discussed in the Appendix. Note that the recommended 

reference level is based on 2,000-hour occupancy, which is not feasible for mine closure activities because 

of inclement weather conditions during the winter. Assuming that work is limited to a 9-month period 

(1,500 hours) the reference PAEC is about 2 × 10–3 mJ∙m–3 (0.1 WL), and the radon concentration is 

about 1 kBq∙m–3 (27 pCi∙L–1 as shown in Table 2.

Discussion

 (continued)

The reference level establishes a threshold for intervention under conservative assumptions. In this 

case, workplace controls are considered if a worker’s annual effective dose from radon exposure is likely 

to exceed 10 mSv if left unmitigated. Compliance with the reference level is assured by average PAEC 

concentrations less than 0.1 WL and yearly occupancy no more than 1,500 hours. Current estimates from 

occupancy records suggest that individuals involved in mine closure activities spend far less than 1,500 

hours per year at the work site because of funding constraints, procurement regulations, and other work 

preparation activities. Reference concentration values may be adjusted upward on the basis of improved 

occupancy estimates and assurances that the annual effective dose goal of 10 mSv (1 WLM) is maintained.

State inactive mine reclamation program staff had established ALARA goals for mine reclamation work 

and have incorporated these goals into its most recent revision to mine safety procedures included in 

contract specifications. The state inactive mine reclamation program goals are similar to, but slightly less 

conservative than, the proposed reference level. The state inactive mine reclamation program goals are 

not to exceed 15 mSv effective dose from radon and 20 mSv dose-equivalent from external irradiation. 

On the basis of an assumed annual occupancy of 800 hours, a DCF of 8.6 mSv∙WLM–1, and allowing for 

about 10% error, state inactive mine reclamation program staff equate the radon ALARA goal to a PAEC 

concentration of 0.33 WL (1.55 WLM∙y–1), which is used to trigger respiratory protection. If one assumes 

the NIOSH-preferred DCF (excluding error), then the state inactive mine reclamation program radon 

goal reduces to 1.5 WLM∙y–1. Conversely, applying the 800 hour occupancy to the NIOSH-recommended 

reference level of 1 WLM∙y–1 results in a PAEC concentration of 0.2 WL.

Table 2. Recommended radon annual reference levels*

Effective dose Exposure PAEC Radon 222 concentration

10 mSv (1 rem) 3.54 J∙h∙m−3 (1 WLM∙y−1) 2 x 10−3 mJ∙m−3 (0.1 WL) 1kBq∙m−3 (27 pCi∙L−1)

*PAEC and radon 222 concentration values assume 75% occupancy in a working year (i.e., 1,500 hours).
Page 14 Health Hazard Evaluation Report 2011-0090-3161

Recommendations

Conclusions

Monitoring results and onsite observations during this HHE 

suggest that ionizing radiation hazards during mine closure 

activities are relatively low overall; however, radon exposures 

necessitating intervention can occur at some work locations. 

Limiting occupancy, simple engineering controls (i.e., barriers, 

ventilation), and the use of respiratory protection in some/

certain situations are the preferred control measures for keeping 

radon exposures ALARA. Extreme differences in observed radon 

concentrations between and within adits suggests that knowledge 

of environmental factors (moisture, ventilation patterns, weather 

conditions) and a rigorous monitoring plan are necessary for 

appropriate hazard characterization. Employers and employees 

together must remain vigilant in identification, characterization, 

and mitigation of radon hazards in the workplace through training, 

monitoring practices, and application of control measures. Lowlevel gamma radiation fields at the surface of waste rock piles 

and near mine adits also may contribute to the ionizing radiation 

hazard; however, on the basis of our observations, intervention is 

rarely needed and may be limited to the case of reducing exposures 

to the declared pregnant worker.

On the basis of our findings, we recommend the actions listed 

below to create a more healthful workplace. Our recommendations 

are based on the hierarchy of controls approach (Appendix). 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.

This hierarchy can be summarized as follows:

● Elimination

● Substitution

● Engineering controls

● Administrative controls

● Personal protective equipment

Control methods at the top of the list are potentially more effective 

and protective than those at the bottom. Following the hierarchy 

normally leads to the implementation of inherently safer systems, 

ones where the risk of illness or injury has been substantially reduced.
Health Hazard Evaluation Report 2011-0090-3161 Page 15

Recommendations

 (continued) Elimination and substitution, while most effective at reducing 

hazards, also tend to be the most difficult to implement in an existing 

process and do not apply to radon exposures during the construction 

of mine closures. Engineering controls, the next tier in the hierarchy, 

are used to remove a hazard or place a barrier between the employee 

and the hazard. The use of engineering controls is hampered by the 

extreme conditions common to the remote mining areas. Permanent 

electrical power is unusual, and the rugged terrain makes transporting 

equipment to job sites difficult and potentially hazardous. 

Nevertheless, simple controls involving portable equipment may 

offer improvements in risk reduction over administrative controls or 

respiratory protection.

Administrative controls are management-dictated work practices 

and policies to reduce or prevent exposures to workplace hazards. 

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.

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.

This HHE addresses the hazards associated with ionizing radiation 

exposure during closure work at the mine entry point, but not within 

the mine. In selecting controls, specific working conditions at these 

remote mine sites must be considered. Undue focus on radiological 

risks at these sites may undermine appropriate consideration of other 

significant occupational hazards. Other uncharacterized closure 

activities, such as assessing wildlife habitation of the mine, may require 

prolonged underground occupancy, which is likely to introduce a 

different set of occupational hazards and require modification or 

addition to the controls recommended in this report. Employers are 

encouraged to use the following recommendations on engineering 
Page 16 Health Hazard Evaluation Report 2011-0090-3161

Recommendations

 (continued) and administrative controls to minimize potential radiation hazards 

during closure activities, but should also conduct task-specific job 

hazard analyses to ensure that ALARA goals are maintained during 

all work activities. In particular, additional hazard characterization is 

needed for jobs requiring mine occupancy.

Engineering Controls

1. Construct temporary barriers at mine openings. Our limited 

evaluation of the barrier at Adit 908A3 suggested that a simple 

temporary barrier may substantially reduce radiation hazards 

from mine outcasting. The barrier could be constructed 

onsite from plastic sheeting and a stick frame. Even a loose 

fitting barrier should reduce airborne radon concentrations 

in the immediate area of an outcasting mine opening. The 

effectiveness of the barrier can be ascertained by confirmatory 

radon monitoring.

2. Use dilution ventilation when possible, particularly when 

average radon concentrations are above 1 WL. Directing vent 

fans toward or across the mine opening may help to offset 

outcasting. Portable battery-powered fans may be sufficient 

to lower average radon concentrations to below 0.1 WL. 

Industrial fans powered by portable gas-powered generators 

may be required in rare situations where average breathing 

zone radon concentrations are likely to exceed 1.0 WL.

Administrative Controls

1. Adopt a reference level or ALARA goal for requiring 

intervention (i.e., engineering, administrative, or PPE 

controls). For example, a reference level of 10 mSv effective 

dose from radon progeny in a working year is consistent with 

the recommendations of the ICRP and existing NIOSH 

recommendations for radon exposures in uranium miners. 

This reference level is well below regulatory limits but is readily 

achievable in occupational settings associated with mine 

closure activities.

2. Increase sample frequency and/or duration to improve the 

precision of exposure estimates. If CWLMs are used, then, 

in addition to lengthening sampling periods, the instruments 

should be tested and/or calibrated in a radon chamber 
Health Hazard Evaluation Report 2011-0090-3161 Page 17

Recommendations 

(continued) over the range of anticipated exposures. Radon monitoring 

protocols currently used by the state inactive mine reclamation 

program may not be sufficient to characterize workplace radon 

concentrations that widely vary with time. Inexpensive passive 

monitors that are designed for estimating long-term averages 

may provide a more suitable alternative to CWLMs.

3. Inform employees about the health risk of ionizing radiation 

exposure in the workplace (Appendix). In this case, the health 

risk is primarily excesses in lung cancers from radon exposure 

and, to a lesser extent, lung and other malignancies from 

exposures to low linear energy transfer radiation. Training 

should emphasize methods to identify conditions that 

can increase the hazard (e.g., outcasting mine, presence of 

standing water), the controls used to keep individual employee 

exposures ALARA (e.g., work restrictions, engineering 

controls, respiratory protection), and personal risk concerns 

such as pregnancy.

4. Ensure that all preparatory work, such as hazards briefing, 

job training, gathering native rock, staging tools, and mixing 

mortar, is conducted in a manner that maximizes work 

efficiency and reduces the duration of work in the immediate 

vicinity of the mine opening where the dose is greatest. Further 

dose reduction may be achieved through work restrictions. The 

employer could restrict the stay time for individuals in the work 

area to ensure that annual doses remain below the reference 

level. Given that none of the closure work we observed resulted 

in occupancies at mine opening approaching a full work 

shift, simple stay time restrictions may be an effective control 

measure. As an alternative, stay times could be assigned to 

individuals rather than to the work group. In most cases, the 

area of greatest hazard is small and cannot be simultaneously 

occupied by many employees. Rotating employees through the 

area during the course of work distributes the collective dose 

from the job more evenly to all employees thereby reducing 

the exposure to any single worker. If stay times are used, the 

employer should keep a record of the times in the area to 

reconstruct doses at a later time.

5. Appropriately use personal dosimetry. On the basis of the 

dose measurements by NIOSH and data obtained from 

the state inactive mine reclamation program, there is little 

evidence supporting the need for personal dose monitoring. 

Nonetheless, some employers may issue dosimetry to their 
Page 18 Health Hazard Evaluation Report 2011-0090-3161

Recommendations 

(continued)

employees as a precaution. If dosimeters are used, the 

equipment and monitoring cycle should be selected on 

the basis of the low daily dose expected (i.e., <40 μSv), and 

employers should keep records of exposures according to 

applicable standards.

Personal Protective Equipment - Radon 

1. Wear NIOSH-approved respirators for radionuclides and 

radon daughters in situations where barriers, ventilation, 

and administrative controls are insufficient to ensure that a 

worker’s annual radon exposure will not exceed 1.0 WLM. To 

ensure proper selection, maintenance, and use of respirators, 

respiratory protection should be provided in the context 

of a written respiratory protection program that meets the 

requirements in the OSHA respiratory protection standard [29 

CFR 1910.134]. More information on the OSHA respiratory 

protection standard is available at http://www.osha.gov/

SLTC/respiratoryprotection/index.html.

References

 

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

Government Printing Office, Office of the Federal Register.

Duraski R [2010]. Construction worker radiation exposure at 

abandoned uranium mines. Bureau of Land Management, New 

Mexico State Office. Unpublished draft.

ICRP [2007]. The 2007 recommendations of the International 

Commission on Radiological Protection. ICRP Publication 103. 

Ann ICRP 37(2-4):1–332.

Kendall GM, Smith TJ [2002]. Doses to organs and tissues from 

radon and its decay products. J Radiol Prot 22(4):389–406.

Kendall GM, Fell TP, Harrison JD [2009]. Dose to red bone 

marrow of infants, children and adults from radiation of natural 

origin. J Radiol Prot 29(2):123–138.

Richardson RB, Eatough JP, Henshaw DL [1991]. Dose to red 

bone marrow from natural radon and thoron exposure. Br J Radiol 

64(763):608–624.
Health Hazard Evaluation Report 2011-0090-3161 Page 19

Appendix: Occupational Exposure Limits and Health Effects

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 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 

from adverse health effects even if their exposures are maintained below these levels.

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 of 1970. 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 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. 

The TLVs are developed by ACGIH committee members from a review of the published, peer-reviewed 

literature. ACGIH TLVs are not consensus standards. 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 2011].

Employers should understand that not all hazardous chemicals and physical agents have specific OSHA 

PELs, and for some agents the legally enforceable and recommended limits may not reflect current healthbased 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 and exposure guidelines when making 

risk assessments 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.
Page 20 Health Hazard Evaluation Report 2011-0090-3161

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Below we provide OELs and exposure guidelines for alpha and gamma radiation, as well as a discussion of 

the potential health effects from exposure to ionizing radiation.

Radon

Radon is a colorless, odorless, inert, radioactive noble gas that has three isotopic forms found ubiquitously 

in nature: 222Rn, which is a member of the 238U decay chain; 220Rn (commonly known as “thoron”), which 

is in the decay chain of 232Th; and 219Rn (known as “actinon”), which results from the decay of 235U. Of 

the three forms, 222Rn and its subsequent radioactive decay products present the greatest risk in most 

environmental and occupational settings because of its natural abundance. 222Rn undergoes radioactive 

decays via a series of solid short-lived radionuclides (i.e., polonium-218, lead-214, bismuth-214, and 

polonium-214), commonly referred to as “radon progeny” or “radon daughters.” These decay products 

appear either as unattached ions or are attached to condensation nuclei or dust particles, forming a 

respirable radioactive aerosol.

Environmental levels of radon in the United States vary widely, with average indoor concentrations in 

U.S. homes of about 46 Bq∙m–3 and in Colorado homes of 96 Bq∙m–3 [Marcinowski et al. 1994]. Outdoor 

radon concentrations tend to be much lower with national and regional (Nevada and Colorado) averages 

of about 15 Bq∙m–3 [Price et al. 1994; Borak and Baynes 1999], but progeny equilibrium is typically 

greater outdoors [NCRP 2009]. NCRP estimates that radon progeny exposure accounts for about 36% 

of the total dose received by the U.S. population annually [NCRP 2009]. The main contributor to tissue 

absorbed dose is densely ionizing radiation in the form of alpha particles from the decay of respired 

short-lived radon progeny; therefore, the organ most at risk from exposure is the lung, primarily from 

deposition of radon progeny in the bronchial epithelium. Dose to other organs and the fetus from inhaled 

radon progeny are at least an order of magnitude less than that of the lung [Kendall and Smith 2002]. 

Numerous studies of underground uranium miners who were exposed to relatively high levels of radon 

have unequivocally established radon as a human lung carcinogen [IARC 1988]. EPA claims that radon is 

the second leading cause of lung cancer in the United States and is the leading cause among persons who 

never smoked. The estimated risk from lifetime exposure at the EPA action level of 150 Bq∙m–3 (4 pCi∙L–1) 

is 2.3% [EPA 2003].

Much less information is available on other health outcomes associated with radon exposure. There is 

sparse evidence suggesting increased leukemia in uranium miners exposed to radon [Darby et al. 1995; 

Rericha et al. 2006] although most miner studies have not shown similar results [Tomasek et al. 1993; 

Laurier et al. 2004; Mohner et al. 2006; Schubauer-Berigan et al. 2009; Lane et al. 2010]. Some researchers 

have postulated that radon progeny that is deposited on skin surfaces can result in non-negligible dose to 

sensitive basal cells, which may result in increased incidence of non-melanoma skin cancer [Sevcova et al. 

1978; Eatough and Henshaw 1991; Denman et al. 2003]. The current weight of evidence is insufficient to 

establish a causal link between radon and skin cancer in humans [Charles 2007a; Charles 2007b].
Health Hazard Evaluation Report 2011-0090-3161 Page 21

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Terminology, Units and Dose 

Airborne radioactivity concentrations specific to radon exposures are typically expressed as PAEC, whereby 

PAEC is the sum of the alpha energy emitted by decay of atoms from any mixture of short-lived radon 

progeny within a unit volume of air. The quantity is expressed in units of J∙m–3 where 1 J∙m–3=6.242 × 1012 

MeV∙m–3. PAEC is often defined in terms of WL where 1 WL=1.3 × 108 MeV∙m–3 (1.3 × 105 MeV∙L–1). 

Radon progeny exposure is typically quantified as the time integral of PAEC and is expressed in units of 

J∙h∙m–3. The historical unit of exposure is the WLM, which is defined as exposure to a PAEC of 1 WL for 

one month (170 hours); therefore, 1 WLM =3.54 mJ∙h∙m–3. Exposures have also been quantified in terms 

of EEC of 222Rn gas, where 1 kBq∙m–3 of 222Rn in 100% equilibrium = 5.56 × 10–6 J∙m–3 (i.e., 100 pCi∙L–1=1 

WL). Feq is the ratio of the EEC to the activity concentration of 222Rn; therefore, a measured radon 

concentration of 1 kBq∙m–3 that is in 40% equilibrium (i.e., Feq=0.4) will give a PAEC of = 2.22 x 10–6 J∙m–3

(0.4 WL).

Radiation exposure standards are typically expressed in units of E, which is a radiation protection quantity 

used to describe “dose” in terms of equivalence to uniform whole-body exposure to low linear energy 

transfer radiation. Thus, by accounting for various types of radiation and body tissues, the risk from 

absorbed dose to lung tissue from densely ionizing radiation (e.g., radon PAEC) can be related to the 

stochastic risk caused by uniform whole body exposures to gamma radiation. By definition:

where wR T T,R

tissue or organ T due to incident radiation R. The unit of effective dose is the joule per kilogram and is 

called the Sv. In the case of radon progeny, the target tissue is the lung (wT=0.12), and alpha particles are 

given a radiation weight of 20 under standard ICRP recommendations [ICRP 1992].

The ratio of effective dose to the total exposure to radon progeny is referred to as the DCF. In its 

Publication 65, ICRP recommended an epidemiologic-based DCF for occupationally exposed individuals 

of 5 mSv∙WLM–1 [ICRP 1993]. This value was obtained by direct comparison of the detriment associated 

with a unit exposure to radon, as determined by studies of uranium miners, to the detriment associated 

with a unit effective dose, as determined principally from studies of Japanese atomic bomb survivors. 

The ICRP [ICRP 2009; ICRP 2010] has recently reexamined the results from epidemiological studies of 

miners and now recommends a lifetime excess absolute risk of 5 × 10–4 per WLM (14 × 10–5 per mJ∙h∙m–3) 

for radon and radon progeny induced lung cancer, compared to the previous value of 2.8 × 10–4 per WLM 

(8 × 10–5 per mJ∙h∙m–3). This change in the detriment suggests a concomitant two-fold increase to the DCF 

value (i.e., 10 mSv∙WLM–1).

Dosimetric-based DCFs for radon are widely available in the literature as a substitute for the risk-based 

approach [Stather 2004; Kranrod et al. 2010; Hofmann and Winkler-Heil 2011]. DCF values vary from 

differences in modeling assumptions, such as breathing rate, unattached fraction, and weighting factors for 

tissues and radiation quality (Table A1). (See Table A2 for radon terms and unit conversions.) Most DCFs 

 and w are the radiation and tissue weighting factors, and D is the mean absorbed dose in 
Page 22 Health Hazard Evaluation Report 2011-0090-3161

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

from reference biokinetic and dosimetric models report a higher dose per unit exposure than that found 

in ICRP Publication 65 and appear more in line with recent ICRP findings on radon-induced lung cancer 

risk. In 2009, the ICRP acknowledged these differences and stated that revisions to the recommended 

DCF are likely to increase the estimate of the effective dose per unit exposure by twofold [ICRP 2009; 

ICRP 2010]. Given these findings, we used a DCF value of 10 mSv∙WLM–1 in subsequent calculations in 

this report.

Table A1. Various radon dose conversion factors (DCFs)

Source Model basis Exposure place DCF (mSv∙WLM−1)

[ICRP 1993] epidemiologic workplaces 5

[ICRP 1981] dosimetric indoors and outdoors 10

[NCRP 2009] dosimetric indoors and outdoors 10

[UNSCEAR 2008]* dosimetric indoors and outdoors 5.7

[Winkler-Heil et al. 2007] dosimetric mines 8.3–11.8, avg. ~9

[Nikezic et al. 2006] dosimetric NS† 14.2

[Porstendörfer and Reineking 1999] dosimetric workplaces 5.7–13

[Kranrod et al. 2010] dosimetric workplaces 10.8

[Marsh et al. 2008] ‡ dosimetric mines 10.8–30.2

[Marsh et al. 2010] epidemiologic workplaces

12 based on lifetime 

detriment

5 based on lung detriment

*UNSCEAR=United Nations Scientific Committee on the Effects of Atomic Radiation

†NS=not specified

‡Assumes wR=20 and wT=0.12

There are some instances in which radon measurements are restricted to radon gas, in which case a 

progeny equilibrium factor is assumed. The degree of disequilibrium is a function of the “age” of the air, 

progeny deposition (plate-out), and ventilation patterns. Therefore, Feq can vary widely and is likely to 

be lowest at the adit brow where progeny diffuses into fresh air and highest deep within the abandoned 

mine where there is little or no ventilation [Gillmore et al. 2011]. Typical values for Feq are 0.4 for indoor 

air and 0.7 for outdoors [ICRP 1993]. Some studies have suggested that values of Feq in air in and around 

abandoned mines and caves range between 0.04 and 0.7, with a mean of about 0.4 [Butterwreck et 

al. 1992; Cavallo 2000; Gillmore et al. 2001; Denman et al. 2003; Gillmore et al. 2011]. A one-hour 
Health Hazard Evaluation Report 2011-0090-3161 Page 23

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Table A2. Radon terms and unit conversions

Metric Description Historic 

units SI units Unit conversions

Activity Radioactive decay events per 

unit time Ci Bq 1 Ci = 3.7 x 1010 Bq

Concentration Radon gas concentration pCi∙L–1 Bq∙m–3 1 pCi∙L–1 = 37 Bq∙m–3

PAEC WL J∙m–3 1 WL = 2.08 x 10–5 J∙m–3

EEC pCi∙L–1 Bq∙m–3 1 Bq∙m–3 = 5.56 x10–9

 J∙m–3

100 pCi∙L–1 = 1 WL

Equilibrium 

factor, Feq

Fraction of potential alpha 

energy of the short-lived radon 

progeny, compared to secular 

equilibrium

NA* NA

Feq = (0.106 cPo-218 + 0.514 cPb-214 + 

0.380 cBi-214) / cRn-222 where cx stands for 

the activity concentration of the nuclide 

x

Exposure Exposure WLM J∙h∙m–3

1 WLM = 3.54 J∙h∙m–3

1 WLM = 170 WL∙h

1 WLM = 800 Bq∙m–3 for 2,000 hour 

occupancy and Feq=0.4

Dose D† rad‡ Gy 100 rad = 1 Gy

E§ rem¶ Sv 100 rem = 1 Sv

*NA=not applicable

†D=Absorbed dose

‡rad=radiation absorbed dose

§E=Effective dose or weighted equivalent dose

¶rem=roentgen equivalent man

Gamma and X-rays 

External Dose

Gamma rays are penetrating high-energy, short-wavelength electromagnetic radiation emitted from the 

nucleus of an atom undergoing radioactive decay. X-rays are electromagnetic radiation originating outside 

of the nucleus; therefore, they are indistinguishable from gamma-rays and differ only by their origin. Both 

exposure to a radon gas concentration of 1 kBq∙m–3 (~27 pCi∙L–1) at an equilibrium factor, Feq, of 0.4 

corresponds to an effective dose of 6.4 μSv; therefore, continuous exposure in a 2,000-hour working year 

at this level results in approximately 12.8 mSv.
Page 24 Health Hazard Evaluation Report 2011-0090-3161

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

gamma and x-rays are a form of ionizing radiation, which is known to cause cancers in most tissues, in 

most species, and in all ages including the fetus [Little 2000].

Gamma and x-ray radiation exposure is ubiquitous and results from both terrestrial and cosmic sources. 

The NCRP estimates that the average U.S. adult male receives 0.56 mSv annually from external irradiation 

by ubiquitous background sources; 0.21 mSv from terrestrial sources; and 0.33 from cosmic radiation 

[NCRP 2009]. Average absorbed dose rates from terrestrial sources in the United States range from <6 

nGy∙h–1 to >83 nGy∙h–1 [Duval et al. 2005]. Cosmic-ray dose rates are slightly higher, ranging from <40 

nGy∙h–1 to >88 nGy∙h–1 [Duval et al. 2005]. Western states tend to have higher dose rates due to increased 

amounts of naturally occurring radioactive material and increased elevation. Typical absorbed dose rates 

in the Four Corner region range from 0.1 μGy∙h–1 to 0.4 μGy∙h–1 [EPA 2008]. Elevated gamma radiation 

is likely in areas around overburden, subeconomic ore (protore), or waste rock piles in which increased 

levels of radium are present. The radium content is roughly proportional to the uranium content in raw 

materials. Dose rate measurements (including background) taken by the EPA at overburden piles in U.S. 

mining sites typically ranged from 0.2 μSv∙h–1 to 3.0 μSv∙h–1 with an average value of 0.5 μSv∙h–1. Dose 

rates at protore piles were higher, with an average value of 3.5 μSv∙h–1 and a range of 0.8 μSv∙h–1 to 12.5 

μSv∙h–1 [EPA 2008].

Occupational Exposure Limits 

NIOSH-Recommended Exposure Limit 

In October 1987, NIOSH published its recommended standard for occupational exposure to radon 

progeny in underground mines [NIOSH 1987]. The primary goal of these recommendations was to 

decrease the risk of lung cancer in underground uranium miners over a working lifetime of 30 years. 

Exposure limits were derived from (1) risk models that used information on protracted radon exposures 

in the mining industry, (2) values that were measureable and reproducible, and (3) limits that could be 

achieved with available technology. NIOSH made several recommendations in this publication, including:

● “Exposure to radon progeny in underground mines shall not exceed 1 WLM per year, and the average work shift 

concentration shall not exceed 1/12 of 1 WL (or 0.083 WL). The REL of 1 WLM per year is an upper limit 

of exposure, and every effort shall be made to reduce exposures to the lowest levels possible...” [Section 2(a)].

● “Grab samples for radon progeny in the workplace shall be taken and analyzed using working level monitors, the 

Kusnetz method, or any other method at least equivalent in accuracy, precision, and sensitivity.” [Section 2(b)].

● “All operators of underground mines shall perform environmental evaluations in all work areas to determine 

exposures to radon progeny...If environmental monitoring in a work area indicates that the average work shift 

concentration of radon progeny exceeds 1/12 WL, the mine operator shall prepare an action plan describing 

the types of engineering controls and work practices that will be implemented to reduce the average work shift 

concentration in that area.” [Section 3(a) (1) and (3)].
Health Hazard Evaluation Report 2011-0090-3161 Page 25

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

● “The mine operator shall institute a medical surveillance program for all miners.” [Section 4(a) (1)].

● “Respiratory protection shall be used by miners (1) when work practices and engineering controls are not 

adequate to limit average work shift concentrations of radon progeny to 1/12 WL, (2) when entering a mine 

area where concentrations or radon progeny are unknown, or (3) during emergencies ...” [Section 7(a)].

The NIOSH REL of 1 WLM∙y–1 results in an annual effective dose that is fivefold less than current U.S. 

occupational standards for whole-body irradiation [10 CFR 835, Appendix C; 10 CFR 20, Appendix B, 

Table 1; 29 CFR 1910.1096; 29 CFR 1926.53] and one half of that observed in most international settings 

(i.e., 20 mSv∙y–1, averaged over 5 years, as recommended by the ICRP [ICRP 1992].

OSHA Standards for General Industry and Construction 

OSHA established a PEL for radon at 3.7 kBq∙m–3 (100 pCi∙L–1) averaged over a work week, which 

results in a cumulative exposure in a working year (2,000 hours) of 12 WLM, assuming 100% progeny 

equilibrium [29 CFR 1910.1096; 29 CFR 1926.53]. The OSHA PEL was based on the now obsolete 

values from the 1969 revision of 10 CFR 20, which relied on the earlier recommendations by the Federal 

Radiation Council of 12 WLM∙y–1. Nevertheless, OSHA also requires employers to limit doses to 12.5 mSv 

in a calendar quarter [29 CFR 1910.1096 (b)(1)]; therefore, an effective radon control limit of 5 WLM per 

year is inferred, provided “dose” includes the effective dose from radon, and the DCF is 10 mSv∙WLM–1.

Whole Body Dose

For those 18 years old or older, OSHA states that persons working in a “restricted area” shall be limited 

to doses to the whole body (head and trunk, active blood-forming organs, lens of eyes, or gonads) to 

no more than 12.5 mSv in any period of one calendar quarter [29 CFR 1910.1096]. A restricted area is 

defined as any area where access is controlled by the employer for purposes of protection of individuals 

from exposure to radiation or radioactive materials [29 CFR 1910.1096(a)(3)]. OSHA allows this limit to 

be exceeded provided that the dose does not exceed 30 mSv in any calendar quarter [29 CFR 1910.1096(b)

(2)(i)]; the lifetime occupational dose remains below 5(N–18), where N is the age of the worker [29 CFR 

1910.1096(b)(2)(ii)]; and the employer maintains adequate exposure records to show that these conditions 

have not been violated) [29 CFR 1910.1096(b)(2)(iii)]. On the basis of these constraints and using the DCF 

of 10 mSv∙WLM–1, one may derive an applicable exposure standard of 1.25 WLM in any calendar quarter 

or a continuous average exposure in a work shift of no more than about 0.4 WL.

Airborne Radioactivity

In regard to airborne radioactivity standards, OSHA currently sets the maximum permissible 

concentration for 222Rn at 3.7 kBq∙m–3 (100 pCi∙L–1) for 40 hours in any work week of 7 consecutive 

days [29 CFR 1910.1096(c)(1)]. Thus, assuming an equilibrium fraction of 100%, continuous workplace 

exposures at the maximum permissible concentration (1 WL) result in 3.0 WLM per quarter.
Page 26 Health Hazard Evaluation Report 2011-0090-3161

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Personal Monitoring 

OSHA requires personal monitoring of adult employees who enter a restricted area and are likely to 

receive a dose in any calendar quarter that exceeds 3.125 mSv [29 CFR 1910.1096(d)(2)(i)].

Other Recommendations and Standards 

The NIOSH REL was intended for the protection of underground uranium miners and was meant for 

consideration in regulations promulgated by MSHA. Nevertheless, MSHA has not revised its annual 

exposure limit of 4 WLM [40 CFR 57.5038], which was initially adopted for U.S. miners in 1971. The 

current MSHA limit has also been embraced by the NRC, which specifies an ALI for occupational 

exposures of 4 WLM [10 CFR 20, Appendix B, Table 1]. The U.S. annual limit of 4 WLM was initially 

selected by the Federal Radiation Council on the basis of the feasibility of managing exposures in the 

uranium mining industry. However, subsequent dosimetric and epidemiologic-based models in ICRP 

Publication 32 [ICRP 1981] concluded that the DCF for radon progeny was ~10 mSv∙WLM–1. That is, 

the effective dose from 4 WLM exposures was approximately equal to the regulatory dose limits in place at 

that time.

Not all U.S. regulating agencies limit annual occupational radon exposures to 4 WLM. DOE uses an 

ALI of 10 WLM, which is based on the ICRP recommendations of a dose conversion convention of 5 

mSv∙WLM–1 [ICRP 1993].

In large part, countries outside of the United States have adopted the most recent recommendations of the 

ICRP [ICRP 1993], whereby occupational radon exposures are summed with other sources of radiation, 

and the total dose is limited to 20 mSv∙y–1 averaged over 5 years and less than 50 mSv in any one year 

(Table A3).

In this context, the DCF is assumed to be 5 mSv∙WLM–1; therefore, the equivalent annual radon 

exposure limit is 14 mJ•h•m–3 (4 WLM) averaged over 5 years, not to exceed 35 mJ•h•m–3 (10 WLM) in 

any one year.
Health Hazard Evaluation Report 2011-0090-3161 Page 27

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Table A3. Occupational radon regulations, guidance, and recommended limits

Agency Regulation / 

Recommendation Covered Facilities Radon Limits Total Exposure Limit

Intervention, 

Reference or 

Control levels

ICRP

[ICRP 2011] Publication No. 115

Domestic and 

occupational 

exposure

Lifetime lung 

cancer EAR*=5 × 

10–4 per WLM

ICRP 

[ICRP 2007] Publication No. 103

Domestic and 

occupational 

exposure

Applicable limits 

consistent with 

ICRP Publication 

65

Applicable 

limits consistent 

with ICRP 

Publication 60

<10 mSv∙y–1

(<1.5 kBq∙m–3)

ICRP 

[ICRP 1992; 

ICRP 1993]

Publication No. 65 

(radon)

Publication No. 60 

(total dose)

Domestic and 

occupational 

exposure

Lifetime lung 

cancer EAR=2.8 × 

10–4 per WLM

 14 mJ·h·m–3 

(20 mSv; 4 WLM, 

3 kBq∙m–3)

35 mJ·h·m–3 

(50 mSv; 10 WLM, 

8 kBq∙m–3)

< 20 mSv∙y–1

averaged over 

5 years, not to 

exceed 50 mSv 

in any year

3–10 mSv∙y–1

 (0.5–1.5 kBq∙m–3)

1 kBq∙m–3=6 mSv 

based on 2,000 

hour occupancy, 

Feq =0.4

NCRP 

[NCRP 1984] Report No. 77

Domestic and 

occupational 

exposure

Lifetime lung 

cancer EAR=1.5 × 

10–4 per WLM

2 WLM∙y–1

5 mSv∙y–1

in areas of 

enhanced levels 

of the uranium 

series. Not 

additive; radon 

considered to 

be controlling

ACGIH 

[ACGIH 2011]

2011 TLVs® and 

BEIs® TLV=4 WLM∙y–1

Limits 

consistent 

with ICRP 

Publication 60

NIOSH 

[NIOSH 1987]

Publication No. 88-

101 Underground mines

REL=1 WLM∙y–1

Not addressed

1/12 WL (0.083 

WL) per working 

shift

IAEA†

[IAIA 2003]

Basic Safety 

Standard 115 and 

Safety Report 

Series No. 33

All workplaces 

(includes exposure 

to naturally 

occurring radon 

not related 

to production 

activities)

Limits consistent 

with ICRP 

Publication 65

Limits 

consistent 

with ICRP 

Publication 60

Potential 

remediation 

measures 

discussed

IAEA

[IAEA 2004]

Safety Guide 

No. RS-G-1.6

Activities involved 

in the mining and 

processing of raw 

materials

Limits consistent 

with ICRP 

Publication 65

Limits 

consistent 

with ICRP 

Publication 60

Respirators 

recommended only 

for short duration 

tasks
Page 28 Health Hazard Evaluation Report 2011-0090-3161

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Agency Regulation / 

Recommendation Covered Facilities Radon Limits Total Exposure Limit

Intervention, 

Reference or 

Control levels

MSHA 

[CFR]

30 CFR Part 57 

57.5038 Underground mines 4 WLM∙y–1

Not addressed

Respiratory 

protection required 

at levels ≥1 WL.

Air-supplied 

respiratory 

protection required 

at levels ≥10 WL

NRC

[CFR]

10 CFR 20, 

Appendix B, 

Table 1

NRC Licensees

ALI=4 WLM (14 

mJ·h·m–3) , gas 

(without progeny) 1 

mCi (37 MBq)

DAC‡: Progeny, 

0.33 WL

(EEC=30 pCi∙L–1); 

radon gas (without 

progeny) 148 kBq∙ 

m–3 (4,000 pCi∙L–1)

Total Effective 

Dose Equivalent 

of 50 mSv (5 

rem)

Posting and 

control at 0.6 ALI, 

12-DAC∙h (0.1 

WL continuous) 

averaged over one 

work week

Individual 

monitoring required 

for adults likely to 

receive 0.4 WLM in 

one year

OSHA 

[CFR]

29 CFR 1910.1096

29 CFR 1926.53

Those not 

regulated by the 

U.S. Atomic Energy 

Act of 1954. 

100 pCi∙L–1 (~3.7 

kBq∙m–3)

12 WLM∙y–1 EEC

Combining 

internal and 

external sources 

is not directly 

addressed; 

however, “dose” 

is limited to 

<12.5 mSv in 

any calendar 

quarter.

Posting and 

control at 25% of 

the exposure limit 

averaged over 

one work week, 25 

pCi∙L–1 (~1 kBq∙m–3) 

in occupied areas

DOE 

[CFR]

10 CFR 835, 

Appendix A DOE facilities

DAC EEC=3 

kBq∙m–3

 (80 pCi∙L–1); 0.83 

WL

ALI=10 WLM

Total Effective 

Dose Equivalent 

of 50 mSv 

(5 rem)

Posting and control 

at 12-DAC∙h (0.25 

WL continuous) 

in one work week 

(0.06 WLM).

Air monitoring 

required for 

individual likely 

to receive an 

exposure of 40 or 

more DAC-hours 

(0.2 WLM) in a year

Individual 

monitoring required 

for adults likely 

to receive a 

committed effective 

dose of 1 mSv (0.2 

WLM) in one year

Table A3. Occupational radon regulations, guidance, and recommended limits (continued)
Health Hazard Evaluation Report 2011-0090-3161 Page 29

Appendix: Occupational Exposure Limits and Health Effects

 (continued)

Agency Regulation / 

Recommendation Covered Facilities Radon Limits Total Exposure Limit

Intervention, 

Reference or 

Control levels

ARPANSA§ 

[ARPANSA 

2005]

Radiation 

Protection Series 

Publication No. 9

Australian facilities

Limits consistent 

with ICRP 

Publication 65

Limits 

consistent 

with ICRP 

Publication 60

Radiation 

protection is 

required if longterm 222Rn >1 

kBq∙m–3

CNSC¶ 

[CNSC 2000] SOR/2000-203 Canadian nuclear facilities

Limits consistent 

with ICRP 

Publication 65

Limits 

consistent 

with ICRP 

Publication 60

HSE**

[HSE 1999]

Ionising radiations 

regulations 1999 

No. 3232

UK†† nuclear 

facilities

Limits consistent 

with ICRP 

Publication 65

Limits 

consistent 

with ICRP 

Publication 60

222Rn gas 

concentration in air, 

averaged over 24-

hour period >400 

Bq∙m–3 (11 pCi∙L–1)

222Rn progeny in 

air averaged over 

8-hour period > 

6.24 x 10–7 J∙m–3

(0.03 WL)

*EAR: Excess absolute risk

†IAEA: International Atomic Energy Agency

‡DAC: Derived air concentration

§ARPANSA: Australian Radiation Protection and Nuclear Safety Agency

¶CNSC: Canadian Nuclear Safety Commission

**HSE: United Kingdom (UK) Health and Safety Executive

††UK: United Kingdom

Table A3. Occupational radon regulations, guidance, and recommended limits (continued)
Page 30 Health Hazard Evaluation Report 2011-0090-3161

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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.

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 organizations 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 Robert D. Daniels of the Occupational 

Energy Research Program, Industrywide Studies Branch, and 

David C. Sylvain of HETAB, Division of Surveillance, Hazard 

Evaluations and Field Studies. Industrial hygiene field assistance 

was provided by Tim Radtke of the Office of Occupational Safety 

and Health, U.S. Department of the Interior. Industrial hygiene 

equipment and logistical support were provided by Donald 

Booher and Karl Feldmann. Health communication assistance was 

provided by Stefanie Evans. Editorial assistance was provided by 

Ellen Galloway. Desktop publishing was performed by Greg Hartle 

and Mary Winfree.

Copies of this report have been sent to representatives at the federal 

agency, the state inactive mine reclamation program, the state health 

department, 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 at 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 [2012]. Health hazard evaluation report: evaluation of exposure to radon 

progeny during closure of inactive uranium mines – Colorado. By Daniels R, 

Sylvain D. 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. [2011-0090-3161].

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