Final Report 

History and Basis of NESHAPs and Subpart W 

 

 Prepared by 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

 Vienna, VA 22182 

 Under 

Contract Number EP-D-05-002 

Work Assignment No. 4-11, Task 4 

 

Prepared for 

U.S. Environmental Protection Agency 

Office of Radiation and Indoor Air 

1200 Pennsylvania Avenue, N.W. 

Washington, DC 20460 

Reid J. Rosnick 

 Work Assignment Manager 

September 25, 2008
In accordance with the Quality Assurance Project Plan: Technical and Regulatory Support to 

Develop a Rulemaking to Modify the NESHAP Subpart W Standard for Radon Emissions from 

Operating Uranium Mills (40 CFR 61.25), this document has been reviewed and approved by the 

following individuals: 

Work Assignment Task Manager: ________________________ Date: __09/25/2008__ 

 Harry Pettengill 

Project Manager: ________________________ Date: __09/25/2008__ 

 Abe Zeitoun 

Corporate Quality Assurance Mgr: ________________________ Date: __09/25/2008__ 

 Gregory Beronja 

Work Assignment QA Manager: ________________________ Date: __09/25/2008__ 

 Stephen Ostrow 

WA 4-11, Task 4 – NESHAPs History i SC&A – September 25, 2008
TABLE OF CONTENTS 

1.0 Introduction......................................................................................................................... 1 

2.0 Background on the Development of Radionuclide NESHAPS .......................................... 1 

2.1 The 1977 Amendments to the Clean Air Act .......................................................... 2 

2.2 Regulatory Activities between 1979 and 1987 ....................................................... 2 

2.3 Regulatory Activities between 1987 and 1989 ....................................................... 4 

2.4 1989 Radionuclide NESHAPs Reconsideration Rulemaking................................. 4 

3.0 Basis for the Subpart W Risk Assessment .......................................................................... 5 

3.1 Existing Impoundments.......................................................................................... 5 

3.2 New Impoundments................................................................................................ 6 

4.0 Comparison of the NESHAPs Risk Assessment with Current Risk Assessment 

Approaches.......................................................................................................................... 7 

5.0 Conclusions....................................................................................................................... 10 

6.0 References......................................................................................................................... 10 

WA 4-11, Task 4 – NESHAPs History ii SC&A – September 25, 2008
1.0 INTRODUCTION 

On December 15, 1989, the Environmental Protection Agency’s (EPA) Office of Radiation and 

Indoor Air (ORIA) promulgated a National Emission Standard for Hazardous Air Pollutant 

(NESHAP) for radon emissions from operating uranium mill tailings (Subpart W). 

Section 112(q) of the Clean Air Act (CAA), as amended, requires the EPA to review, and if 

appropriate, revise or update the Subpart W standard on a timely basis (10-year interval). To 

date, EPA has not revisited this standard since its promulgation and now desires to do so. The 

purpose of this work assignment is to provide essential technical and regulatory support 

necessary for EPA to review the technical basis on which the standard was based and the 

decision-making process that was used to determine the requirements of the standard. 

Furthermore, this effort will assist the EPA in determining the appropriateness of the standard for 

this type of pollutant, and to decide if modification of Subpart W is warranted. 

The facilities covered by Subpart W are uranium mills licensed and regulated by the U.S. 

Nuclear Regulatory Commission (NRC) or its Agreement States. There are two separate 

standards outlined in Subpart W. The first states that existing sources must ensure emissions 

from tailings impoundments not exceed 20 pCi/m2

-sec of radon-222 (Rn-222). The second 

prescribes a work practice specifying one of two types of impoundment structures that new 

sources must construct to handle tailings during operations. The work practice also applies to 

operations at existing sources once their existing impoundments can no longer accept additional 

tailings. As part of these two standards, Subpart W also requires that existing sources file an 

annual report of the facility's emissions. 

The standard as promulgated was a health-based standard. It is important to review how the 

initial standard was developed and to ascertain if it is still valid. Furthermore, it needs to be 

determined if the standard requires revision to satisfy the requirements in CAA Section 112(d), 

which requires a technology-based standard. It is also important to determine if this type of 

standard is appropriate for this type of pollutant. 

The reports collected for this review include the original risk assessment material used by EPA to 

develop the Subpart W standard, including the final rulemaking package for Subpart W (FR 

1989), the background information document (BID) for the Subpart W rulemaking (EPA 1989, 

Volumes 1 and 2), and the economic impact analysis for the Subpart W rulemaking (EPA 1989, 

Volume 3). This regulatory information was evaluated and compared and contrasted to current 

risk assessment modeling methodologies. The purpose of this report is to illuminate the 

differences, if any, and detail what impact this information would have on the original radon risk 

standard as provided in Subpart W. 

2.0 BACKGROUND ON THE DEVELOPMENT OF RADIONUCLIDE 

NESHAPS 

In the following subsections, we present a brief history of the development of environmental 

radiation protection standards by the EPA, with particular emphasis on the development of 

radionuclide NESHAPs. 

WA 4-11, Task 4 – NESHAPs History 1 SC&A – September 25, 2008
2.1 The 1977 Amendments to the Clean Air Act 

On January 13, 1977, the EPA established environmental protection standards for nuclear power 

operations pursuant to its authority under the Atomic Energy Act (AEA) (FR 1977). The 

standards, Title 40, Part 190, of the Code of Federal Regulations (40 CFR 190), which covered 

all licensed facilities that are part of the uranium fuel cycle, established an annual limit of 

exposure and the requirement that licensees keep all exposures “as low as reasonably 

achievable” (ALARA). Because of the uncertainties associated with the risk of inhaled radon, 

the standards exempted Rn-222 from the annual limit. 

After the promulgation of 40 CFR 190, the 1977 amendments to the Clean Air Act (CAA) were 

passed. These amendments included the requirement that the Administrator of the EPA 

determine whether or not radionuclides should be regulated under the Act. 

In December of 1979, the Agency published its determination in the Federal Register (FR 1979) 

that radionuclides constitute a hazardous air pollutant within the meaning of Section 112(a)(1). 

As was stated in the Federal Register, radionuclides are known to cause cancer and genetic 

defects, and contribute to air pollution that may be anticipated to result in an increase in 

mortality or an increase in serious irreversible or incapacitating reversible illness. The Agency 

further determined that the risks posed by emissions of radionuclides into the ambient air 

warranted regulation, and listed radionuclides as a hazardous air pollutant under Section 112. 

Section 112(b)(1)(B) requires the Administrator to establish NESHAPs as a “level which (in the 

judgment of the Administrator) provides an ample margin of safety to protect the public health” 

or find that they are not hazardous and delist them. 

2.2 Regulatory Activities between 1979 and 1987 

To support the development of radionuclide NESHAPs, the Agency developed a BID to 

characterize “source categories” of facilities that emit radionuclides into ambient air (EPA 1979). 

For each source category, information needed to characterize the exposure of the public was 

developed. This included characterization of the facilities included in the source category 

(numbers, locations, proximity of nearby individuals); radionuclide source terms [curie/year 

(Ci/y)] release rates by radionuclide, solubility class, and particle size); release point data (stack 

height, volumetric flow, area size); and effluent controls (type, efficiency). Doses to nearby 

individuals and regional populations caused by releases from either actual of model facilities 

were estimated using the computer codes AIRDOSE-EPA and DARTAB. 

In 1983, the EPA proposed radionuclide NESHAPs for four source categories based on the 

results reported in a new BID (EPA 1983). The four source categories for which NESHAPs were 

proposed were Department of Energy (DOE) and Non-NRC-Licensed Federal Facilities, NRCLicensed Facilities, Elemental Phosphorus Plants, and Underground Uranium Mines. For all of 

the other source categories that it had considered in its BID (i.e., coal-fired boilers, the phosphate 

industry, other extraction industries, uranium fuel-cycle facilities, uranium mill tailings, highlevel waste disposal, and low-energy accelerators), the Agency found that NESHAPs were not 

necessary. In reaching this conclusion, the EPA found that (1) the levels of radionuclide 

emissions did not cause a significant dose to nearby individuals or the regional populations, 

WA 4-11, Task 4 – NESHAPs History 2 SC&A – September 25, 2008
(2) the costs of additional effluent controls were not cost effective, or (3) existing regulations 

under other authorities were sufficient to keep emissions at an acceptable level. 

During the public comment period on the proposed NESHAPs, the Agency completed its 

rulemaking efforts under the Uranium Mill Tailings Remedial Control Act (UMTRCA) to 

establish standards (40 CFR 192) for the disposal of uranium mill tailings. With respect to the 

emission of Rn-222, the UMTRCA standards established a design standard calling for a Rn-222 

flux rate of no more than 20 pCi/m2-sec. 

In February of 1984, the Sierra Club sued the EPA in the U.S. District Court for Northern 

California (Sierra Club v. Ruckelshaus, No. 84-0656) (EPA 1989), demanding that the Agency 

promulgate final NESHAPs or delist radionuclides as a hazardous air pollutant. The court sided 

with the Sierra Club and ordered the EPA to promulgate final regulations. In October of 1984, 

the EPA withdrew the proposed NESHAPs for Elemental Phosphorus Plants, DOE-Facilities, and 

NRC-Licensed Facilities, finding that existing control practices protected the public health with 

an ample margin of safety (FR 1984). The EPA also withdrew the NESHAP for Underground 

Uranium Mines, but stated its intention to promulgate a different standard, and published an 

Advance Notice of Proposed Rulemaking (ANPR) to solicit additional information on control 

methods. It also published an ANPR for licensed uranium mills. Finally, the Federal Register

notice affirmed the decision not to regulate the other source categories identified in the proposed 

rule, with the exception that it was doing further studies of phosphogypsum stacks to see if a 

standard was needed. 

In December of 1984, the District Court for Northern California found the EPA's action 

withdrawing the NESHAPs to be in contempt of its order. Given the ruling, the EPA issued the 

final BID (EPA 1984) and promulgated final standards for Elemental Phosphorus Plants, DOEFacilities, and NRC-Licensed Facilities in February of 1985 (FR 1985a), and a work practice 

standard for Underground Uranium Mines in April (FR 1985b). 

The Environmental Defense Fund (EDF), the Natural Resources Defense Council (NRDC), and 

the Sierra Club (SC) filed court petitions seeking review of the October 1984 final decision not 

to regulate the source categories identified above, the February 1985 NESHAPs, and the April 

1985 NESHAP. The American Mining Congress (AMC) also filed a petition seeking judicial 

review of the NESHAP for underground uranium mines. 

On September 24, 1986, the Agency issued a final NESHAP for Operating Uranium Mill 

Tailings, establishing an emission standard of 20 pCi/m2

-sec for Rn-222 and a work practice 

standard requiring that new tailings be disposed of in small impoundments or by continuous 

disposal. The justifications for the work practices included the fact that while large 

impoundments did not pose an unacceptable risk during active operations, the cyclical nature of 

the uranium milling industry could lead to prolonged periods of plant stand-by and the risk that 

the tailing impoundments could experience significant drying with the resulting increase in 

Rn-222 emissions. Furthermore, the two acceptable work practices were believed to actually 

save the industry the significant costs of constructing and closing large impoundments before 

they were completely filled. With the promulgation of the NESHAP for Operating Uranium Mill 

WA 4-11, Task 4 – NESHAPs History 3 SC&A – September 25, 2008
Tailings, releases of radionuclides to air during operations and tailings disposal at uranium mills 

were covered by three EPA regulations, 40 CFR 190, 40 CFR 192, and 40 CFR 61, Subpart W. 

In November of 1986, the AMC and the EDF filed petitions challenging the NESHAP for 

operating uranium mill tailings. 

2.3 Regulatory Activities between 1987 and 1989 

While the petitions filed by the EDF, NRDC, SC, and AMC were still before the courts, a 

decision by the U.S. District Court for the District of Columbia in NRDC v. EPA (FR 1989), 

found that the Administrator had impermissibly considered costs and technological feasibility in 

promulgating the NESHAP for Vinyl Chloride. The court outlined a two-step decision process 

that it would find acceptable, first establishing a standard based solely on an acceptable level of 

risk, and then considering additional factors, such as costs to establish the “ample margin of 

safety.” Given the Court's decision, the Agency reviewed how it had conducted all its NESHAP 

rulemakings and requested that the Court grant it a voluntary remand for its radionuclide 

NESHAPs. As part of an agreement with the Court and the NRDC, the Agency agreed to 

reconsider all issues that were currently being litigated, and it agreed that it would explicitly 

consider the need for a NESHAP for two additional source categories; radon from 

phosphogypsum stacks and radon from DOE facilities. The subsequent reconsideration became 

know as the radionuclide NESHAPs reconsideration rulemaking. 

2.4 1989 Radionuclide NESHAPs Reconsideration Rulemaking 

In the radionuclide NESHAPs reconsideration rulemaking, the Administrator relied on a “bright 

line” approach for determining whether or not a source category required a NESHAP. Namely, 

no NESHAP was required if all individuals exposed to the radionuclide emissions from the 

facilities in the source category were at a life-time cancer risk of less than 1 in 1,000,000, and 

less than 1 fatal cancer per year was estimated to be incurred in the population. For source 

categories that did not meet this “bright line” exclusion, a two-step multi-factor approach to 

setting the emission standards was adopted. 

The first step established a presumptively acceptable emissions level corresponding to a 

maximum individual risk (MIR) of about 1 in 10,000 life-time cancer risk, with the vast majority 

of exposed individuals at a lifetime risk lower than 1 in 1,000,000, and with the total fatal 

cancers per year in the exposed population of less than 1. If the baseline emissions from a source 

category met these criteria, they were presumptively adequately safe. If they did not meet these 

criteria, then the Administrator was compelled by his non-discretionary duty to provide adequate 

safety to determine an emission limit that would correspond to risks that were adequately safe. 

After baseline emissions were determined to be adequately safe, or an adequately safe alternative 

limit defined, the analysis moved to the second step, where reduced risks for alternative emission 

limits were evaluated, along with the technological feasibility and costs estimated to be 

associated with reaching lower levels. In the two-step approach, the Administrator retained the 

discretion to decide whether or not the NESHAP should be set at these lower limits. 

WA 4-11, Task 4 – NESHAPs History 4 SC&A – September 25, 2008
As with previous radionuclide NESHAPs, exposures were estimated using a combination of 

actual site data and model or representative facilities and computer models. For all radionuclides 

other than Rn-222, the CAP88 computer code, which included updates to AIRDOSE-EPA and 

DARTAB, and the addition of RADRISK, was used to calculate total effective dose equivalent 

and lifetime fatal cancer risk, and the distribution of risks in the exposed populations. For 

Rn-222, the CAP88 computer codes were used to established ambient concentrations (pCi/m3) in 

each of the sectors in a 0-80 km radius of the source. The concentration within each sector was 

then converted to working level months (WLMs), based on a 0.70 equilibrium fraction between 

Rn-222 and its decay products, and a respiration rate appropriate to members of the general 

public. Using risk factors derived from human epidemiological studies, the WLM exposure data 

were converted to risks. 

Of the NESHAPs promulgated in 1989, those for Phosphogypsum Stacks (Subpart R), Radon 

from DOE Facilities (Subpart Q), and Operating Uranium Mills Tailings Disposal (Subpart T) 

are the most relevant for comparison with Subpart W, in that they all address the emission of 

Rn-222 from relatively large area sources. Furthermore, the basic post-disposal Rn-222 emission 

limit is consistent at 20 pCi/m2

-s. Regarding the operational phase, no standard was deemed to 

be necessary for phosphogypsum stacks, since the large ponds that cover most to the top of the 

stacks and are part of the plants normal operation to control Rn-222 emissions to an acceptable 

level. For DOE facilities, there is no operational phase, as all these site are simply storage sites 

for radon-emitting wastes that were generated in the past (most as part of the Manhattan Project). 

The NESHAP for disposal of uranium mill tailings (Subpart T) was determined to be necessary 

by the Administrator to assure that the tailings did not remain unreclaimed for long periods of 

time, as was possible under the UMTRCA standard (40 CFR 192). Additionally, the design 

standard of the UMTRCA regulations were enhanced by the requirement that the 20 pCi/m2-sec 

Rn-222 design standard be demonstrated to have been achieved by post-disposal radon 

monitoring. 

3.0 BASIS FOR THE SUBPART W RISK ASSESSMENT 

3.1 Existing Impoundments 

As noted previously, the NESHAP for operating uranium mill tailings addressed both existing 

and future tailings impoundments. For the existing impoundments, the radon emissions and 

estimated risks were developed using site-specific data for each of the 12 mills that were 

operating or operable at the time the assessment was made. These data included the average 

radium-226 (Ra-226) content of the tailings; the overall dimensions and areas of the 

impoundments (developed from licensing data and areal photographs); areas of dry (unsaturated) 

tailings; the existing populations within 5 km of the centers of the impoundments (identified by 

field enumeration); 5–80 km populations derived from U.S. Census tract data; meteorological 

data (joint frequency distributions) from nearby weather stations; mixing heights; and annual 

precipitation rates. 

The CAP88 codes were used to estimate airborne concentrations based on the calculated Rn-222 

source term for each facility. Radon-222 source terms were estimated on the assumption that a 

Rn-222 flux of 1 pCi/m2 -sec results for each 1 pCi/g Ra-226 in the tailings and the areas of dried 

WA 4-11, Task 4 – NESHAPs History 5 SC&A – September 25, 2008
tailings at each site. The radon flux rate of 1 pCi/m2

-sec per pCi/g Ra-226 was derived based on 

both theoretical radon diffusion equations and the available radon emissions measurements. 

For each sector in the 0–80 km grid around each facility, the estimated Rn-222 airborne 

concentration was converted to cumulative WLMs assuming a 0.70 equilibrium fraction between 

radon and its decay products, an average respiration rate appropriate for members of the general 

public, and the assumption of continuous exposure over a 70-year lifetime. Using a risk 

coefficient of 760 fatalities/106 WLM, lifetime risk, fatal cancers per year, and the risk 

distribution were calculated for the exposed population. 

The baseline risk assessment for existing uranium tailings showed MIR of 3×10-5, with 0.0043 

committed cancers in the 2 million persons living within 80 km of the mills. The distribution of 

the cancer risk showed that 240 persons were at risks between 1×10-5 and 1×10-4, and 60,000 

were at risks between 1×10-6 and 1×10-5. The remainder of the population of about 2 million 

were at a risk of less than 1×10-6. Based on these findings, the EPA concluded that baseline risks 

were acceptable. 

The decision on an ample margin of safety considered all of the risk data presented above plus 

costs, scientific uncertainty, and the technical feasibility of control technology necessary to lower 

emissions from operating uranium mill tailings piles. As the risks from existing emissions were 

very low, the EPA determined that an emission standard of 20 pCi/m2

-sec, which represented 

current emissions, was all that was necessary to provide the ample margin of safety. The 

necessity for the standard was explained by the need to assure that mills continued the current 

control practice of keeping tailings wet and/or covered. Finally, in order to assure that 

groundwater was not adversely affected by continued operation of existing piles that were not 

synthetically lined or clay lined, the NESHAP ended the exemption to the requirements of 

40 CFR 192.32(a), which protects water supplies from contamination. 

3.2 New Impoundments 

The risk assessment for new mill tailings impoundments was based on a set of model mills, 

defined so that the impact of alternative disposal strategies could be evaluated. For the purpose 

of estimating the risks, the model mills were characterized to reflect operating mills, and the 

dispersion modeling and population exposures were based on the arid conditions and sparse 

population density that characterize existing impoundments in the southwestern states. 

The results of the modeling exercise indicated an MIR of 1.6×10-4, a fatal cancer incidence of 

0.014 per year, and only 200 persons at a risk greater than 1×10-4. Given the numerous 

uncertainties in establishing the parameters for the risk assessment and in modeling actual 

emissions and exposures, the Administrator found that the baseline emissions for new tailings 

impoundments met the criteria for presumptively safe. 

The decision on an ample margin of safety for new tailings considered two alternatives to the 

baseline of one large impoundment; phased disposal using a series of small impoundments and 

continuous disposal. The evaluation of these alternatives showed a modest reduction in the MIR 

and the number of fatal cancers per year, but a significant increase in the number of individuals 

at a lifetime risk of less than 1×10-6. The costs estimated for the two alternatives showed that 

WA 4-11, Task 4 – NESHAPs History 6 SC&A – September 25, 2008
phased disposal would lead to an incremental cost of $6.3 million, while continuous disposal was 

believed to actually result in a modest cost saving of $1 million. 

Given the large uncertainties associated with the risk and economic assessments performed for 

the new tailing impoundments, and considering the boom and bust cycles that the uranium 

industry has experienced, the Administrator determined that a work practice standard was 

necessary to prevent the risks from increasing if an impoundment were allowed to become dry. 

Finally, although continuous disposal showed slightly lower over-all risks and costs than phased 

disposal, the Administrator recognized that it was not a proven technology for disposal of 

uranium mills tailings. Therefore, he determined that the work practice standard should allow 

for either phased disposal (limited to 40-acre impoundments, with a maximum of 2 

impoundments open at any one time) or continuous disposal. 

4.0 COMPARISON OF THE NESHAPS RISK ASSESSMENT WITH 

CURRENT RISK ASSESSMENT APPROACHES 

Since it is neither feasible nor practical to directly measure the exposure of individuals in the 0–

80-km area surrounding an emission source, risk assessments rely on modeling exposures. The 

essential elements in assessing the risk from a source emitting Rn-222 to the ambient air are as 

follows: 

• The source term (Ci/y) 

• The dispersion of the Rn-222 from the source to the receptor 

• The ingrowth and depletion of the short-lived Rn-222 decay products from the source to 

the receptor 

• The location of the receptors within the assessment area 

• The duration of the receptors' exposures 

• The fatal cancer risk/unit of exposure (risk/cumulative WLM) 

The following paragraphs discuss how each of these elements were derived for the risk 

assessments performed in support of the Subpart W rulemaking and suggest where different 

values might be used if the assessments were performed now. 

The source terms for existing impoundments were based on radon flux rates (pCi/m2

), Ra-226 

concentrations in the tailings, and the areas of unsaturated tailings exposed at each site. The 

radon flux rate that was used, 1 pCi/m2

 Rn-222/pCi/g Ra-226, was selected based on theoretical 

diffusion rates from thick sources. As diffusion rates depend on the porosity of the matrix and its 

moisture content, the specific rate that was selected was chosen to reflect site conditions that 

pertain in the southwestern part of the country where the industry is concentrated. The 

concentrations of Ra-226 in the tailings reflect measurement data, while the areas of unsaturated 

tailings were estimated based on both photographs of the impoundments and information 

WA 4-11, Task 4 – NESHAPs History 7 SC&A – September 25, 2008
supplied by the industry during the public comment period. Given that the NESHAP imposed an 

annual requirement for the facilities to measure and report their Rn-222 emissions, it should now 

be possible to develop the source term for each mill based on measurement data. If this approach 

were to be taken, it would be necessary to know the configuration of the impoundment during 

each measurement period. 

The dispersion of the Rn-222 from the tailings impoundments to the receptors in the assessment 

area were estimated using the AIRDOSE-EPA model in the CAP88 assessment code. 

AIRDOSE-EPA uses a Gaussian plume dispersion model to calculate air concentrations at the 

locations of the receptors. The model uses meteorological data (in the form of joint frequency 

distributions of wind speed and direction by stability class), annual precipitation rate, and lid 

height supplied by the user, and it accounts for removal from the plume by dry deposition and 

scavenging. The model allows for the source to be characterized as either a point source or an 

area source, and allows the user to input the effective release height. For the risk assessments, 

impoundments were characterized as area sources with an effective release height of 1 meter. As 

on-site meteorological data were not available, joint frequency data representing long-term 

averages were obtained from the nearest weather station. Annual precipitation rates and lid 

heights were site-specific. 

While the EPA has developed a number of dispersion models, including some that are more 

sophisticated than the AIRDOSE-EPA model, the AIRDOSE-EPA model is preferred for 

assessments involving radionuclides. When its predicted concentrations have been benchmarked 

against measured values, it has demonstrated good agreement; in fact, it has often surpassed the 

performance of more “sophisticated” models. 

The ingrowth and decay of the Rn-222 decay products is very important to estimating the risks 

from Rn-222 exposure. When the Rn-222 emanates from the tailings, the fraction of its shortlived decay products is zero, as they are retained in the tailings matrix. However, their ingrowth 

begins immediately, and theoretically could reach 100% (total equilibrium) at some distance. As 

a practical matter, 100% ingrowth is unlikely to ever be attained, due to dry deposition and 

scavenging during plume transport. For the Subpart W assessments, the equilibrium fraction of 

decay products was set at 0.70. The equilibrium fraction of 0.70 is appropriate for distances 

beyond approximately 15,000 meters from the impoundments (where the majority of the exposed 

populations are located) and assumes that an individual spends 75% of their time indoors. For 

individuals nearer to the impoundments than 15,000 meters, the assumption of a 0.70 equilibrium 

fractions will over-state their exposure and resulting risk. 

Since the NESHAP Subpart W was promulgated, there has been a great deal of work done to 

better characterize the appropriate equilibrium fraction for assessing exposure to Rn-222. In 

general, these studies suggest a somewhat lower fraction than was used in the Subpart W 

assessments, which would have the effect of lowering the estimated risks (UNSCEAR 2000). 

The locations of the receptors in the 80-km assessment area around each site were determined in 

one of two ways. For the 0–5 km radius, actual site visits were conducted, and the populations in 

each of 16 directions were determined for distances of 0.5, 1.0, 1.5, 2.0, 2.5, and 5.0 km. For 

distances between 5 and 80 km, increments of 5 km and 10 km were used, and U.S. Census tract 

data were used to estimate the population in each sector. 

WA 4-11, Task 4 – NESHAPs History 8 SC&A – September 25, 2008
Locating the nearby receptors by actual enumeration is seldom possible when conducting risk 

assessments. Generally, the best that can be done is to identify nearby sectors where persons are 

presumed to reside. Using the U.S. Census tract data to populate the 5–80 km assessment grid, 

while not perfect, as it places all of the tract's population at the centroid of the tract, is as accurate 

a means of calculating collective exposure as we have. 

For the NESHAPs risk assessment, individuals in the population are presumed to experience a 

constant level of exposure over a 70-year period (i.e., lifetime). While the Agency has often 

been criticized for making this assumption, it has repeatedly found that it is appropriate for 

evaluating radiation risks where age at exposure is an important factor in the over-all risk. 

The final step in the assessment of the risks from Rn-222 released from operating uranium mill 

tailings impoundments is to correlate the exposures with risk. For almost all radionuclide risk 

assessments, the risk is based on dosimetry models. However, for Rn-222, it is based on 

epidemiology. Specifically, based largely on work presented by the Nation Council on Radiation 

Protection (NCRP 1984) and the International Commission on Radiological Protection (ICRP 

1977), the NESHAP used a risk of 760 fatal cancers/106

 cumulative WLM (reflecting a risk 

coefficient of 7.6×10-4/WLM). 

As with the equilibrium fraction, considerable additional studies have been conducted to 

determine the appropriate risk coefficient for Rn-222. EPA’s current recommended risk 

coefficient is 5.38×10-4 per WLM. This value is based primarily on the National Academy of 

Sciences Report BEIR VI (NAS 1999). The EPA recommended risk coefficient is reported in 

Section VI of “EPA Assessment of Risks from Radon in Homes,” EPA 402-R-03-003, June 2003. 

Table 10 of that report, reproduced below, provides a more detailed breakdown of the risks. 

Table 10: Estimates of risk per WLM by smoking category and gender for a stationary 

population in which 53% of males and 41% of females are ES (Authors note: In this table, 

ES refers to ever smoked and NS refers to never smoked). 

Gender Smoking Category Risk per WLMa

 (10-4) Expected Life Span 

(years) 

Male ES 

NS 

ES and NS 

10.6 

1.74 

6.40 

71.5 

72.8 

72.1 

Female ES 

NS 

ES and NS 

8.81 

1.61 

4.39 

78.0 

79.4 

78.8 

Male & Female ES 

NS 

ES and NS 

9.68 

1.67 

5.38 

74.2 

76.4 

75.4 a Based on 1990 adult (ages greater than or equal to 19 y) ever smoked prevalence data (58.7 males and 42.3 

females are ES) and assumption that 37% males and 36% females or children (ages < 18y) will become ES). 

If the risk assessments for Subpart W were done today, the over-all risks that would be estimated 

would likely be somewhat lower than those estimated in 1989. 

WA 4-11, Task 4 – NESHAPs History 9 SC&A – September 25, 2008
5.0 CONCLUSIONS 

The risk assessments that were performed for the NESHAP for Operating Uranium Mill Tailings 

were based on the best data and science that were available at the time. While it is unlikely that 

the over-all risk profile would change significantly if the assessments were redone today, 

additional data and new scientific findings have become available. Most significantly, measured 

emissions data should be available, and the current scientific consensus on both equilibrium 

fractions and the radon risk coefficient could be used in the assessment. 

6.0 REFERENCES 

40 CFR 61, Title 40, Part 61: National Emission Standards for Hazardous Air Pollutants, 

Subpart W: National Emissions Standards for Radon Emissions from Operating Mill Tailings. 

40 CFR 190, Title 40, Part 190: Environmental Radiation Protection Standards for Nuclear 

Power Operations. 

40 CFR 192, Title 40, Part 192: Health and Environmental Protections Standards for Uranium 

and Thorium Mill Tailings. 

NAS (National Academy of Sciences) 1999. Biological Effects of Ionizing Radiation (BEIR) VI: 

The Health Effects of Exposure to Indoor Radon. National Academy of Sciences, National 

Research Council. 1999. 

EPA (Environmental Protection Agency) 1979. Radiological Impacts Caused by Emission of 

Radionuclides into Air in the United States, EPA 520/7-79-006, Office of Radiation Programs, 

Environmental Protection Agency, Washington DC. August 1979. 

EPA (Environmental Protection Agency) 1983. Draft Report, Background Information 

Document Proposed Standards for Radionuclides, EPA 520/1-83-001, Office of Radiation 

Programs, Environmental Protection Agency, Washington DC. March 1983. 

EPA (Environmental Protection Agency) 1984. Final Background Information Document 

Proposed Standards for Radionuclides, EPA 520/1-84-022-1, Office of Radiation Programs, 

Environmental Protection Agency, Washington DC. October 1984. 

EPA (Environmental Protection Agency) 1986. Final Rule for Radon-222 Emissions From 

Licensed Uranium Mill Tailings Background Information Document, EPA 520/1-86-009, Office 

of Radiation Programs, Environmental Protection Agency, Washington DC. August 1986. 

EPA (Environmental Protection Agency) 1989. Risk Assessment Methodology Environmental 

Impact Statement NESHAPS for Radionuclides Background Information Document, Volumes 1–

3, EPA 520/1-89-005, Office of Radiation Programs, Environmental Protection Agency, 

Washington DC. September 1989. 

WA 4-11, Task 4 – NESHAPs History 10 SC&A – September 25, 2008
WA 4-11, Task 4 – NESHAPs History 11 SC&A – September 25, 2008

EPA (Environmental Protection Agency) 2003. EPA Assessment of Risks from Radon in Homes, 

EPA 402-R-03-003, June 2003. 

FR (Federal Register) 1977. EPA established environmental protection standards for nuclear 

power operations pursuant to its authority under the Atomic Energy Act (AEA). Volume 42, 

page 2858. January 13, 1977. 

FR (Federal Register) 1979. EPA determination that radionuclides constitute a hazardous air 

pollutant within the meaning of Section 112(a)(1). Volume 44, page 78738. December 27, 1979. 

FR (Federal Register) 1984. EPA withdrew the proposed NESHAPs for Elemental Phosphorus 

Plants, DOE-Facilities, and NRC-Licensed Facilities. Volume 49, page 43906. October 23, 

1984. 

FR (Federal Register) 1985a. EPA promulgated final standards for Elemental Phosphorus Plants, 

DOE-Facilities, and NRC-Licensed Facilities. Volume 50, page 7280. February 8, 1985. 

FR (Federal Register) 1985b. EPA established a work practice standard for Underground 

Uranium Mines. Volume 50, page 15385. April 17, 1985. 

FR (Federal Register) 1989. Final rulemaking package for Subpart W. Volume 54, page 51679. 

December 15, 1989. 

ICRP (International Commission on Radiological Protection) 1977. Recommendations of the 

International Commission on Radiological Protection, ICRP Publication 26, Ann. ICRP, 1, (1), 

Pergamon Press, 1977. 

NCRP (National Council on Radiation Protection and Measurements) 1984. Evaluation and 

Occupational and Environmental Exposures to Radon and Recommendations, NCRP Report 

No. 78, Washington, DC. 

UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) 2000. 

United Nations Scientific Committee on the Effects of Atomic Radiation Report to the General 

Assembly, with Scientific Annexes. United Nations. New York 2000.