DISCLAIMER: This is a working document provided by the Centers for Disease Control and Prevention (CDC) 

technical support contractor, SC&A, for use in discussions with the National Institute for Occupational Safety and 

Health (NIOSH) and the Advisory Board on Radiation and Worker Health (ABRWH), including its Working Groups 

or Subcommittees. Documents produced by SC&A, such as memorandum, white paper, draft or working documents 

are not final NIOSH or ABRWH products or positions, unless specifically marked as such. This document prepared 

by SC&A represents its preliminary evaluation on technical issues.

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

MEMORANDUM

TO: Savannah River Site Work Group 

FROM: SC&A, Inc.

DATE: October 11, 2018 

SUBJECT: SC&A Review of RPRT-0081 Methods for Thoron Dose Reconstruction 

Introduction and Background 

The National Institute for Occupational Safety and Health (NIOSH) issued Addendum 3 to its 

evaluation of Special Exposure Cohort (SEC) Petition SEC-00103 for the Savannah River Site 

(SRS) on November 20, 2012 (NIOSH 2012), which dealt specifically with the feasibility of 

dose reconstruction of thorium exposures from October 1, 1972, through December 31, 2007. 

Subsequently, SC&A reviewed this addendum and identified 32 total findings (SC&A 2013). 

One of the 32 findings (Finding 4) concerned the potential radon-220 (also known as “thoron”) 

exposures at SRS during the period under evaluation. Specifically, Finding 4 of SC&A 2013 

(p. 10) states:

Finding 4: NIOSH has not discussed the radon-220 source term derived from the 

storage of thorium-232. The radon-220 dose could be important in some 

circumstances where there was significant residual thorium or where thorium was 

stored in significant amounts. This includes at least two high-level waste tanks.

This finding was first discussed at the teleconference meeting of the SRS Work Group on 

February 26, 2014. At that time, the discussion centered around a 1997 study (Sigg et al. 1997) 

of thoron releases and associated exposure levels at the H-Area Tank Farms (specifically, 

Tanks 12 and 15). These areas are particularly important due to the large amounts of thorium in 

storage and thus the most likely source of thoron emissions and associated exposure potential. 

While the source term would not appreciably change during the period 1972–1997, SC&A raised 

concerns that thoron emissions in the late 1990s may not reflect the exposure potential from the 

earlier periods due to the potential for modification of engineering controls that may have altered 

the exposure potential to thoron from the periodic venting of the tanks. 

Rather than attempt to characterize any changes in the tank farm equipment and engineering 

controls, NIOSH proposed to perform additional data captures at SRS to obtain a set of 

representative air sampling reports from the relevant tank farm areas. This data could then be 

used to evaluate the thoron exposure potential during years prior to the 1997 study, which was 

proposed for use in dose reconstruction. Specifically, NIOSH stated during the SRS Work Group 

meeting on February 26, 2014: 
Memo – SC&A Review of Thoron Model 2 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

Our path forward would be for us to go to the site and take a representative 

sampling of air samples over time from the tank farm area and evaluate those 

initial counts, the six-hour counts, the 24-hour counts, and come up with what the 

thoron component of the total radon would be. And that would be the dose we 

would assign to the workers in that area. [ABRWH 2014, p. 178] 

Subsequently, NIOSH produced ORAUT-RPRT-0081, Appropriateness of Using 1997 Gross 

Alpha Air Sampling Data as a Method of Bounding Thoron Intakes at the SRS H-Area Tank 

Farm Between 1972 and 1994, Revision 00 (NIOSH 2017a; hereafter “RPRT-0081”), which 

calculated the lognormal distributions of the air concentration due to thoron for three different 

periods (1982–1983, 1984–1985, and 1990) for comparison to the 1997 data (Sigg et al. 1997). 

The results of the air sample analysis were presented in Table 3-1 of RPRT-0081 and are 

reproduced in Table 1, below, for convenience 

Table 1. Results of NIOSH’s Analysis of Air Sampling Data at Thorium Locations at the 

SRS Tank Farm (from NIOSH 2017a, Table 3-1) 

Evaluated 

Period

Geometric Mean

(μCi/cm3)

95th Percentile

(μCi/cm3)

99th Percentile

(μCi/cm3)

1982–1983 1.24×10-12 9.03×10-12 2.05×10-11

1984–1985 3.97×10-12 2.89×10-11 6.58×10-11

1990 8.98×10-13 6.95×10-12 1.62×10-11

1997 2.84×10-11 2.67×10-9 1.76×10-8

Based on the data analysis shown in Table 1, RPRT-0081 (pp. 11–12) concludes: 

Air concentrations of thoron measured at H Area tanks in the 99th-percentile for 

the periods 1982-83, 1984 to 1985 and 1990 are less than 1% of the derived air 

concentration for thoron when its progeny are present (9.0 × 10-9 μCi/cm3). The 

majority of the data used to arrive at these values are from measurements taken at 

the point of exhaust, maximizing the concentrations compared to open air.... the 

exhaust values represent a conservative upper bound of potential thoron progeny 

exposures... 

NIOSH concludes that concentrations of thoron daughters reported in the 1997 

study are sufficient to bound thoron exposures of workers in thorium storage 

areas.

At its 122nd meeting, the Advisory Board on Radiation Worker Health (ABRWH 2018) tasked 

SC&A with reviewing the methodology and rationale presented in RPRT-0081; this 

memorandum presents SC&A’s review of that document. 

Discussion

While SC&A’s original concerns centered around changes in engineering controls occurring at 

the tank farms prior to the extensive study of thoron exposure in 1997 (Sigg et al. 1997), SC&A 

agreed that a more pragmatic approach to the issue would be to capture a representative number 
Memo – SC&A Review of Thoron Model 3 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

of air sample results from the tank farms that could be used to directly characterize the relevant 

exposure potential to thoron and its progeny over time. As stated in the introduction, NIOSH 

evaluated data from three different time periods—1982–1983, 1984–1985, and 1990—to 

characterize the thoron levels at the tank farm for the period from late 1972 through 1995.1

1 The data proposed for dose assignment presented in Sigg et al. 1997 were taken in January, March, June, October, 

and November 1996.

However, the stated time periods do not cover the entirety of the years stated. Table 2 shows the 

air sampling references analyzed in RPRT-0081 and the actual temporal coverage of NIOSH’s 

source data, which represents only ~8% of the period of interest (October 1, 1972–December 31, 

1995). 

Table 2. Temporal Coverage of Air Sampling Data Evaluated in RPRT-0081 

Time Period Cited Reference Actual Temporal Coverage

1982–1983 DuPont 1982–1983 1/4/1982, 2/8/1982–1/19/1983

1984–1985 DuPont 1984–1985 12/26/1984–9/11/1985

1990 * WSRC 1990a; WSRC 1990b ** 12/27/1989–2/28/1990; 12/17/1990–1/2/1991

* While RPRT-0081 indicates the air sampling data were all from 1990, a small fraction of air samples were from 

late 1989 and early 1991. 

** WSRC = Westinghouse Savannah River Company.

RPRT-0081 did note that sporadic air sampling data was captured for the period 1974–1979; 

however, the data did not provide the requisite level of detail to allow for evaluation of thoron 

exposure potential. It is unknown if additional air sampling data are available for the relevant 

areas of the tank farms that might be used to better characterize the temporal gaps in the analysis 

presented in RPRT-0081. 

Observation 1: To characterize and bound potential thoron exposures for the period October 

1972–1995, NIOSH analyzed data from three periods: 1982–1983, 1984–1985, and 1990. 

However, the actual temporal coverage of the data constitutes only approximately 8% of the 

period of interest. It is not known whether additional air sampling data are available to evaluate 

the temporal gaps in the NIOSH’s analysis. 

SC&A acknowledges that the data comparison presented for the years that were analyzed

demonstrates that the 1996 derived thoron exposures significantly bound the limited earlier data 

by an order of magnitude or more for these periods. Therefore, further data capture and analysis 

may not be deemed necessary to assure the approach is bounding. 

SC&A compiled the air sampling data for the three evaluated periods period as cited in 

RPRT-0081 and used the methods outlined in ORAUT-RPRT-0084, Two-Count Filter Method 

for Measurement of Thoron Progeny in Air, Revision 00 (NIOSH 2017b), to determine the 

airborne concentration of lead-212 (Pb-212). NIOSH (2017b) provides the following equation to 

determine the airborne concentration of Pb-212 (CPb): 

 
Memo – SC&A Review of Thoron Model 4 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

CCPPPP = �

λλBBBB − λλPPPP

λλBBBB

� ∗ (AA24 − AA6)

(ee−λλPPPP∗24 − ee−λλPPPP∗6)

∗ λλPPPP

FF ∗ �1 − ee

−λλPPPP ∗ TT�

where:

CPb = airborne concentration of Pb-212 (picocurie per liter [pCi/L]) 

λBi = decay constant for bismuth-212 (Bi-212) (0.687 per hour [hr]) 

λPb = decay constant for Pb-212 (0.065 per hr) 

A24 = total alpha activity at 24 hours (pCi)

A6 = total alpha activity at 6 hours (pCi)

F = flow rate of air sampler (L/hr)

T = collection time for the air sampler (hr) 

Figure 1 shows an example air sample record from Dupont 1982–1983. 

Figure 1. Example Air Sampling Record for Tank 42 from the H-Area Tank Farms in 

October 1982 (Dupont 1982–1983) 

To obtain the alpha activity in pCi from the example record in Figure 1, the values provided in 

counts per minute (cpm) are first multiplied by the scaling factor (4.1) to obtain the 

disintegrations per minute (dpm) and then divided by 2.22 dpm/pCi. This yields the following 

values of alpha activity:

A6 = (28 × 4.1) / 2.22 = 51.7 pCi 

A24 = (15 × 4.1) / 2.22 = 27.7 pCi 

The flowrate of the air sampler (F) in L/hr can be obtained by multiplying the reported flow rate 

of 3 cubic feet per minute (ft3/min) by 28.32 L/ft3 and 60 min/hr, which yields the value:

F = 3 × 28.32 × 60 = 5,097 L/hr 
Memo – SC&A Review of Thoron Model 5 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

Finally, the value for the collection time is simply the number of hours between 9:00 am on 

October 6, 1982, and 9:00 am on October 13, 1982 (168 hours of collection time). Therefore, the 

concentration of Pb-212 can be calculated using the NIOSH 2017b formula as follows: 

CCPPPP = �

0.687 − 0.065

0.687 � ∗ (27.7 − 51.7)

(ee−0.065∗24 − ee−0.065∗6)

∗ 0.065

5,097 ∗ (1 − ee−0.065∗168) = 5.9xx10−4ppCCpp/LL

To convert the Pb-212 concentration to microcuries per cubic centimeter (μCi/cm3), the above 

value must be divided by 103 cm3/L and divided by 106 pCi/μCi, which results in the following 

airborne concentration:

CPb = 5.9×10-4 / 103 / 106 = 5.9×10-13 μCi/cm3

SC&A’s full analysis of the cited air sampling references in RPRT-0081 is shown in Tables 3–5 

for each period and measurement location evaluated. Table 6 shows a comparison of the period 

totals to the proposed thoron levels in 1996 for use in dose assignment. Notably, the number of 

air sampling results evaluated in SC&A’s assessment was greater than the number reported in 

RPRT-0081. For example, for the period 1982–1983, SC&A evaluated 210 air sample results 

while RPRT-0081 appears to have evaluated 184. Similarly, SC&A evaluated 211 and 160 air 

sample results for the periods 1984–1985 and 1990, while NIOSH only reported 184 and 134 

samples for these same periods, respectively. The source of the discrepancy in total available air 

sample results available for analysis is not known at this time.

Table 3. Overview of SC&A Analysis of DuPont 1982–1983 Air Sampling Data at the 

Relevant Locations of the Tank Farms 

Measurement Location Number of 

Samples Analyzed

Average Pb-212 

Concentration 

(μCi/cm3

) * 

Maximum 

Pb-212 

Concentration 

(μCi/cm3)

Tank 13 General Air 3 4.07×10-11 1.06×10-10

Tank 13 Other 8 1.92×10-11 4.57×10-11

Tank 14 General Air 21 7.96×10-13 5.63×10-12

Tank 14 Other 14 1.56×10-12 4.55×10-12

Tank 15 Berm Area 32 1.00×10-12 9.15×10-12

Tank 15 General Air 30 6.21×10-13 7.68×10-12

Tank 15 Other 11 1.14×10-12 3.17×10-12

Tank 42 Annulus 46 1.15×10-11 7.03×10-11

Tank 42 Exhaust 45 1.54×10-11 2.64×10-10

Total 210 7.61×10-12 2.64×10-10

* Zero results censored at 0.2×10-12 μCi/cm3 are not included. 
Memo – SC&A Review of Thoron Model 6 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

Table 4. Overview of SC&A Analysis of DuPont 1984–1985 Air Sampling Data at the 

Relevant Locations of the Tank Farms 

Measurement 

Location

Number of 

Samples Analyzed

Average Pb-212 

Concentration 

(μCi/cm3) *

Maximum Pb-212 

Concentration 

(μCi/cm3)

Tank 11 Exhaust 28 5.54×10-12 1.08×10-11

Tank 12 Exhaust 37 5.56×10-12 2.54×10-11

Tank 14 Exhaust 35 4.07×10-12 1.16×10-11

Tank 15 Exhaust 37 1.25×10-11 4.92×10-11

Tank 42 (Primary) 37 8.70×10-12 1.52×10-11

Tank 42 (Annulus) 37 3.66×10-12 9.44×10-12

Total 211 6.75×10-12 4.92×10-11

* Zero results censored at 0.2×10-12 μCi/cm3 are not included. 

Table 5. Overview of SC&A Analysis of WSRC 1990a and WSRC 1990b Air Sampling 

Data at the Relevant Locations of the Tank Farms

Measurement 

Location

Number of 

Samples 

Analyzed

Average Pb-212 

Concentration 

(μCi/cm3) *

Maximum Pb-212 

Concentration 

(μCi/cm3)

Tank 11 Annulus GA 9 1.17×10-12 5.57×10-12

Tank 11 Unspecified 8 1.51×10-12 9.11×10-12

Tank 12 Annulus GA 9 1.69×10-12 9.05×10-12

Tank 12 Unspecified 8 4.38×10-12 1.52×10-11

Tank 13 General Area 18 7.05×10-13 2.85×10-12

Tank 13 Unspecified 8 1.26×10-12 5.67×10-12

Tank 14 General Area 18 1.32×10-12 9.05×10-12

Tank 14 Unspecified 8 6.87×10-12 3.80×10-11

Tank 15 Annulus GA 9 1.76×10-12 6.96×10-12

Tank 15 Unspecified 8 9.24×10-12 3.98×10-11

Tank 40 Annulus GA 8 1.25×10-11 6.17×10-11

Tank 40 General Area 9 1.56×10-12 8.36×10-12

Tank 40 Primary GA 8 3.37×10-13 1.52×10-12

Tank 42 Annulus GA 8 8.68×10-12 2.28×10-11

Tank 42 Primary GA 8 1.70×10-12 7.59×10-12

Tank 51 Annulus GA 8 5.43×10-12 2.09×10-11

Tank 51 Primary GA 8 1.40×10-12 7.08×10-12

Total 160 3.24×10-12 6.17×10-11

* Zero results censored at 0.2×10-12 μCi/cm3 are not included. 
Memo – SC&A Review of Thoron Model 7 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

Table 6. Comparison of SC&A Analysis of Earlier Time Periods to the 1996 Thoron Levels 

Proposed for Dose Reconstruction

Time Period Average Air 

Concentration

95th 

Percentile

99th 

Percentile

Maximum Air 

Concentration

SC&A Analysis of 1982–1983 7.61×10-12 Not 

Calculated

Not 

Calculated 2.64×10-10

SC&A Analysis of 1984–1985 6.75×10-12 Not 

Calculated

Not 

Calculated 4.92×10-11

SC&A Analysis of 1990 3.24×10-12 Not 

Calculated

Not 

Calculated 6.75×10-11

NIOSH Proposed Dose 

Reconstruction Analysis based on 

1996 Data

2.84×10-11 * 2.67×10-9 1.76×10-8 Not Presented

* This value represents the geometric mean rather than the arithmetic average

As shown in Tables 3–6, the average thoron levels prior to 1996 are at least a factor of four lower 

than the geometric mean presented in Table 3-1 of RPRT-0081. Furthermore, the maximum 

observed values during the evaluated periods are at least an order of magnitude lower than the 

95th percentile value calculated for 1996 (at the 99th percentile, the difference is nearly two 

orders of magnitude). 

Observation 2: Based on the comparison of the limited air sampling data for the period of 

interest, it is clear that the 1996 thoron assessments bound the exposure potential for the earlier 

periods evaluated in RPRT-0081. 

Finally, RPRT-0081 does not specify how NIOSH intends to assign thoron exposures to the 

relevant claimant population at SRS. RPRT-0081 (p. 12) states:

NIOSH concludes that concentrations of thoron daughters reported in the 1997 

study are sufficient to bound thoron exposures of workers in thorium storage 

areas. [Emphasis added.] 

It is not clear if this methodology is intended to apply only to tank farm workers or any worker in 

an area of SRS where thorium might have been stored. Additionally, it is not clear whether 

NIOSH intends to assign the geometric mean, the 95th percentile, or the 99th percentile as 

presented in Table 3-1 of RPRT-0081. RPRT-0081 should explicitly define how the proposed 

dose reconstruction approach would be implemented. 

Observation 4: RPRT-0081 should explicitly define how the proposed thoron exposures would 

be implemented in an actual dose reconstruction (e.g., the magnitude of exposure and 

workers/locations included).

Conclusion

SC&A noted significant temporal gaps in the analysis of air monitoring reports for the period 

from October 1, 1972, to the end of 1995 because NIOSH only analyzed approximately 8% of 
Memo – SC&A Review of Thoron Model 8 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution. 

the relevant time period. However, SC&A acknowledges that the calculated airborne 

concentrations reported in Table 3-1 of RPRT-0081 (and shown in Table 1 of this memo) are

significantly lower during the sampled periods prior to 1996; therefore, additional data capture 

and analysis are likely unnecessary as the 1996 thoron characterization represents a bounding 

scenario for dose reconstruction. If it is determined that the 1996 thoron exposure values are 

appropriate for dose reconstruction, NIOSH should specify how it intends to implement the 

proposed approach as it pertains to magnitude of exposure and location of affected workers. 

References

ABRWH 2014. Transcript: Teleconference Meeting of the Advisory Board on Radiation and 

Worker Health, Savannah River Site Work Group. February 26, 2014 

ABRWH 2018. Transcript: 122nd Meeting of the Advisory Board on Radiation Worker Health, 

Oak Ridge, TN. April 11, 2018. 

DuPont 1982–1983. Process Air Sample Results H Area 1982–1983, E.I. du Pont de Nemours 

and Company, Savannah River Plant, Aiken, SC. [SRDB Ref. ID 159268] 

DuPont 1984–1985. Process Air Sample Results H Area 1984–1985, E.I. du Pont de Nemours 

and Company, Savannah River Plant, Aiken, SC. [SRDB Ref. ID 159270]

NIOSH 2012. SEC Petition Evaluation Report Petition SEC-00103, Addendum 3, National 

Institute for Occupational Safety and Health, Cincinnati, OH. November 20, 2012. 

NIOSH 2017a. Appropriateness of Using 1997 Gross Alpha Air Sampling Data as a Method of 

Bounding Thoron Intakes at the SRS H-Area Tank Farm Between 1972 and 1994, ORAUTRPRT-0081, Revision 00, National Institute for Occupational Safety and Health. Cincinnati, OH. 

April 4, 2017. 

NIOSH 2017b. Two-Count Filter Method for Measurement of Thoron Progeny in Air, ORAUTRPRT-0084, Revision 00, National Institute for Occupational Safety and Health, Cincinnati, OH. 

March 27, 2017. 

SC&A 2013. SC&A Review of Addendum 3 to the NIOSH Savannah River Site Special Exposure 

Cohort (SEC-00103) Evaluation Report, SC&A, Inc., Vienna, VA. September 12, 2013. 

Sigg et al. 1997. R.A. Sigg, R.H. Ross, and A.H. Weber, Personnel Exposures from Periodic 

Venting of Radon Gas from High-Thorium-Content Tanks at H-Tank Farm, WSRC-TR-97-0033, 

Revision 0, Westinghouse Savannah River Company, Aiken, SC. April 8, 1997. [SRDB Ref. 

ID 49677] 

WSRC 1990a. Special Air Sample Results Building 241-24 H December 1990 – January 1991, 

Westinghouse Savannah River Company, Savannah River Site, Aiken, SC. [SRDB Ref. 

ID 153886] 
Memo – SC&A Review of Thoron Model 9 SC&A – October 11, 2017

NOTICE: This document has been reviewed to identify and redact any information that is protected by the 

Privacy Act 5 USC §552a and has been cleared for distribution.

WSRC 1990b. Weekly Air Monitoring Data, Tank Farm January – February 1990, 

Westinghouse Savannah River Company, Savannah River Site, Aiken, SC. [SRDB Ref.

ID 153887]