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MEMO 

TO: Fernald Work Group 

FROM: Joyce Lipsztein and John Stiver, SC&A 

DATE: April 17, 2012 

SUBJECT: SC&A Comments on Slide 7 of NIOSH Presentation 

Some Comments on NIOSH Presentation delivered to the Fernald Work Group April 9, 2012. 

Slide 7 from NIOSH Presentation: 

DRAFT

MIVRML Thorium Algorithm

• Where ROI is the count rate in a region of interest

• For example: ROI0.208‐0.248 is the count rate in the

portion of the spectrum between 0.208 MeV and

0.248 MeV (from Pb‐212)

• 3.23 is the factor that represents the empirical

background data for 1100 unexposed workers

• and 8.84 is the thorium coefficient to convert to units of

mass (milligrams thorium)

There is no scientific basis for the equation presented on the slide above. It is an empirical 

equation derived for the lung measurement results of a specific thorium source used for 

calibrations at Y-12 and was intended to be used only for screening. 

 The value 3.23 (3.23 ± 0.7) is an average value of summed ratios of counts in the three 

ROIs obtained for about 1,100 non-exposed persons. It was derived for the fixed wholebody counter at Y-12. The ratio for non-exposed persons is dependent on and strongly 

influenced by the counting room background. The background of the Mobile In-Vivo 

Radiation Monitoring Laboratory (MIVRML) was higher than the background at the 

fixed facility. There are no scientific papers showing that the value of 3.23 ± 0.7 is 

appropriate and applicable for people measured in a higher background counting room. 
Memo to the Fernald Work Group 2 SC&A – April 17, 2012 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

 The sum of the counts under the Pb-212 and Ac-228 peak areas is strongly dependent on 

the amount of Pb-212 and Ac-228 in the source material, which in turn depends on the 

age of the source since chemical separation (degree of disequilibrium between parent and 

progeny). 

Example: 

Let’s assume that a worker was exposed to Type M thorium source for 60 days and that he was 

monitored at the MIVRML. Let’s assume he was monitored in the year he was working with 

thorium, on one of these dates: 

 The middle of his exposure period (30 days after the first day of exposure) 

 The last day of exposure 

 90 days, 120 days, 180 days or 360 days after first day of exposure 

 Minimum Detectable Activities (MDA) in units of nanocuries (nCi) for Pb-212 and 

Ac-228 were not reported for the mg thorium data. For illustrative purposes, this 

example applies the MDAs reported for Pb-212 and Ac-228 post-1978. 

If the lung burden of Th-232 was 10 mg at 30 days after the first day of exposure, his daily 

intake would have been 28.7 Bq. If the lung burden of Th-232 was 10 mg at 60 days after the 

first day of exposure, his daily intake would have been 17 Bq. If the lung burden of Th-232 was 

10 mg at 90 days after the first day of exposure, his daily intake would have been 23 Bq. If the 

lung burden of Th-232 was 10 mg at 120 days after the first day of exposure, his daily intake 

would have been 30 Bq. If the lung burden of Th-232 was 10 mg at 180 days after the first day 

of exposure, his daily intake would have been 47 Bq. If the lung burden of Th-232 was 10 mg at 

360 days after the first day of exposure, his daily intake would have been 137 Bq. 

(1) If the worker was exposed to a thorium source in equilibrium with the daughters, the 

activity of both Pb-212 and Ac-228 would have been equal to 1.1 nCi (40.5 Bq or 

2,430 dpm) on all monitoring dates. Thus, the sum of the counts in the regions of the 

Pb-212 and the Ac-228 peaks would be higher than background in all three regions. 

 

(2) If the worker was exposed to thorium material that went through three chemical 

separations (NIOSH’s worst-case scenario) producing a large disequilibrium ratio of 

Th-232 to Th-228 (1:0.19), then the activities of Pb-212 in the lung corresponding to 

10 mg Th-232 lung burdens would vary from 0.16 to 0.198 nCi, all below detection 

limits indicated for post-1978 (MDA equals 0.23 nCi). The activities of Ac-228 in the 

lung corresponding to 10 mg Th-232 lung burdens would vary from 0.005 to 0.11 nCi, 

all below detection limits indicated for post-1978 (MDA equals 0.24 nCi). As activities 

of Pb-212 and Ac-228 are both below the MDA, it would not be possible to distinguish 

the counts in the regions of interest (ROIs) of Pb-212 and of the two ROIs of Ac-228 of 

the workers from the counts of non-exposed persons. The result of the equation would 

lead to a conclusion that the workers were not exposed, instead of indicating a lung 

burden of 10 mg. 
Memo to the Fernald Work Group 3 SC&A – April 17, 2012 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

(3) If the worker was exposed to thorium material that went through a single chemical 

separation, immediately after separation, when the only isotopes present are Th-232 and 

Th-228 in equal activities but without the presence of daughters, the activities of Pb-212 

in the lung corresponding to 10 mg Th-232 lung burdens would vary from 0.81 to 

1.0 nCi. The activities of Ac-228 in the lung corresponding to 6 mg Th-232 lung 

burdens would vary from 0.003 to 0.0674 nCi, all below detection limits (MDA equals 

0.24 nCi). Activities of Ac-228 are below the MDA, and thus it is not possible to 

distinguish the counts in the two ROIs of Ac-228 of the workers from the counts of nonexposed persons. The only peak region that has higher counting than from non-exposed 

people is the Pb-212 peak. The equation described in Slide 7 cannot be used directly as 

posted. 

‐ The “thorium coefficient” of 8.84 is an empirical value that lacks a technical 

explanation, yet is the linchpin that translates the dimensionless sum of ratios to 

thorium mass for the sources and conditions of calibration at Y-12, as indicated in the 

“Rule of Thumb” document (letter from L. Max Scott to C.M. West dated November 

21, 1961). It cannot be applied directly to FMPC, or for that matter to most Y-12 

exposures, because the coefficient varies depending on the equilibrium rates from 

Th-232 and daughters for the sources comprising the lung burden in any given 

worker. The Rule of Thumb that NIOSH cites as “the algorithm used to calculate the 

thorium mass results from the MIVRML measurement data” does not explain how the 

coefficient was derived or its limitations as a screening tool. In the West paper, 

Health Physics Considerations Associated with Thorium Processing (West 1965), it 

is stated that a rise in the ratio of 1 is equivalent to 33% of the lung burden of the 

listed mixture. West does not explain how the 33% of the lung burden was derived,

but he points to its applicability to the particular mixture of daughters of the source he 

was using.

‐ In addition, the mean sum of ratios of 3.23 for non-exposed persons has a variation of 

0.7. This variation was used to derive the “MDA” for Th-232 in the lung (6 mg). 

Thus, all results between -0.7 and 0.7 multiplied by the 8.84 (-6 mg to 6 mg), would 

produce values within the variation of the population, with a mean of 0. It is 

important to note that this is not a true MDA based on counting statistics, as is the 

customary accepted practice in health physics. The “MDA” range of values as seen 

above would greatly underestimate a true exposure of 10 mg, as seen in the second 

scenario above (three chemical separations). It is not possible to give an upper value 

to this underestimation; much larger intakes could have been missed all together. On 

the other hand, if the worker was not exposed, his lung burden would be within the 

range of μg of Th-232 in the lung and some values could be an overestimation of the 

Th-232 burdens by 2 to 3 orders of magnitude. 

‐ Three main issues remain concerns to SC&A. First, the sum of ratios method 

depicted in Slide 7 can potentially miss very large intakes, depending on the number 

of purifications and the age of the source since separation. Second, the thorium 

coefficient of 8.84 is narrowly defined for a set of conditions unique to Y-12 and is 
Memo to the Fernald Work Group 4 SC&A – April 17, 2012 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and Worker 

Health for factual accuracy or applicability within the requirements of 42 CFR 82. 

not transferrable to FMPC or any other facility. Third, Th-232 was present at FMPC 

in both soluble Type M (TNT) and insoluble Type S (oxide and metal) materials. 

Ra-228 produced in a Type S matrix leaves the lung more rapidly than thorium and 

exhibits properties closer to Type M. These physico-chemical disparities are not 

captured by the sum of ratios screening method. 

Lack of Coherence between Slide 6 and Slide 7: 

‐ In addition, slide 6 of the presentation proposes the use of a correction factor of 5.25 

to be applied to measured results. This factor of 5.25 was derived assuming that 

Pb-212 was measured, and that the Pb-212 result was used to derive the mg of Th-232 

results. The same correction factor is not applicable to the empirical method 

described in slide 7. 

‐ In summary, if the empirical equation in slide 7 of the NIOSH presentation was 

applied without modifications, that would account for the source terms encountered at 

FEMP; the mg of Th-232 results were not derived correctly and carry huge 

uncertainties that cannot be reconciled. Thus, SC&A’s position regarding the 

veracity of the mg thorium chest count data, as articulated in the recent memorandum 

titled, Summary of SC&A Concerns Regarding the Latest Documents Posted by 

NIOSH to Complement their White Papers on In-Vivo Thorium Bioassay (SC&A 

2012), remains unchanged. 

REFERENCES 

SC&A 2012. Summary of SC&A Concerns Regarding the Latest Documents Posted by NIOSH 

to Complement their White Papers on In-Vivo Thorium Bioassay, Response to NIOSH white 

paper prepared for the Advisory Board’s Fernald Work Group. SC&A, Inc., Vienna, Virginia. 

April 6, 2012. 

Scott, L.M., 1961. Letter from L.M. Scott (Union Carbide) to C.M. West (Union Carbide): Rule 

of Thumb for Computing Thorium Body Burdens from In-Vivo Counts, November 21, 1961, 

Document Capture No. Y/AB-NIOSH-2007-046. SRDB 32615. 

West, C.M., 1965. Health Physics Considerations Associated with Thorium Processing, Report 

Number Y-KB-53, Union Carbide Corporation, Nuclear Division, Oak Ridge, Tennessee, Y-12 

Plant, 1965. SRDB 11596.