SC&A Review of Proposed NIOSH 

Methods for Reconstructing 

Thorium Doses at Fernald 

(1979–2006)

Bob Barton - Health Physicist

Joyce Lipsztein - PhD

S. Cohen and Associates

Contractor to:

Advisory Board on Radiation and Worker Health/ABRWH

Centers For Disease Control and Prevention

December 4, 2014

Cincinnati, OH
Thorium Internal Dose Reconstruction 

at Fernald

• Two separate SEC periods were granted for Fernald 

on the basis of the inability to reconstruct internal 

thorium exposures

• 1968–1978 recommended by ABRWH on 5/29/2012

• 1953–1967 recommended by ABRWH on 8/28/2013

• 6/26/2014: NIOSH releases a white paper detailing 

dose reconstruction methods from 1/1/1979 

through 12/31/2006

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Summary of Proposed Dose 

Reconstruction Methods (1979–2006)

• DR methods can be split into three distinct periods: 

1979–1989, 1990–1994, and 1995–2006

• Three periods delineated by the monitoring 

methods employed, data availability, and 

monitoring coverage in each time period

• Table on pages 12 and 13 of NIOSH white paper 

summarizes the methods for each period

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Summary of Proposed Dose Reconstruction 

Methods (1979–2006) continued

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Summary of Proposed Dose Reconstruction 

Methods (1979–2006) continued

• Selection of “thorium workers” during initial period 

(1979–1989)

• NIOSH white paper indicates the following job types: 

chemical operators, fork truck drivers, laborers, 

transportation laborers, operations, production workers, 

maintenance personnel

• Discussion during 9/3/2014 WG, Mr. Hinnefeld: “And we’d be 

pretty encompassing about that. You figure almost anybody in 

operations could have done that, most anybody in maintenance. 

Transportation could have been involved in it. You have safety and 

health people. Might have security people there. So you’ve got to be 

pretty inclusive...”

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Summary of Proposed Dose Reconstruction 

Methods (1979–2006) continued

• Selection of unmonitored “thorium workers” during 

initial period (1990–1994)

• Summary Table from NIOSH white paper who will be 

assigned thorium doses based on 10% of the DAC

• Main text of the whitepaper states: “1990–1994: Thorium 

workers with no in vivo results, but with pre-job fecal sample results 

during this employment period at Fernald are recommended to be 

assigned a dose...” 

• Unmonitored workers from 1995–2006 are not 

assigned a thorium coworker dose

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Summary of Proposed Dose Reconstruction 

Methods (1979–2006) continued

• NIOSH white paper also provides methods for 

calculating thoron exposure to thorium-related 

activities

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Summary of Proposed Dose Reconstruction 

Methods (1979–2006) continued

• White paper does not necessarily specify a specific 

worker or location that would restrict application of 

thoron exposures

• White paper states: “The dates and bounding levels of 

calculated potential exposures represent recorded operational 

history. However, thorium was present on site for most of its 

history. For unknown work locations and time periods of concern, 

Dose Reconstructors should assume that thoron exposure potential 

existed, as a claimant-favorable assumption, and assign thoron 

doses based on the guidance from the table.”

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Completeness Evaluation of Thorium In 

Vivo Records (1979–1989)

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Completeness Evaluation of Thorium In Vivo 

Records (1979–1989) continued

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Completeness Evaluation of Thorium In Vivo 

Records (1979–1989) continued

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Completeness Evaluation of Thorium In Vivo 

Records (1979–1989) continued

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• SC&A evaluated the number of days that would elapse 

between a given worker’s samples
Adequacy Evaluation and Interpretation of 

Thorium In Vivo Records (1979–1989)

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• Four main facets for interpreting the in vivo monitoring 

data:

1. Assumption of triple-separated thorium

2. Use of Pb-212 to calculate Th-232 and Th-228 intakes after 

results are adjusted for bias (median/mean of the normal 

analytical background adjusted to zero)

3. Exposures to unsupported Ra-228 to the monitored and 

unmonitored worker

4. Calculation of the OPOS Statistic 

• Use of Ac-228 chest burden results to assign Ra-228 intakes.

• Evaluate the Ac-228 chest burden (MDA/2 for missed dose or the 

measured result adjusted for bias) with Ra-228 biokinetic model 

and assign it as intake rate of Type M Ra-228.
Adequacy Evaluation and Interpretation of 

Thorium In Vivo Records (1979–1989) continued

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1. Assumption of Triple-Separated Thorium

• Discussed in numerous work group meetings, 

including the June 13, 2013 deliberations

• SC&A 2012 had the following statement concerning 

triple separated thorium: “SC&A agrees that the 

triple-separation hypothesis (Th-228/Th-232 = 0.19) is 

claimant favorable, for the period 1979–1988, when 

Pb-212 results are used to calculated the doses.”

• SC&A’s position remains unchanged
Adequacy Evaluation and Interpretation of 

Thorium In Vivo Records (1979–1989) continued

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2. Use of Pb-212 to calculate Th-232 and Th-228 

intakes after a correction for bias:

• SC&A 2012 had expressed concerns that there was an 

observed negative bias in the data: “Most of the Th-232 

progeny results above the MDA are for Ac-228, and in 

most cases, Ac-228 activities are higher than the Pb-212 

activities in the lung.” 

• Subsequently, NIOSH calculated an adjustment for the 

observed bias in the chest counts of Ac-228 and Pb-212.

• SC&A agrees with the adjustments for bias calculated by 

NIOSH.
Adequacy Evaluation and Interpretation of 

Thorium In Vivo Records (1979–1989) continued

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3. Exposures to unsupported Ra-228: 

• SC&A agrees with the use of Ac-228 results to calculate 

intakes of Ra-228, as described by NIOSH for the 

monitored worker.

• Evaluate the Ac-228 chest burden (MDA/2 for missed 

dose or the measured result adjusted for bias) with Ra228 biokinetic model and assign it as an intake rate of 

Type M Ra-228.

• NIOSH has not proposed a method for estimating 

unsupported Ra-228 exposures to the unmonitored 

worker.
Adequacy Evaluation and Interpretation of 

Thorium In Vivo Records (1979–1989) continued

4. Calculation of the OPOS (one person – one sample) 

Statistic:

• Thorium coworker model currently assumes a “postweighted” OPOS approach

• SC&A currently recommends a “pre-weighted” OPOS 

approach

• Currently under discussion by the SEC Issues Work 

Group

• Issue should be tabled pending the results of those 

discussions

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SC&A Analysis of Job Types Potentially 

Exposed to Thorium

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• Page 16 of the NIOSH whitepaper provides a 

short list of job types considered thorium 

workers for the purposes of assigning coworker 

intakes (1979–1989):

• Chemical Operator

• Fork Truck Drivers

• Laborers

• Transportation Laborers

• Operations

• Production Workers

• Maintenance Personnel
SC&A Analysis of Job Types Potentially 

Exposed to Thorium (continued)

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• SC&A analyzed available claimant files (CATI, 

DOE Response, DOL Case Files) to determine the 

appropriateness of the list of job types

• SC&A only examined claims with a POC <50%

• Workers were classified into four categories of 

workers based on the current guidance

1. Not likely to be assigned coworker intakes

2. Likely to be assigned coworker intakes

3. Unknown coworker applicability

4. Ambiguous coworker applicability
SC&A Analysis of Job Types Potentially 

Exposed to Thorium (continued)

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• The number of claims in each category is shown 

in the table below:

• Claims that fell into the third and fourth 

category are most germane to this investigation

• SC&A examined 20 such claimants in these 

categories
SC&A Analysis of Job Types Potentially 

Exposed to Thorium (continued)

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• Observations based on the examination of 20 noncompensated claims:

1. Job categories included: engineers, fire protection, technicians, 

analytical chemists, supervisors, inventory control, clerks, 

laundry, and various types of trades workers.

2. 13 of 20 surveyed claims indicated exposure potential to thorium 

in the CATI report.

3. Several claimants who worked after 1988 were monitored by the 

IVEC system, but were not monitored or sporadically monitored 

prior to this time. 

4. 10 of the 20 claimants indicated that work locations were highly 

variable and they “worked all over the site.”

5. 6 claimants specifically indicated direct work with thorium or in 

thorium areas (Building 64 and 65, thorium overpack, “thorium 

warehouse”) 
Review of Coworker Assignments (1990–1994)

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• Monitored worker doses based on the IVEC in vivo 

monitoring system

• Unmonitored worker thorium intakes are assigned 

based 10% of the class W thorium DAC value 

(5 × 10-13 μCi/ml)

• DAC value for solubility class Y is a factor of two higher 

(1 × 10-12 μCi/ml)

• NIOSH should consider the limiting DAC value based on 

solubility class Y unless information exists to the 

contrary
Review of Thorium Dose Assignments 

(1990–1994) continued

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• Many workers were monitored via the IVEC system 

during this period

• SC&A examined the records of claimants who have 

a POC <50% and worked for at least 3 months in 

the period of interest

• 252 total claimants identified for examination
Review of Coworker Assignments (1990–1994) 

continued

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• Observations based on claimant review:

1. Nearly 75% of the examined claims were monitored 

via the IVEC system

2. 67 claims were not monitored via the IVEC system in 

the 1990–1994 period

3. 45 of 67 can be considered job titles with little 

exposure potential including: clerk, secretary, 

contract administrator, HR representative, computer 

programmer, occupational health nurse, contract 

attorney, mail courier, intermittent auditor, 

estimator, data entry/analyst
Review of Coworker Assignments (1990–1994) 

continued

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• Observations based on claimant review (cont):

4. The remaining claims (22 out of 67) may have had exposure 

potential including: laborers, maintenance, painters, iron 

workers, heavy equipment operators, technologist, quality 

assurance, health physics, and engineers.

5. 9 of 22 unmonitored claims indicated “work all over the site”

6. 11 of 22 unmonitored claims indicated exposure in 

radiological areas was intermittent or non-existent

7. External badging was intermittent to non-existent in 10 of 22 

cases

8. One claim indicated involvement in thorium overpacking,

however the operation likely occurred after 1994 based on 

available breathing zone data.
Review of Coworker Assignments (1990–1994) 

continued

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• NIOSH white paper indicates that coworker intakes 

based on 10% of the DAC should be applied to workers 

who submitted thorium fecal samples but were not 

monitored via the IVEC system

• Based on sampling of unmonitored claimants, it is 

unlikely workers would have been chronically exposed at 

a level above 10% of the DAC for the duration of relevant 

employment

• Based on the sampling of unmonitored claimants, SC&A 

feels it is inappropriate to restrict coworker intakes 

based on the presence of a pre-employment fecal 

sample
Analysis of Breathing Zone Data for Dose 

Reconstruction (1995–2006)

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• Available breathing zone samples are used to 

reconstruct exposures to monitored workers

• No coworker dose assignment has been proposed after 

1994

• Issue was discussed at 9/3/2014 work group: “the 

thorium area would be defined... And if you’re going into 

this, into the thorium radiological area or the airborne, 

potential airborne area, everybody had a BZ with them.” 

- Mr. Hinnefeld
Analysis of Breathing Zone Data for Dose 

Reconstruction (1995–2006) continued 

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Analysis of Breathing Zone Data for Dose 

Reconstruction (1995–2006) continued

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Analysis of Breathing Zone Data for Dose 

Reconstruction (1995–2006) continued

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• Example provided before of a claimant who stated they were 

involved in thorium overpack operations

• From CATI: “[redacted] worked in the Thorium Overpack site 

where [redacted] remotely operated a device that would move 

drums around. [Redacted] had to dress out and enter the building 

to get an electric forklift, went over to the actual boxes they 

loaded the drums in (overpacks), [redacted] put a lid on the boxes 

and set them in an area for the Chemical Operators to clean, then 

the Rad Techs came in to survey them, if they were clean they 

were sent out to a driver on the "clean" area on process side and 

then they were sent to an area to be readied to ship offsite. 

[Redacted] ... In the Thorium Overpack [redacted] had to wear 

double sets of cloth coveralls. [Redacted] had to wear a cloth 

hood... [redacted] always wore a full-face respirator in the 

Thorium Overpack area... [redacted] had lapel monitoring done 

when [redacted] was in Thorium Overpack when [redacted] was 

dressed out in double sets of anti-contamination clothing.
Analysis of Breathing Zone Data for Dose 

Reconstruction (1995–2006) continued

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• Numerous thorium BZ samples were identified 

for this claimant both in the claim file and the 

HIS_20

• BZ samples covered the assumed period in 

question and were pulled and measured 

approximately every 6–7 days
Review of Thoron Exposure Estimates

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• Major assumptions

1. The white paper appears to contradict itself on the 

assumption of 300 mT thorium in the storage sites (the 

preceding page appears to quote 450 mT, the introduction 

quotes over 2000 mT of thorium materials in addition to 

the plant 8 silos and bins and pilot plant storage).

2. The release fraction should be better established. Quoted 

release fractions in the white paper appear to range from 

10-6 all the way to 10-3.

3. The equilibrium factor of 0.02 (or 2%) is not well founded. 

The stated reference indicates equilibrium factors could 

range from 2%–10%, but also notes “more precise studies 

are warranted.”
Review of Thoron Exposure Estimates 

(continued)

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• Major assumptions (continued)

4. SC&A could not determine the rationale behind occupancy 

times of 3 months (up through 1989) and 1 month (during 

final closure).

5. The specific activity of thoron was given as 6.4 × 10-4 pCi/g 

assuming exposures occurred 6–12 months after separation 

and an equilibrium fraction of Th-228/Th-232 of 0.65. The 

equilibrium fraction of materials stored in Building 65 was 

at least 0.95.

• NIOSH should evaluate potential exposure to thoron 

and progeny associated with storage and/or handling 

of Ra-228. 
Main Conclusions

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• SC&A finds that dose reconstruction for internal thorium 

exposures is feasible and can be performed in a 

claimant-favorable manner from 1979 through 2006.

• For 1979–1994, claimants should only be restricted from 

application of coworker intakes if it is clearly 

demonstrable that no thorium/thoron exposure 

potential existed for the worker.

• The main parameters for estimating exposure to thoron 

should be better described and established to assure 

that thoron exposures are calculated in a scientifically 

defensible, claimant-favorable and/or bounding 

approach.
Questions?

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