ORAU Team

Dose Reconstruction Project for NIOSH

Estimation of Radium-226 Activity in the Body from Breath 

Radon-222 Measurements

Document Number:

ORAUT-OTIB-0025

Effective Date: 04/05/2005

Revision No.: 00

Controlled Copy No.: ______

Page 1 of 5

Subject Expert: Michael G. Stabin

Document Owner

Approval: Signature on File Date: 03/31/2005

Cindy W. Bloom, Team Leader

Approval: Signature on File Date: 03/31/2005

Judson L. Kenoyer, Task 3 Manager

Concurrence: Signature on File Date: 04/01/2005

Richard E. Toohey, Project Director

Approval: Signature on File Date: 04/05/2005

James W. Neton, Associate Director for Science

Supersedes:

None
Effective Date: 04/05/2005 Revision No. 00 Document No. ORAUT-OTIB-0025 Page 2 of 5

RECORD OF ISSUE/REVISIONS

ISSUE 

AUTHORIZATION 

DATE

EFFECTIVE 

DATE

REV. 

NO. DESCRIPTION

Draft 12//03/2004 00-A A new technical information bulletin to provide 

information for converting radon breath analysis 

results to radium whole body activities. Initiated 

by Cindy W. Bloom.

Draft 12/09/2004 00-B Updated to address ORAU Team comments. 

Includes modification of radon-222 release 

fraction. Initiated by Michael G. Stabin.

Draft 12/15/2004 00-C Unit changes in table on page 3 and first equation 

on page 4 based on comment by Donald Bihl. 

Initiated by Cindy W. Bloom.

04/05/2005 04/05/2005 00 First approved issue. Initiated by Cindy W. 

Bloom.
Effective Date: 04/05/2005 Revision No. 00 Document No. ORAUT-OTIB-0025 Page 3 of 5

Radium-226 is a member of the 238U decay series. It has a half-life of about 1,600 years, and decays 

by α emission to 222Rn (“radon”), which in turn decays by α emission to 218Po and several other 

members of this decay series, before reaching stable 206Pb (see Figure 1).

Figure 1. Ra-226 decay series (from ICRP 1998).

Radium-226 entering the body via inhalation is deposited primarily in bone, but is also deposited in 

soft tissues (ICRP 1998). The radon formed by 226Ra decay can escape from the body by virtue of its 

being a noble gas, which is soluble in blood and can thus pass to the lungs, pass the lung/blood 

interface, and exit the body through exhalation. Srivastava et al. (1986) estimate that about 84% of 

226Ra in the body can escape from the body, based on the assumption that the equilibrium 

percentages of 226Ra in the body after inhalation intakes are 33%, 39%, 14%, and 14% in lungs, 

cortical bone, trabecular bone, and other soft tissues, respectively, and that 100% of radon formed in 

lung and soft tissue will be released and 70% of that formed in bone will escape (this results in a 

radon release fraction of 84% in breath). International Commission on Radiological Protection (ICRP)

Publication 67 cites the work of a number of investigators who note that the long-term retention of 

radon in bone will result in a 222Rn/

226Ra ratio of about one third at long times after intake, which is 

consistent with the above assumption (ICRP 1994). In addition, ICRP Publication 67 presents a 

recycling model with predicted results for adults that are in reasonable agreement with the values 

cited above for bone/soft tissue ratios of activity.

Srivastava et al. (1986) note that the body content of 226Ra can be estimated from the amount of 

exhaled radon by the relationship:

f

C I Q

Rn

Rn

λ =

where

Q = the quantity of 226Ra present in the body (pCi)

I = the breathing rate of the subject (L/h)

λRn = the decay constant of 222Rn (per h)

CRn = the concentration of 222Rn in the breath sample (pCi/L)

f = the release fraction for 222Rn

Subjects should breathe radon-free air (from a tank of oxygen, for example) for a few minutes before 

performing the test. The test is best performed at the beginning of a workday on a Monday, so that 

radon breathed in the workplace is not still present in the lungs and available to influence the results. 
Effective Date: 04/05/2005 Revision No. 00 Document No. ORAUT-OTIB-0025 Page 4 of 5

The location where the test is performed should be as radon-free as possible. Environmental radon, 

from soil, building materials, and other sources fluctuates considerably and can interfere with 

interpretation of the test results. Toohey, Keane, and Rundo (1983) noted that “the exhalation of 

radon increases by a factor of two and then returns to ‘normal’ in a period of 1-2 hr following a meal,” 

so measurements might be better made before work on a Monday but some time after a morning 

meal. For retrospective dose assessment, of course, no further control is possible over the data 

gathering, but dose reconstructors should keep these possible interferences in mind when evaluating 

individual cases.

As noted above, Srivastava et al. (1986) cite a value of 0.84 for f. This value is probably most 

applicable to workers who have been in mines for several years and thus chronically exposed for an 

extended period. An Oak Ridge Associated Universities memorandum suggests instead a value of 

0.65 for f, use of which will result in larger estimates of the 226Ra activity in the body (Dolan 1989). 

This is consistent with assuming an emanating fraction of 0.63 ± 0.06, which has been shown in a 

number of radium dial workers with long-term body burdens of 226Ra (ingested and assumed to all be 

in bone) (Toohey 1983). In these subjects, both 226Ra body content and 222Rn exhalation rates were 

measured at the same time, which lends additional credibility to the values. Assuming a value of 0.63

and a standard breathing rate of 1.2 m3

/hr [“light work” from ICRP Publication 66 (ICRP 2004)], a 

value of 1 pCi/L of radon in the breath corresponds to approximately 0.25 μCi of 226Ra in the body:

Ci

pCi h

h

L

L

pCi

0.252 μ

252,000

0.63

1

ln(2)

1 1200 91.8

=

=

This indicates that 1 pCi/L in breath would indicate the presence of 0.25 μCi in the whole body and 

approximately 0.13 μCi in the bone. Based on the above calculation, the conversion factor for breath 

222Rn in pCi/L to whole-body activity of 226Ra in pCi is 2.52 × 105

. 

The method discussed here relates only to quantification of 226Ra activity in the body and calculation 

of dose from this activity (and the activity of its progeny, including 222Rn, which can be formed in the 

body). This method is not applicable to quantifying radiation doses due to exposure to 222Rn gas (and 

its progeny) in the workplace.

Example

A breath radon-222 measurement gives a value of 0.05 pCi/L. The estimated 226Ra activity in the 

whole body is: 

E pCi pCi L

E pCi

L

pCi 1.26 4

/

0.05 2.52 5 = ++

The radiation dose to various organs from 226Ra in the body can be derived by selecting an 

appropriate intake scenario and applying appropriate dose factors to the derived 226Ra activity level.
Effective Date: 04/05/2005 Revision No. 00 Document No. ORAUT-OTIB-0025 Page 5 of 5

REFERENCES

Dolan, B., 1989 memorandum to P. Groer, “Relationship between Breath Radon Measurements and 

Skeletal Radium Burdens,” ORAU (Oak Ridge Associated Universities), September 1.

ICRP (International Commission on Radiological Protection), 1994, Age-Dependent Doses to 

Members of the Public from Intake of Radionuclides: Part 2. Ingestion Dose Coefficients, ICRP 

Publication 67, Pergamon Press, New York, New York.

ICRP (International Commission on Radiological Protection), 1998, Individual Monitoring for Internal 

Exposures of Workers, ICRP Publication 78, Pergamon Press, New York, New York.

ICRP (International Commission on Radiological Protection), 2003, Basic Anatomical and 

Physiological Data for Use in Radiological Protection: Reference Values, ICRP Publication 89, 

Pergamon Press, New York, New York.

Srivastava G. K., M. Raghavayya, P. Kotrappa, and S. Somasundram, 1986, “Radium-226 body 

burden in U miners by measurement of Rn in exhaled breath,” Health Phys. 50(2), pp. 217-

221.

Toohey R. E., A. T. Keane, and J. Rundo, 1983, “Measurement techniques for radium and the 

actinides in man at the Center for Human Radiobiology,” Health Phys. 44, Suppl. No. 1, pp 

323-341.