Health Effects of Exposure to Radon BEIR VI (1999) / Chapter Skim
Currently Skimming:
Appendix B Comparative Dosimetry
Appendix B Comparative Dosimetry
Pages 176-223
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 176... ...
In chapters 2 and 3, the committee reviewed the biological and epidemiological data suggesting that lung-cancer risk varies directly with exposure to radon and its decay products It is now assumed that lung-cancer risks varies directly with the dose of alpha energy delivered to the appropriate cellular targets, and that the dose can be estimated from the exposure using a dosimetric model. A dosimetric model is employed to estimate the dose received by particular classes within the general population such as adult males (not miners)
|
From page 177... ...
As used in the monitoring of uranium mines, an equilibrium mixture of these decay products
|
From page 178... ...
i is the average concentration of radon decay products during exposure interval expressed in WE and ti is the number of hours of the exposure. The cumulative exposure when spending all of the time in a house at a given decay product concentration is more than four times that for occupational exposure (8766 compared to 2000 hours worked on an annual basis)
|
From page 179... ...
ranges from 0.2 to 0.8 with typical values of 0.35 to 0.4 (Hopke and others 1995~. Ventilation Air Cleaner Radon Una ached Pa c e Ventilation Atom · <~ Deposition Air Cleaner Attachment Recoil Ventilation Deposition Air CleanerAttached ~>> Atom ~ FIGURE B-2 Behavior of radon decay products in indoor air.
|
From page 180... ...
To obtain estimates of the activity-weighted size distributions, the number distributions are multiplied by the probability of attachment for the unattached radon decay product (Porstendorfer and others 1979~. Exposure in Normally Occupied Homes Over the past decade, improvements in measurement technology have allowed the direct measurement of the activity-weighted size distribution of the radon decay products in normally occupied homes.
|
From page 181... ...
using several different types of diffusion batteries as well as cascade impactors. They again observed modes around 10 rim and 130 rim in the PAECweighted size distribution measured with a low-volume screen diffusion battery.
|
From page 182... ...
However, the minor mode was always < 5 nm, distinctly smaller than in the New York distributions. One of the problems with the extension of screen diffusion batteries to smaller particle sizes is the substantial collection efficiency of the high mesh-number screens typically used in diffusion batteries designed to cover the range of particle size from 5 to 500 nm.
|
From page 183... ...
Tu and Knutson (1988a,b) have used the 25 L mind screen diffusion batteries to measure the 2~8Po-weighted size distributions in the presence of several specific aerosol sources.
|
From page 184... ...
FIGURE B-6 Activity-weighted size distribution of the indoor aerosol in a closed room without active aerosol sources. Figure taken from Reineking and Portstendorfer (1986~.
|
From page 185... ...
185 rot ..
|
From page 186... ...
have also developed these graded-screen diffusion batteries for activity-weighted size distribution measurements. Intercompanson experiments between the measurement methods have been performed (Hopke and others 1992~.
|
From page 187... ...
187 .~ a' a' Cq a' Cq 3 oo oo _' bC o VO a' Cq a' a' Cq a' a' ·_4 Cq ·_4 .= o VO ¢ ^ C)
|
From page 188... ...
For both algonthms, he has developed a Monte Carlo method for determining the stability of the inferred size distnbutions. New input values for the concentrations found on each stage are chosen from a normal distributions using the measured radon decay-product activity as the mean value and the measured uncertainty as the standard deviation of the distnbution.
|
From page 189... ...
189 o ·_4 o .= .0 5 Cq a' ·_4 Cat a' be ·_4 a' 3 ·_4 ·_4 a' ·_4 s° Cut o .5 C)
|
From page 190... ...
However, these results are not representative of the activity-weighted size distributions produced in normally occupied homes. In all cases radon concentrations were also measured using standard continuous radon monitors.
|
From page 191... ...
However, the activity is found in particle sizes around 200 to 300 rim that are not deposited in the respiratory tract at a high rate and thus produce low doses per unit exposure (as will be presented later in this chapter)
|
From page 192... ...
. _ ~ 0.0 0.2 0.4 0.6 0.8 1.0 Equilibnum Factor FIGURE B-13 Cumulative frequency distribution for equilibrium factor based on the set of measurements for houses with a smoker (Hopke and others 1995~.
|
From page 193... ...
In both of these reports, efforts were made to measure activity-size distributions using a Casella cascade impactor (May 1945~. Simpson and others examined the airborne activity in the Beaverlodge Mine.
|
From page 194... ...
The hourly doses per unit radon concentration were calculated for each
|
From page 195... ...
195 o .0 4= · ~ ca m C~ C~ 00 C~ l R a' ~ C .g ~ V Cq V bC a' O O ~ ~n VO l ~ ~ ~ ~ a, ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ O O O O O O O O a, ¢ E~ CM CM oo ~ ~ O ~ oo ~ ~ ~ ~ ~ O ~ ~ ~ ~ oo oo CM CM ~ ~ ~ O CM ~ ~ CM CM ~ CM ~ CM ~ ~ CM ~ O ~ O O .
|
From page 196... ...
The size-dependent dose conversion coefficients can then be multiplied by the activity at each particle size and summed to yield the total dose. The reference size distributions in homes were taken from those measured by Hopke and others (1995)
|
From page 197... ...
0.5 ~ 0.4 a C;. 0 3 ¢ o on 0.1 0.0 - r//////J~ Po-2lS - ~\~ Pb-214 ~ Bi-214 - Il~llll~ll PAEC 100 1000 Particle Diameter (nary)
|
From page 198... ...
also uniform and calculation of absorbed dose can proceed without complication. However, nonuniformities in the spatial and temporal distributions of the radionuclide, coupled with heterogeneous tissue composition, make the calculation of absorbed radiation dose complex and uncertain, as in the case of inhaled radon and radon progeny in the respiratory tract.
|
From page 199... ...
The approach has been to develop an in vitro/in vivo experimental animal respiratory tract model cellular system to understand the relationships that exist between cytogenetic damage in different populations of respiratory tract cells and radiation dose. The damage in these cell types has been measured as cell-killing (Thomassen and others 1992)
|
From page 200... ...
A comparison of the dose-response relationships for these exposure conditions resulted in an RBE of 65 for protracted exposures to radon vs low-LET gamma radiation (Brooks and others 1995~. In addition to physical factors, there are many biological factors that can influence observed dose and exposure response relationships.
|
From page 201... ...
Studies related the initial radon-induced cellular damage to the cells at risk for radon-induced lung cancer. In these studies the frequency of micronuclei was measured in the deep-lung fibroblasts, deep-lung epithelial cells, and in nasal and tracheal epithelial cells following exposure to either 60Co or radon inhalation.
|
From page 202... ...
Thus, a dosimetric model will be needed to relate exposure to dose. Dosimetric Models Various models have been used to evaluate doses to the lungs from inhaled radon progeny (for example, Jacobi and Eisfeld 1980; Harley and Pasternack 1982; Hofmann 1982; NEA 1983; James 1988~.
|
From page 203... ...
To use the newly adopted ICRP lung model to evaluate the equivalent lung dose as a function of the radon progeny activity-size distribution, the software code LUDEP (Jarvis and others 1993) , which calculates regional lung deposition of activity per unit exposure, was extended to calculate tissue doses from the short-lived radon progeny as described by Birchall and James (1994~.
|
From page 204... ...
Diffusional Radon progeny undergo Brownian diffusion a random motion caused by their collisions with gas molecules; this motion can lead to contact and deposition on respiratory surfaces. Diffusion is significant for particles with diameters of less than 1 ,um; only then does their size approach the mean free path of gas molecules.
|
From page 205... ...
The surface area of the lung increases greatly as the air moves down the airways to the alveolar region. Particle deposition in the various portions of the respiratory tract varies with particle size, breathing rates, relative amounts of oral and nasal breathing, and the dimensions of the various regions of the respiratory system.
|
From page 206... ...
TABLE B-7 Parameters used in the ICRP (1994) lungdeposition model Functional Residual Capacity Extrathorasic Dead Space Bronchial Dead Space Bronchiolar Dead Space Tidal Volume Height Tracheal Diameter First Bronchiolar Diameter Ventilation Rate (m3 h-1)
|
From page 207... ...
Measurements have been made on other input parameters that are needed for the mathematical lung model. The results of the calculation using the ICRP model for a typical male breathing through his nose with a breathing rate with a typical mix of sleeping, resting, and light exercise (0.779 might)
|
From page 208... ...
For particle sizes above 500 nm, deposition starts to increase again because impaction can now help to deposit particles in the respiratory tract. For larger particles (> 2.5 Am)
|
From page 209... ...
. Deposition of radon progeny in the respiratory tract causes alpha irradiation of several epithelial tissues; in the nasal passages, the bronchi, the bronchioles, and the alveolated part of the lungs (NRC 1991~.
|
From page 210... ...
These dose conversion coefficients will be the basis for comparing the effects of exposure received from radon progeny arising from different exposure scenarios. Comparative Dosimetry The objective of the comparative dosimetry is to calculate the differences in dose that are deposited in the respiratory tract between males working underground and males under more normal conditions as given above, women, children at the age of 10, and infants at the age of 1.
|
From page 211... ...
FIGURE B-20 Physical Lung Dose Conversion Factor between airborne concentration of the short-lived radon progeny (in Bqm-3 and dose rate (in nGy hat) , as a function of carrier particle diameter (in nary)
|
From page 213... ...
fBronchiolar dead space volume. gExtrathoracic dead space volume.
|
From page 214... ...
The dose-rates per unit radon are calculated for men, women, children, and infants based on the activity-weighted size distributions measured. Each value of the dose per unit radon concentration in homes is divided by the dose per unit radon concentration in mines to yield a series of K values.
|
From page 215... ...
based on the set of measurements in smoker homes.
|
From page 216... ...
In the case of the homes with a smoker, there is increased radon-progeny activity per unit radon gas concentration as shown by the increased equilibrium factor, but because the activity tends to be carried by larger particles, it is less effective in delivering dose to the respiratory tract. In the case of the non-smoker homes, the number of the larger particles is smaller and the amount of airborne progeny activity per unit radon gas concentration is also less, but because it tends to be carried by smaller particles, the average dose rate per unit activity concentration is higher.
|
From page 217... ...
. K Factor 3 FIGURE B-26 Cumulative frequency distribution for the K factor for adult females based on the set of measurements in non-smoker homes.
|
From page 218... ...
. o 3 K Factor FIGURE B-28 Cumulative frequency distribution for the K factor for 1 year old infants based on the set of measurements in non-smoker homes.
|
From page 219... ...
The decay chain, including half-lives and the energies of the emitted radiation, is outlined in Table B- 13. Homes There have only been limited measurements of 220Rn and its decay products in indoor air.
|
From page 220... ...
220 ant o a' Cal a' o C)
|
From page 221... ...
The available data for the ratio of PAEC arising from 220Rn decay products to that from 222Rn are summarized in Table B-14. Rannou (1987)
|
From page 222... ...
Few health studies have been conducted on the possible health effects of inhaling thoron decay products. Among residents in high-background areas of Brazil, China, and India, statistically significant increases have been observed for chromosome aberrations.
|
From page 223... ...
To reduce the uncertainties concerning the effects of 220Rn decay products, we will need to better characterize dosimetry of thoron progeny and obtain more data on exposures. CONCLUSIONS In this chapter we have reviewed the basis for the dosimetric extrapolation of risk from miners to the general population, taking into account differences in the aerosol characteristics, breathing rates, and the respiratory physiology of the various segments of the population (women, children, infants, as well as men)
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.