Answer to Question #817 Submitted to "Ask the Experts"

Q

I have measured simultaneously (using LR-115 Type II SSNTD) the indoor radon and thoron concentration in Bq m^-3 and total radon, thoron, and their daughter concentration (PAEC) in mWL. For examble, at a location I have measured the following: radon: 75 Bq m^-3; thoron: 150 Bq m^-3; and PAEC: 20 mWL. Clarify for me how to calculate the dose to population. Also, I have measured the daughter concentration of radon and thoron using air sampler. For examble: Radon daughters: 5 mWL and Thoron daughters: 10 mWL at the same location. Which data can I use for dose calculation or can I use both?

A

By "dose to the population", we assume you mean dose to individuals who are continuously exposed to these indoor airborne concentrations in the home. The radiation dose from inhalation of radon and thoron and their daughter products can be expressed several ways, including (a) absorbed dose to the irradiated long tissue (primarily the bronchial epithelium), (b) average absorbed dose to the whole lung, (c) dose equivalent (or equivalent dose) to the whole lung, or (d) effective dose (i.e., the average uniform whole-body dose that is estimated to represent the same risk as the actual partial-body irradiation). For this answer, we will assume that you are interested in the effective dose. DOSES TO INDIVIDUALS You give three different sets of measurements that might be used for estimating dose to an individual in the measured location from airborne radon and thoron and their daughter products: 1. A set of radon and thoron concentrations (in Bq m^-3), 2. The potential alpha energy concentration (PAEC) of the airborne daughter products, (in mWL) – presumably the combined value for radon and thoron, and 3. A pair of separate radon-daughter and thoron-daughter PAECs (in mWL) from air sampling -- presumably for the same location as for measurement sets 1 and 2. The dose is primarily from the radon and decay products and, thus, the PAEC measurements are the ones most directly related to dose; however, it is often more expeditious to make measurements of the precursor radon gas, and thus, with the application of additional factors or assumptions, dose can also be estimated from airborne radon and thoron concentrations. The relationship between airborne concentration of radon, thoron, and/or radon and thoron decay products and the associated radiation dose is affected by a number of factors, the dosimetry continues to be reviewed, and as yet there is not complete unanimity as to the appropriate conversion factors. For the purpose of this response we will use some published values as examples. Alternative 1; Calculating Effective Dose from radon and thoron gas concentrations. Given: Radon = 75 Bq m^-3 Thoron = 150 Bq m^-3 The 1993 report of UNSCEAR (the United Nations Scientific Committee on the Effects of Atomic Radiation), "Sources and Effects of Ionizing Radiation," uses a radon conversion factor of 25 µSv y^-1 per Bq m^-3 for an indoor occupancy factor of 0.8 Or about 7,000 h y^-1). From the same report a thoron conversion factor of 25 µSv y^-1 per Bq m^-3 it can be inferred for the same occupancy conditions. Effective dose = (75 x 25) +(150 x 22) = 5,175 µSv y^-1 or 5.2 mSv y^-1. Alternative 2; Calculating Effective Dose from radon plus thoron PAEC. Given: PAEC =20 mWL. For this case we need a PAEC-to-dose conversion factor. Factors have been published for estimating effective dose from the cumulative exposure in WLM (working level months), where a WLM is exposure to one WL for 170 h. Usually an occupancy factor of 0.7 for the home is assumed, and thus for 1 year in a home with 1 WL, the exposure in WLM is (365×24×0.7/170)=36 WLM. We are not aware of a published single factor for radon daughters and thoron daughters combined. The ICRP (International Commission on Radiological Protection), in its Publication 65, "Protection Against Radon-222 at Home and at Work" (1993), presents a radon daughter dose coefficient for members of the public of 3.9 mSv per WLM. ICRP 65 does not give a dose coefficient for thoron, but ICRP Publication 32, "Limits for Inhalation of Radon Daughters by Workers" (1981), gives an effective dose equivalent of 3.4 mSv per WLM of thoron daughters. These two reports are based on different dosimetry models and the two conversion factors are not necessarily directly comparable. However, in the absence of an alternative, let us use the ICRP thoron values for the purpose of making an estimate. The values for radon daughters and thoron daughters are not greatly different, and thus for an estimate for the radon-thoron aggregate we will use an average value of 3.6 mSv per WLM. This gives a dose estimate for radon daughters +thoron daughters at 20 mWL: 0.020 WL×36 WLM/WL-y×3.6 mSv WLM^-1 = 2.6 mSv y^-1. Alternative 3; Calculating Effective Dose from radon PAEC and thoron PAEC. Given: Radon daughter PAEC = 5 mWL; Thoron daughter PAEC = 10 mWL. Effective dose equivalent estimates then are: Radon daughters at 5 mWL: 0.005 WL×36 WLM/WL-y x 3.9 mSv WLM^-1 = 0.70 mSv y^-1. Thoron daughters at 10 mWL: 0.010 WL x 36 WLM/WL-y x 3.4 mSv/WLM = 1.22 mSv y^-1. Radon daughters + Thoron daughters: 1.9 mSv y^-1. Comment on the Three Alternative Estimates. The three estimates, 5.2, 2.6, and 1.9 mSv y^-1 are different but of the same order of magnitude. Differences probably reflect differences in precision and accuracy of the three measurement methods, the uncertainties in the three difference calculational methods, and differences in dose conversion conventions developed and published at different times by different groups. POPULATION DOSE If all the individuals in the population of interest are exposed to the same concentrations, then the estimates of individual dose represent the average dose rate (mSv y^-1) for members of the population and the population dose (person-Sv) is the average dose times the number of persons in the population. If different subgroups of the population are exposed to different concentrations (and hence receive different average doses), then a calculation should be made of the typical individual dose for each population group. The average dose for the whole population is the person-weighted average across all groups and total population dose (person-Sv) is the sum of the population doses for the individual subgroups.