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The distribution and sources of radon and thoron gas in homes in Western Australia

Toussaint, L.F. (2001) The distribution and sources of radon and thoron gas in homes in Western Australia. PhD thesis, Murdoch University.

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The largest contributor to public radiation dose (and hence cancer risk) is radon gas. There was thus a need for a study in Western Australia to determine base-line data for radon-in-homes concentrations and "map" the distribution for the State. Such a map can indicate the location of high radon concentrations. Remedial action can then be applied, where deemed necessary, to reduce the concentrations and hence the dose to the occupants in the identified areas.

The study was undertaken by conducting a Western Australia-wide survey of radon concentrations (and gamma radiation levels) measured over a one-year period for homes in Western Australia on a population-weighted basis. From this data, the map was produced showing both the radon and gamma distribution data. One purpose for also collecting gamma information was to determine whether or not gamma radiation levels might be a useful indicator of higher radon concentrations.

As there is sometimes confusion in publications on whether it is radon or radon progeny (Equivalent Equilibrium Concentration) that is being measured, this was clarified at the outset. A historical perspective of radiation dose was first discussed and this was used as a basis to deal with the concepts of radon and radon progeny and the method of converting these to a likely radiation dose based on intake when breathing.

The radon map produced from the study was used to select areas of higher radon concentration for further study, including measurement of tl10ron concentrations. A dwelling in the identified "high radon" area was selected for an individual study of daily radon progeny concentration patterns within the home, the likely radon generating sources (soils and building materials) and the exhalation of radon gas from the soils into the atmosphere.

Other specific radon-generating sources were also measured. Of particular note was the potential contribution of radon released from ground water at water treatment plants. In such plants radon concentrations in excess of 500 Bq.m'3 were measured over a full year of study. From measurements outside the plants, however, there was no indication that this radon source contributed significantly to radon-in-homes concentrations.

For the 1912 homes surveyed, the mean radon concentration was found to be 16.6 Bq.m'3 (SD: 20.7). This was found to contribute to an approximate annual dose of 0.28 mSv. The mean annual radiation absorbed dose in air from gamma radiation exposure was found to be 1083 microgray (SD: 349). For the Darling Scarp area, there was correlation of 0.81 between the higher radon concentrations and the higher gamma radiation levels, indicating that gamma measurements are a useful initial indicator of possible elevated radon concentrations.

The radon study and resulting map indicated that the higher radon areas occurred in parts of Gosnells on the old flood plain and in an area in the Darling Scarp. stretching from Kalamunda and Roleystone through York and to Beverley. Radon concentrations in these areas were sometimes in excess of 100 Bq.m'3.

The higher radon areas indicated in the map in the Darling Scarp provided the basis for selecting areas for thoron studies. A total of 20 passive monitors were used. The mean thoron concentration in homes in the Darling Scarp was 14.0 Bq.m'3 (SD: 11.6), while for the coastal plain it was 3.4 Bq.m'3 (SD: 2.8). For outdoors in the Darling Scarp, the thoron mean was 76.3 Bq.m'3 (SD: 136.3). These measurements show that the main source of the thoron in homes on the Scarp is from the underlying soil rather than from the building materials. Although the outside thoron concentrations close to the ground might be considered excessive, the thoron concentrations measured within the houses were considered to not be excessive.

An individual home in an elevated radon area in Gosnells was identified and selected for further study with a view to reducing the radon concentrations in the air breathed by its inhabitants. Specific radiological parameters were measured, including the radon progeny profile over a weekend period at 6-minute intervals. It was noted that the higher concentrations of radon (and hence its progeny) occurred between midnight and dawn in the home measured. Measurements showed that the source of the radon was primarily from the soil beneath the house, rather than from the building bricks. The progeny profile study indicated that slight pressurisation of the home using modem “evaporative” air conditioners is likely to be beneficial in reducing the radon entry into the home.

As a minor study, 6 measurements were made of the emanation (or exhalation) of the radon flux from soils. Although the data was limited, the results tended to confirm that the soil was the main source of radon and that exhalation in the coastal plain area was about 10% of the exhalation measured in the Darling Scarp.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Division of Science and Engineering
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Jennings, Philip
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