Radon-222
(updated 2022)
Guideline
Based on the consideration of the potential health impact from radon released from tap water to the air inside a dwelling, the activity concentration of radon-222 in drinking water should not exceed 100 Bq/L.
The screening value applies to the concentration of radon at the point of use of the water, not at the source, because of the significant decrease in concentration which can occur due to radioactive decay and degassing during storage, treatment and reticulation.
Monitoring of radon activity concentrations in water is generally only warranted when groundwater is directly reticulated to an enclosed building.
General description
Radon-222 is a radioactive gas produced from the decay of radium-226 in soil and minerals. It has a half-life of 3.8 days.
Radon concentrations in surface water supplies are very low due to the gas being rapidly lost to the atmosphere. Elevated concentrations of radon-222 may occur in drinking water derived from groundwater, due to the release of radon from aquifer rocks and minerals, particularly in granitic areas. Dissolved radon 222 in drinking water may be released to air during domestic use and contribute to indoor radon concentrations (WHO 2009).
Screening value for radon
The screening value was determined following consideration of these points:
ingestion of radon in water does not pose a sufficient risk to health to warrant consideration of this pathway in setting a reference level based on ingestion (UNSCEAR 2000).
the main sources of radon in indoor air are the subjacent ground and building materials, with radon in tap water being normally only a small contributor. The release of radon from tap water into household air will depend upon the volume and nature of water usage in the dwelling. The overall radon concentration in air will be influenced by factors such as the construction of the dwelling, ventilation rates and domestic practices.
given the indirect nature of the exposure pathway and the number of assumptions that must be made to assess the dose to an individual arising from the inhalation of radon released to household air from tap water, it is not appropriate to use a level of dose as the basis for a screening value for radon in drinking water.
the existing published data estimates that the transfer coefficient of radon from water to indoor air has an accepted value of (UNSCEAR 2000), although the actual value is highly variable. On this basis, a concentration of radon in tap water of 100 Bq/L would give rise to a concentration in air of 10 , which is 5% of the national reference level of 200 for radon in air in a dwelling (ARPANSA 2017). A screening value of 100 Bq/L would ensure that radon in drinking water would not be a significant contributor to indoor radon, even in the event the transfer coefficient was significantly higher than the accepted value.
Typical values in Australian drinking water
The data on the concentrations of radon-222 in Australian drinking water supplies are limited, but sufficient to indicate that radon may be significant in some rural groundwater supplies (Kleinschmidt et al. 2011).
Treatment of drinking water
The most effective way of eliminating dissolved radon-222 from water is by aeration, either actively by processes such as spraying, or by passive processes such as open-air storage. It is important that radon released from the water during aeration is extracted to the outdoors. Radon concentrations will also decrease by radioactive decay in water stored before use.
Analysis
The concentration of radon-222 in drinking water can be determined by liquid scintillation counting of a small volume of water (APHA 2017, ISO 2015, ASTM 2016). The limits of determination for these methods are approximately 1ā2 Bq/L. Samples must be collected and transported in a way that avoids outgassing of radon, or delays in the analysis; thus coordination with the receiving laboratory is recommended. Jobbagy et al. (2017) provides an overview of laboratory methods for the measurement of radon in drinking water. On site determinations of radon concentration in water can be made using portable equipment described in ISO 13164-3 2015.
Health considerations
The main health risk from radon arises from inhalation of the gas, particularly when it accumulates inside dwellings. Radon-222 has several short-lived radioactive progeny that can give rise to an increased risk of lung cancer. Epidemiological studies have established a relationship between the incidence of lung cancer and exposure to radon in both occupational and residential settings (WHO 2009).
Radon is soluble in water. When a tap or shower is turned on, dissolved radon is released into indoor air. Therefore, controlling the inhalation pathway rather than the ingestion pathway is the most effective way to control doses from radon in drinking water.
No link has been demonstrated in either experimental or epidemiological studies between ingestion of radon in drinking water and increased cancer rates.
References
APHA Method 7500-Rn (American Public Health Association) (2017). Standard Methods for the Examination of Water and Wastewater. 23rd edition, Washington DC.
ARPANSA (Australian Radiation Protection and Nuclear Safety Agency) (2017). Guide for Radiation Protection in Existing Exposure Situations, Radiation Protection Series G-2, ISBN 978-0-9873183-8-1.
ASTM (ASTM International) (2016). D5072-09 Standard Test Method for Radon in Drinking Water. Pennsylvania.
ISO (International Organization for Standardization) (2015). Water quality ā Radon-222. International Standard ISO 13164 Parts 1-4, Geneva, Switzerland.
Jobbagy V, Altzitzoglou T, Malo P, Tanner V, Hult M (2017). A brief overview on radon measurements in drinking water. Journal of Environmental Radioactivity, 173: 18-24.
Kleinschmidt R, Black J, Akber R (2011). Mapping radioactivity in groundwater to identify elevated exposure in remote and rural communities. Journal of Environmental Radioactivity, 102: 235-243.
UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) (2000). Sources, effects and risks of ionising radiation. Report to the General Assembly. ISBN 92-1-142143-8, New York.
WHO (World Health Organization) (2009). WHO handbook on indoor radon: a public health perspective. ISBN 978-92-4-154767-3, Geneva, Switzerland.
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