Water does not readily absorb radon but can be made to do so by placing it in contact with radium rich material. As the radium atom decays into radon, it ejects an alpha particle at high speed. This has the effect of imparting a force on the newly formed radon atom in the same way that the firing of a relatively small projectile from a gun will exert a recoil action on the gun itself. When this happens at the water/rock interface, the radon atom can be forced into the water where it will remain trapped until encountering an air/water interface, i.e. an air space in the soil structure or the water surface, where by choice it will migrate into the air.

When the water containing radon is released into an enclosed container, (such as a building) the radon will migrate into the air until it reaches equilibrium with the amount of radon remaining in the water. Radon concentrations in water can be very high, in the hundreds of thousands of Becquerels per cubic meter (Bq/m3), and is therefore described as the radon concentration per litre (Bq/L.)

In a building, the actual radon concentration in the air will depend on the amount of water exposed to the air and the ventilation rate or air changes taking place. In the domestic environment the radon in water to air transfer ratio is taken as 10,000 to 1. That is 10,000 Bq/m3 of radon in the water will cause a 1 Bq/m3 rise in the house air concentration. As stated previously, 10,000 Bq/m3 Rn in water is described as 10 Bq/L. Therefore, 10 Bq/L. in the water can result in 1 Bq/m3 in the house air.

A water supply with 1000 Bq/L. radon concentration can result in 100 Bq/m3 in the house air, above that resulting from radon emanating from the ground directly.

Industrial processes utilising a lot of water, depending on ventilation rates, might have higher airborne radon resulting from lower Rn concentrations in the water.

Radon released from water usage into the air will contribute to the lung cancer risk in the same way as that drawn from the ground.

In the domestic environment some water is also drunk and radon present in this can also increase intestinal cancer risks such as stomach cancers. For a person drinking one litre a day of tap water containing radon, the cancer risk is approximately equal to the risk from inhalation of radon released into the air from the same water source. That is to say that the risk at a particular level, using the 10,000 to 1 rule, is approximately double when both ingestion and inhalation risks are taken into account.

In a dwelling with 200 Bq/m3 radon air concentration, if half of this is due to radon emanating from the ground and half from radon released from water usage then the total risk would be equivalent to an air concentration of 300 Bq/m3 if the radon came from the ground only.

Radon is a Class �A� carcinogen and the US Congress has mandated that the EPA reduce exposures to Class �A� carcinogens to a risk of only 1 in 10,000. Based only on the inhalation risk using the 10,000 to 1 rule, the EPA is proposing a maximum contaminant level of 300 pCi/L (11.1 Bq/L) for public water supplies. This level is proposed because it corresponds to an estimated lifetime cancer risk of 2 in 10,000, and is consistent with risk guidelines for other regulated drinking water contaminants. However if a municipality is pursuing other radon reduction programs then a level of 4000 pCi/L (148.1 Bq/L) may be applicable.

In the UK the guideline for public water supplies is 100 Bq/L (2700 pCi/L) and for commercial water supplies 1000 Bq/L (27,000 pCi/L.)