There are five possible sources of radon in buildings: the ground underneath the building. the building materials, the water supply, the gas supply and outside air. In the UK, most domestic water surface waters, and no public supplies with high radon levels have been found (Wrixon et al., 1988). Similarly, gas supplies have very low concentrations of radon and do not contribute significantly to indoor radon levels (Wilkins, 1980). Radon in the outside air contributes a background to radon indoors because of exchange with indoor air. NRPB (1993) found that the best estimate of the radon level in Northern Ireland of 19 Bq m-3 compares with the original UK national survey (Wrinox et al., 1988). Building materials in the UK are not usually a significant source of radon and aretocontribute no more than a few Becquerels per cubic metre in dwellings (Cliff et al., 1984).
Most of the radon indoors is contributed by the ground underneath buildings. This is particularly true of buildings with high radon levels. NRPB (1994) found that no dwelling with high radon levels has been found in the UK where radon has come from sources other than the ground. The amount of radon r. entering buildings from the ground is influenced by the following four factors.
a) Radon concentrations in soil gas: This depends on the concentration of the immediate precursorof Rn-222, Ra-226, in rocks and soils. Elevated levels of radium are found in some granites, limestone’s and sandstone’s and other geologies
b) Permeability of the ground: This depends on the nature of the rock and soil under the building Disturbed ground can have greatly increased permeability. Usually the radon comes from the ground within a few metres of the building, but if the ground is particularly permeable or fissured it may come from a greater distance.
c) Entry routes into homes: Concrete floors often have cracks around the edges and gaps around services entries such as mains water supply, electricity or sewage pipes. If homes have suspended timber floors the gaps between the floorboards are the major route of entry. Pathways for soil gas to enter houses are often concealed, and vary between apparently identical houses.
d) Underpressure of homes: Atmospheric pressure is usually lower indoors than outdoors owning to the warm indoor air rising; this creates a gentle suction at ground level in the building through the so-called `stack effect’. Wind blowing across chimneys and windows can also create an underpressure (the `Bernoulli effect’). The result is that the building draws in outside air, typically at the rate of one air change per hour. Most of this inflow comes through doors and windows, but perhaps 1% or so comes from the ground. In an average house, this amounts to a couple of cubic metres of soil gas entering the house each hour.
The radon concentration in a building depends on the rate of entry of the radon and the rate at which it is removed by ventilation. Increasing the ventilation rate will not always decrease the radon concentrations, however, because ventilation rate and underpressure are related, and some ways of increasing ventilation, such as the use of extract fans or opening upstairs windows, can also increase the underpressure.
The factors described above vary greatly from one dwelling to another and lead to large differences in radon concentrations. The underpressure and ventilation rate also vary with time in all buildings. Underpressure tends to be highest in cold weather and at night because the difference in temperature between indoors and outdoors is greatest. At these times, ventilation routes such as windows and doors are generally closed, so higher proportion of the air drawn in by underpressure comes from the soil, thus causing higher radon concentrations.