Radon concentrations indoors tend to differ among countries and even individual buildings because of differences in climate, construction techniques, types of ventilation provided, domestic habits and, most importantly, geology.
After released from bedrock material, radon passes through the soil, diluting in the air before entering buildings. Granites, migmatites, some clays and tills are particularly rich in uranium and radium, which decay into radon. Radon exhalating from the ground beneath buildings is the main source of radon in indoor air.
Radon may enter buildings through cracks in the floor, gaps in construction, windows, drains or spaces around cables and pipes. This is particularly common in temperate and cold regions due to the pressure driven flow of gas which arises because buildings are normally at a slight underpressure compared to pressure under the building.
Radon does not dilute in indoor air as quickly as outside and tends to accumulate in the enclosed spaces of buildings, serving as a significant source of public exposure to radiation.
