Most members of the general public tend to regard their homes and the buildings in which they work as relatively safe havens from the physical and chemical stresses of the ambient environment. However, during recent decades a hazardous phenomenon concerning the built environment has become apparent. it can have a detrimental effect on occupants’ health and has implications for energy usage. Radon gas is the culprit! It has no taste, smelt or colour and its presence is therefore neither immediately apparent or readily detectable. Uranium and thorium, which occur naturally in rocks (e.g. granite) and elements, radon gas. This radioactive gas rises from the ground, enters the atmosphere and is then dispersed largely in the air so giving rise to low outdoor activity-concentrations, which are, at present, regarded as of little consequence for human health. However, radon gas can enter buildings, through cracks, gaps and joints in the ground floor, as well as from some of the building materials employed, such as granite. As a result, on numerous occasions relatively high indoor activity-concentrations ensue in some regions of the UK, notably Cornwall and Devon. All buildings, including dwellings, offices, schools and workplaces have been found to have radon gas accumulating in their interior environments and arising mainly from below ground level. An activity-concentration in excess of 200 Bq m(-3) of air of radon is now acknowledged to be a health hazard to occupants within the affected building. Indoor radon gas has been cited as the second most frequent cause of lung cancel - tobacco smoke being the most virulent - with approximately 2500 premature deaths per annum in the UK being attributable to the ionising radiations transpiring from naturally-occurring radon. Indoor radon gas currently accounts for approximately half of the average UK population’s exposure to ionising radiation, which is known to be responsible for carcinogenesis. The indoor radon problem has been exacerbated over recent decades clue to the implementation of certain energy-thrift measures, which have led to relatively airtight buildings and reduced ventilation air-exchange rates. Replacement of the air within buildings effectively reduces the concentrations of radon gas that would other wise occur. Energy-thrift measures therefore can represent a health hazard for those occupying buildings which are prone to high indoor-radon activity-concentrations.