Radon is a colourless, odourless gas which is radioactive. It is formed as a by product of the natural radioactive decay of radium, which in turn comes from the decay of uranium, and where present radon can move through cracks and fissures in the subsoil, and ultimately into spaces under and within buildings and enclosed structures. High concentrations can pose a risk to human health. Uranium is found in small quantities in all soils and rocks, although the amount varies geographically. Radon levels vary not only between different parts of the country but even from building to building.

Increasingly it has become apparent that radon can be present in areas other than southwest England where it is traditionally associated with the granitic rocks of Cornwall. Whilst it is apparent that the air inside all buildings and confined spaces contains radon, some in recognised sections of the country might have unacceptably high concentrations unless precautions are incorporated into new and existing construction. In eastern England high risk areas include sections of Northamptonshire, Lincolnshire and Leicestershire and are associated with specific geological formations. One such horizon is the Northampton Sand Ironstone which outcrops along a distinct southwest to northeast arc across the above counties.

Current Building Regulations require precautions to be "taken to avoid danger to health and safety caused by substances found on or in the ground to be covered by building". Recent guidance published by the BRE outlines those areas where protective measures are required for new dwellings to protect against radon and also gives information upon protective measures. When radon decays it forms microscopic radioactive particles which can be inhaled into the lungs. The radiation from the particles can cause lung cancer.

A F Howland Associates have through a natural extension of our site investigation activities become increasingly involved in hazard assessment and advising upon remedial and protective measures. As matter of course we now include in all site investigation reports comments upon hazardous gases, and in particular radon. This is clearly pertinent to many structures for the water industry.

Whilst current guidance is focused upon new dwellings (i.e. houses), we do spend a significant part of our lives in "the workplace" and increasingly the need for protective measures in non-domestic buildings is needed in high risk areas. However, in non-domestic situations the protective measures need to be of a more general nature to allow for the far larger range of building types, uses, size and construction details. In high risk areas protective measures are recommended and in their simplest form comprise a membrane or barrier of low air permeability.

• damp proof membranes can be utilised to act as a radon barrier by extending them across the entire plan area of the building including foundation walls, and sealing all service penetrations and joints in the membrane.
• 1200 gauge sheeting is regarded as generally adequate but where there is a risk of puncturing the membrane reinforced polythene sheet should be utilised. Other materials can prove equally acceptable provided that they can produce an equivalent barrier to gas ingress.

Additional or secondary precautionary measures can also be incorporated as a back up should the membrane proof faulty or inadequate and include: -

• incorporating beneath ground bearing concrete floor slabs a sump with a pipe to the outside which can, if required, be used for slab depressurisation at a later date. For large plan area structures more than one sump will be required.
• providing a clean granular fill beneath the slab to maximise the influence zone of the sump.
• utilising a void beneath fully suspended slabs, which will on their own act a secondary protection.