Most people have experienced stuffiness and odours in buildings at some time. The air may have a high humidity and may contain particles, gases and vapours at a concentration higher than outdoors. Some of them, such as carbon monoxide, may be odourless. Spending time in such environments can cause discomfort, headaches and tiredness; some pollutants may be a risk to health either through direct exposure or indirectly by the high humidity encouraging mould growth and an increase in the house dust mite population. Poor indoor air quality may be improved by increased ventilation but this may cause draughts and heat loss. The solution is to balance the rate of release of moisture and pollutants into the indoor air with the rate of exchange of indoor and outdoor air. This information paper is a brief guide to using ventilation to achieve good indoor quality. It also advises on the sources of pollutants that may be present or introduced into your home. 4 pages.