“Health-based standards exist for outdoor air, and there are good arguments for developing equivalent indoor air quality guidelines” It seems self evident that if there are health based standards for outdoor air quality, there is no reason that there should not also be standards applied to the indoor environment. After all, it is indoors that we spend most of our time; the home environment is particularly important because potentially vulnerable people—including sick, elderly, and very young people—spend a very large proportion of time there. Considering the attention and resources currently directed to improving the quality of outdoor air, it is perhaps surprising that—in the United Kingdom at least—so little importance seems to have been given to the (non-industrial) indoor environment. The indoor environment is important not only because of the amount of time spent inside buildings but because there are some very important indoor sources of pollution; including, for example, heating and cooking appliances, open fires, building and insulation materials, furniture, fabrics and furnishings, glues, cleaning products, other consumer products, and various biological sources—for example, house dust mites, fungi, and bacteria. There is also the inflow of polluted outdoor air through windows, evaporation of substances from water, and, in some locations, infiltration of radon and other gases into the building from the underlying soil and bedrock. Important chemical pollutants include combustion products such as nitrogen dioxide, fine particles, and carbon monoxide, formaldehyde from furnishings and furniture (especially particle board constructions), volatile organic compounds from paints, glues, flooring materials, and consumer products, and organochlorines and other substances from pesticides used in the home. Tobacco smoking is of course …