When estimating the numbers of people affected, authors should bear in mind that a proportion of the population will be indoors. This will provide a degree of protection against the effects of the release as compared to being outdoors. The level of protection is related to the rate at which air and toxic material enters the building and may be measured in air changes per hour (ACH). Models exist (see Davies and Purdy, 1986) to determine the outdoor concentration required to give an indoor SLOT or SLOD DTL dose. This (usually higher) outdoor concentration effectively defines the hazard range for people inside buildings.