In western Europe and the USA the prevalence of asthma in both adults and children is rising; an increase that can not be reconciled simply by changes in diagnostic fashion (Cullinan, 1988; Gergen et al., 1988; Hahhtela et al., 1990; Burney et al., 1990; Weitzman et al.9 1992). Such increased prevalence has been too rapid to be explained by alterations in the gene pool and for this reason there has been a growing interest in the possible association between the environment and asthma (Newman Taylor, 1995). There are many ways in which the external environment or environments encountered in the home or workplace could impact on the incidence of asthma and other respiratory diseases. These include alterations in exposure to environmental or occupational aeroallergens, respiratory virus infection, indoor air quality and tobacco smoking (Newman Taylor, 1995). In addition, it has been proposed that outdoor air pollution plays a role in the pathophysiology of asthma and it is this latter association upon which the present chapter will focus.