A review of the literature indicates that there is an increase in the prevalence of self-reported symptoms suggestive of seasonal allergic rhinitis over the last 70 years. The reason for this remains unclear. Epidemiological studies which relate to the effect of pollution suggest that while pollution can exacerbate respiratory tract symptoms, there is no consistent evidence that high levels result in an increase in the prevalence of allergic rhinitis. The increased prevalence of house-dust mite, an alteration in immunity--possibly related to the number of childhood respiratory tract infections, and increased disease awareness, are all factors which may influence the reported prevalence of allergic rhinitis. At present the inconsistencies which exist in the literature mean that it is possible to 'pick' publications which support one view and ignore the rest, so that a variety of views may be held, each with apparently well referenced endorsement. The criteria defined by Bradford Hill (1965) to establish causation and not mere association between any of these factors and allergic rhinitis have, as yet, not been met (see Table V).