In Japan the highest incidence of hay fever was found in populations living along busy roads indicating that vehicle exhaust was involved. This is likely since diesel particles have been shown to make people more susceptible to allergens . Also high concentrations of nitrogen dioxide and sulphur dioxide can irritate the nose and airways. Some types of photochemical pollutants are more abundant in the countryside. For example, high concentrations of ozone occur in rural areas and can effect breathing even in healthy people .Research suggests that it makes the symptoms of hay fever worse but many other aspects are involved such as lifestyle and diet.
More work is needed to sort out the relationships between air pollution and allergies.
Although inheritance is important environmental factors also influence the development of hay fever. The best advice is to avoid exposures to high concentrations of pollen or spores. These may sensitise a person predisposed to allergy. They will not react to it on this first encounter but will have hay fever on subsequent exposures. For example people from northern Europe can develop allergy to Olive pollen if they are exposed to high concentrations during visits to the Mediterranean.