The incidence of hay fever has steadily increased and it now affects almost 25% of the population in the UK. The peak age for contracting hay fever is 20 years, although many children suffer, and it may develop at any age.

Two of the factors that contribute to hay fever are pollution and weather, and surprisingly hay fever is twice as common in towns as in the country. This is largely as a result of road traffic pollution and the effect of sunlight on traffic pollution, which is referred to as "photochemical smog". In effect, this creates a "lid" on the air at ground level, so the pollens released are unable to escape into the upper atmoshphere; instead they are more concentrated at lower levels where they cause the most problems for hay fever sufferers. So although the although city pollen counts have dropped, the number of people treated for hay fever has risen over 400% in the last 20 years.

Some research suggests that it is the particular type of air pollution that can make hay fever worse but the evidence is not clear-cut. Studies in many countries including Sweden, Italy and Spain have shown that hay fever rates are greater in cities than in the surrounding rural areas where actual pollen counts are higher. 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 pollution and ozone can make 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.

Research courtesy of The National Pollen and Aerobiology Research Unit, University of Worcester and Breakspeare Medical Group.

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