Chemical sensitivity which precedes the pollen trigger is increasingly being recognised and can also be treated. Nitrogen oxide, released from vehicle exhausts, can damage the sensitive lining of the nose at levels of 0.4 to 2.0 parts per million making sufferers more vulnerable to attack. Particles in diesel engine fumes can enhance pollen sensitivity and produce allergic reactions in their own right. Recent research shows that urban air may contain up to 100,000 nanometre sized particles per ml. These particles are coated with chemicals, such as ammonium sulphate derived from fossil fuel combustion, and these chemicals are irritants to the membranes of the nose, eyes, throat and lungs.
Many people suffer in sunny, bright weather as a result of the photochemical effect described above.
A proportion of people, however, suffer more at the onset of wet, rainy weather and researchers in Australia have discovered that starch granules packed tightly within pollen grains are released on contact with water and these are sufficient to provoke allergic reactions in susceptible people. In the presence of rain water and high humidity each pollen grain bursts releasing about 700 granules of starch. These are each approximately 1 millionth of a metre across and can easily penetrate the airways causing asthma and hayfever. Some people are affected more as a result of the dramatic increase of mould spores which occurs during rain and thunder storms. Patients may be more susceptible to moulds and dust during the hayfever season although they are able to tolerate them normally when they are not reacting to pollen.