LONDON plane trees may produce 75 per cent of the pollen blanketing some parts of Sydney but only 25 per cent of people who believe they are allergic to the trees' pollen are right. And 12 per cent cent are not allergic to anything at all.
A Sydney University academic tested 64 residents of inner city Sydney who claimed to be allergic to the trees' pollen to find whether these pollution-resistant trees were really to blame for the sneezing, wheezing and weepy eyes that many Sydneysiders suffer from in spring.
During September, in areas where plane trees were flowering, people claiming to be allergic to them were asked to keep a diary and wear a sampling device on their nose.
The results were surprising, said Associate Professor Euan Tovey, a research leader at the Woolcock Institute of Medical Research.
''Only a quarter were technically allergic to plane trees. Some weren't allergic at all,'' he said.
About 56 per cent were allergic to ryegrass, and 41 per cent to Bermuda grass. About 86 per cent were allergic to something, such as mites, cats and pollen, but not necessarily plane trees.
Professor Tovey said that while plane tree pollen doesn't cause as many allergies as most people presumed, their trichomes were a powerful irritant.
''Those white hairy things,'' he said, pointing at microscopic fibres on the back of the plane tree's leaves, ''are very sharp. They're trichomes. They come off and float off in the air. ''
More research was needed but it was very likely it was the trichomes that were causing the bulk of the problems, especially given intense exposure could make symptoms worse. That could explain why arborists often complained about plane trees.
Another problem was the trichome season, running for about 12 weeks from September to November, was twice as long as the pollen season for six weeks from late August.
Professor Tovey, who left his native Christchurch, in New Zealand, partly because of an allergy to ryegrass, says people have a tendency to think bright flowering plants, such as the bright purple tibouchina flower or jasmine, are the cause of allergies.
''If they can see and smell it, they blame it,'' he said. ''Up in Lismore, people blame the tibouchina flowers when it is the ragweed causing most of the problems.
''People presume the cause of the problem is a bright flower when it is really the dull boring ones that are designed to be windborne, like the trichome on the plane tree, that produces the problem.''
Professor Tovey said another problem was that allergies were highly regional, varying across the Sydney region, and most plane trees were in the inner city.