If you thought the equation was simple - spring = rise in pollen = hayfever = much sneezing and sniffling round about now - think again. It's all much more complicated than that.
OK, pollen causes hayfever. That much is true. But which pollen? And when? And why were you sneezing your way through February, when everyone around you won't get going until June?
The answer is that there are many kinds of pollen, they arrive at different times of year, and some people are more allergic to some types of pollen than they are to others. It is true that 95% of hayfever sufferers are allergic to grass pollen, which begins to be released in large quantities at the end of May - so they get most of the publicity. But "for people who are allergic now, it's actually trees," says Catherine Pashley, a University of Leicester research associate in aerobiology who also works for the Midlands Asthma and Allergy Research Association. "People who suffer from grass allergies will probably be kicking off in a couple of weeks' time." There is a bit of grass pollen in the air, but, unless you're really very sensitive, it hasn't yet hit allergenic levels, which are highest in June.
A phenomenon called cross-reaction complicates issues further. Birch is the most allergenic tree - but a "lot of people who are allergic to birch will also have reactions to hazel and alder, which are in the same botanical family," says Pashley. Those who react to ash and olive trees can react to forsythia, privet and jasmine. People sensitive to oak can have problems with birch, alder, hazel, beech, sweet chestnut, hornbeam and hopbeam; plane trees are linked with birch.
Helpfully, the fact that trees are in the same family does not mean that they reach peak pollination at the same time: alder pollen release peaks in February and March; birch can pick up in March, followed by oak from May to June. Except if the weather plays up. "Birch was late this year because we had a pretty miserable start to the season," says Pashley. "It was quite damp and rainy.
And now that we've suddenly got this sun, we've actually got oak starting early, while we've still got birch in the air from the late season." Time for a perfect storm of sneezing, then.
