Hay fever - also known as seasonal allergic rhinitis, an immune system reaction triggered when the body is exposed to one or more types of pollen - affects about 12 million people every year in Britain. But exactly when it strikes depends on the type of pollen or fungal spores to which your body responds.
Symptoms can range from runny eyes and sniffles to headaches, disturbed sleep, listlessness, fatigue and even depression, but no one pollen causes any one symptom. So establishing which irritant (or irritants) is causing your hay fever does not mean you can choose an antihistamine treatment to suit. However, if sufferers know the lifespan of the pollens they react to they can pinpoint how long they need to carry around a supply of tissues, eyedrops and nasal sprays.
The National Pollen and Aerobiology Unit at the University of Worcester has been measuring pollen and fungal spores in the air since the 1960s. It has 33 sites on rooftops around the country, from Scotland to the Isle of Wight. The unit uses data from these sites to form a ''pollen calendar" (see right) that highlights which irritants are most active in which months.
While the hazel and yew tree start flowering as early as January, pollen from the mugwort weed, which thrives in uncultivated places such as roadsides and rubbish tips, peaks as late as July.
"The calendar is a useful tool for people who want to know what might be causing their hay fever," says Professor Jean Emberlin, of the The National Pollen and Aerobiology Unit. "Some people are affected by many different types of pollen and suffer for months, whereas others are allergic to a specific pollen and may only suffer for a couple of weeks. If they can detect which pollen is causing their symptoms, they can go out of their way to avoid it. "For instance," she says, "around 95 per cent of sufferers are affected by grass pollen, which usually starts to flower towards the end of May." This at least gives Britain's sneezing menfolk a couple of weeks to unleash the Flymo before mowing the grass becomes an impossibility until mid-September.
About a quarter of all sufferers are also affected by tree pollen. "Right now," says Prof Emberlin, "during the first two weeks of May, it's likely to be oak or plane trees that are to blame. Plane trees in particular have a small, heavy pollen grain that tends not to disperse very well, particularly in built-up areas."
Although the condition got its name because pollen binds itself to hay, it is an increasing problem in non-rural areas. Growing urbanisation has been cited as a key reason for the dramatic increase in cases (hay fever was barely heard of 200 years ago). With more people living in cities, a greater slice of the population is now exposed to diesel fumes, to which pollen also binds itself.
Given regional, seasonal and air quality differences, using a pollen calendar can be an imprecise science. "Temperatures and conditions differ up and down the country," explains Prof Emberlin. "Plants that flower in May in London may take another two to three weeks to flower in Edinburgh. But as a general guide, it's useful."
Pharmacist Martin Crisp of Superdrug agrees. "Sometimes people who come into the store in January with what they believe is a viral cold will actually have hay fever," he says. "But a little bit of detective work - such as asking them if they have a willow or yew tree in their garden, which both flower early - might go a long way towards diagnosing and treating them properly."
The classic way to treat mild cases is with an over-the-counter antihistamine tablet or spray: Britons buy 30 million packs of hay fever remedies a year. In severe cases, a GP might prescribe steroids to dampen down the inflammatory process, or send a patient to a special immunology clinic to undergo a series of desensitising injections.
But a new prescription-only treatment may offer hope to all sufferers. "There's good evidence that Grazax, a tablet made mostly of grass pollen, is proving effective, and with few side effects," says Dr Jenkins. "However, you have to take it for several months before your hay fever starts for its desensitising properties to work - and it's only in the second year of taking it that it seems to kick in.
"The real trouble is that it's very expensive - about £100 for a course - so primary care trusts are not keen for doctors to prescribe it."
So for now, sufferers should just keep taking the over-the-counter tablets.
HOW TO BEAT HAY FEVER
Put Haymax inside the lower nostrils. This traps pollen entering the nasal passages.
Wear wraparound sunglasses. This reduces sensitivity to light, which can trigger sneezing, and protects the eyes from allergens.
Dry your washing indoors to avoid bringing pollen inside.
Keep windows shut, especially in the late afternoon, when the pollen count rises. Also, don't drive with the windows down.
Take breaks by the coast. Sea breezes reduce pollen count.
Shower before bed.
Take clothes off before entering the bedroom - and sleep with windows closed.
For more tips, visit www.allergyuk.org
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