Here is an unnerving image. Any motorist driving at 70mph who starts to sneeze and is forced by reflex action to close his eyes for a couple of seconds will, in effect, be "driving blind" for nearly 80 metres. A motorist stricken by hayfever, however, can sneeze uncontrollably a dozen times in succession. Add to that, says Dr Richard Turner, fatigue and lack of concentration caused by disturbed sleep, and hayfever begins to take on a far more menacing image than the familiar one of being little more than a nuisance.

"In circumstances like those, the condition can actually be dangerous to others," says Turner, an allergy specialist at the North Hampshire Hospital in Basingstoke, and medical adviser to Action Against Allergy, a charity whose aim is to advance understanding of the complaint. "There is also an economic cost in terms of lost working days or below par performance, plus, of course, the very considerable distress it causes to the people who have it. I think outsiders underestimate how much misery it brings. When hayfever patients have been asked to fill in quality of life scores which measure an illness's effect on things like work, sleep and social interaction, they have significantly low scores, lower than asthma patients, in fact."

In hayfever, or seasonal allergic rhinitis, the body overreacts to pollen grains mistakenly treating them as attackers and produces antibodies to fight them. The antibodies, in turn, trigger the release of histamines causing inflammation and irritation of the delicate lining of the nose and eyes. Symptoms include constant sneezing, running, itchy or blocked nose, and watery, itchy, gritty-feeling eyes, sometimes with swelling of the whites, headaches and a general feeling of being unwell.

It is not known why some individuals are more susceptible than others, but hayfever tends to run in allergy-prone families. It usually starts in late childhood or early teens and peaks in the early 20s. Like many other allergies, including asthma and eczema, the condition is on the increase: an estimated 25-30% of the population will now have it at some point in their lives. The physiological reasons are unclear, though there is growing interest in the theory that we are becoming too clean: as the immune system has fewer real enemies, such as germs, to fight, the more likely it is to invent them.

The environmental causes are more easily measured. Professor Jean Emberlin, director of the National Pollen Research Unit at University College, Worcester, was a member of the World Health Organisation's working party on allergies and climate change across Europe which found that the season was extending in most areas. The UK birch pollen season, for example, has been growing by five to 10 days a decade.

"The trend towards warmer winters and springs means trees are flowering earlier," she explains. "Grasses require longer daylight to start releasing pollen, so they are not affected by earlier warm weather, but because summers are also becoming warmer and wetter they tend to flower for longer. So the pollen season can start in April now and last until the middle of August. This means an extra burden on the health service and on the sufferer."

She believes it may be time to rethink some of our public space planting. "A lot of councils plant birch, for instance, in parks and streets because it looks pretty and it's good for the soil, but like all wind-pollinated trees it needs to release a lot of pollen. Oak is the same, and London plane. We may be better planting insect-pollinated trees which don't need to produce so much because the insects carry it more efficiently. We're talking about flowery, showy trees like the cherry, but wildlife experts aren't so keen as they don't support as many varieties of insect."

The unit, which supplies the data for the Met Office's daily pollen count, can also provide information about specific dates and locations for students who believe that their exam performance was affected and want mitigating evidence.

Better still, says Muriel Simmons, chief executive of Allergy UK, would be to switch the nation's exam timetable to autumn or winter. "It is ridiculous to have exams slap bang in the middle of the hayfever season. These are young people's futures we're dealing with and they can be grossly handicapped and stressed by the condition. People tend to pooh-pooh it as just a few snuffles unless they have it themselves or are close to someone who does. Then they see the difficulties and distress it causes. It also tends to occur at an age when you're very self-conscious about how you look.

"It's awful having a red, dripping nose, or not being able to wear eye make-up, or trying to play sport when you're all blocked up. It's very depressing, and although it sounds melodramatic, I'd go as far as to say it is life-changing," says Simmons.

Simmons cites the case of her niece who left school early, abandoning her ambition to become a nurse because she would always have to sit exams when she was at her worst. "She was horrendously affected, sore streaming eyes, running nose that needed mopping all the time. She couldn't sleep properly, she couldn't concentrate properly and, as she said, even if she managed to deal with the exams who would want a nurse sneezing over them for a good part of the year."

Her niece is now 36 and has carved out a successful career as an occupational therapist but this is the first year she has not had hayfever. Her 37-year-old cousin and 60-year-old father still have it. "Most people tend to grow out of it in their early 20s but some don't," says Simmons. "And although the treatments are so much better these days, they don't work for some people, and for others they're often not 100% effective, particularly on very high pollen count days. Even for those for whom they do work, there is concern about long-term use of medication; and, of course, there's the cost."

There have been enormous strides in treatment for hayfever in recent years. The recognition of allergy medicine as a speciality in its own right by the NHS in 2000 has also helped, though the number of such doctors is still small, and waiting lists long for desensitising treatments, for example. The future, however, looks reasonably bright. A pilot study of sublingual immunotherapy, where grass pollen is given in liquid droplets placed under the tongue, has produced good results without adverse effects - further trials are underway. "The advantage is that people can do this themselves; they don't have to come to hospital," says Dr Amolak Bansal, consultant immunologist at St Helier Hospital in Carshalton, Surrey. "It would also be useful for children who obviously don't like injections."

The other path is via the creation of an anti-IgE. IgE is the antibody produced in an allergic reaction and an anti-IgE would bind to it and render it ineffective. Such a compound would have the enormous advantage of working against all allergens, not just pollens. An ongoing trial with peanuts in the United States looks promising. "I think we have grounds for optimism," says Bansal. "A solution isn't just round the corner but we think the next five years should bring significant advances."

· Aller-eze, the makers of nasal sprays and eye drops, are running a text message service during the season: subscribers receive pollen count information and alerts direct to their mobiles for 10p a message. Send a text message with full postcode to 07781 480206 to register for the service.

Prevention and cures

How to avoid it

· Keep an eye on the pollen count and stay indoors on bad days, especially early evening when count peaks.

· Keep doors and windows closed. Use air conditioning in house and car.

· Fit an air filter to car ventilation system.

· Wear wraparound sunglasses.

· Time holidays to coincide with peak season. The coast is always better than inland as are hot dry countries abroad.

· Don't hang out washing.

· Plant a low allergy garden.

· Get someone else to mow the lawn.

· Set the hose to a fine spray to dampen down pollen.

· Shower before bed to wash away pollens from hair and body.

Treatments

· Antihistamines and antiallergic tablets, capsules or liquids. Some are available from pharmacies, others need a doctor's prescription. Avoid ones that cause drowsiness unless you want a good night's sleep.

· Eye drops and nasal sprays work locally. Nasal sprays should be used as prevention. Avoid decongestants - they can make the problem worse.

· Saline douches or a little vaseline up the nose can be a first aid remedy.

· Steroid tablets or injections are a one-off solution if you have an important event such as an exam or wedding.

· Desensitising injections, where tiny amounts of the allergen are given to build up resistance, require long-term commitment: the patient must have a dozen or more over a period of two to three years. Currently only done in hospital outpatients in case of anaphylactic (extreme allergic) reaction.

· Complimentary therapies include the herb butterbur, acupuncture and a teaspoon of honey a day produced from hives within a five-mile radius of home.

· Homeopathic remedies combine euphrasia, allium cepa and sabadilla. The Royal London Homeopathic Hospital offers EPD (enzyme potentiated desensitisation) injections.