Pollen allergies cause misery for more than a quarter of Britons - and the season's already underway. Jeremy Laurance explains the causes and cures

Has the sneezing season started already?

For some sufferers, it has. The season begins in March and April with tree pollen, of which birch is the worst, affecting 25 per cent of hay fever sufferers. Birch, ash and willow are now flowering across the UK and pollen levels will be highest in dry spells and in sunshine. The grasses that cause most hay fever, affecting 95 per cent of sufferers, start to flower in May, releasing their pollen into the atmosphere. The season peaks in June.

What is the cause?

Pollen is nature's weapon of mass destruction. In sensitive people it lays waste to the nose, eyes, throat, lungs, skin and mouth. The tiny grains provoke an allergic reaction that makes tissues swollen, red, hot, itchy and irritable.

How many people suffer?

Record numbers are succumbing. A recent study of 9,000 people in six countries showed that 26 per cent of the UK population - one in four - are hay fever sufferers, twice the level of two decades ago. The study also showed that Belgium had the highest rate, with 29 per cent of the population affected, and Italy the lowest at 17 per cent.

So, pollen counts are increasing?

Actually, they have declined steadily since the early 1960s. This is one of the many paradoxes about hay fever. The acreage of grassland in counties around London fell by more than a third up to the early 1990s.

So what lies behind the rise?

One factor is pollution, especially traffic pollution. Chemicals from vehicle exhausts are thought to be sensitising the airways of susceptible individuals, making them more prone to allergies. At the same time, proteins on the pollen grains may be washed off and adhere to particles in the polluted air, which, because they are so much smaller, can be drawn more deeply into the lungs, increasing the risk of an allergic reaction. Another factor may be our sterile modern environments (see box below).

Could modern medicines be a cause?

The growth of antibiotic use in childhood and the growing schedule of vaccinations that keeps viruses at bay may also play a role. But at the same time children are spending more time indoors than in the past, in centrally heated, double-glazed homes where they are exposed to higher levels of certain allergens, including house dust mites and cat hair - a form of indoor pollution. This in turn sensitises them to pollen.

How long is the hay fever season?

As the number of hay fever sufferers has grown, so has the length of the pollen season. According to the National Pollen and Aerobiology Research Unit, which runs 28 monitoring stations round the country and provides a daily pollen forecast, hay fever now starts earlier and lasts longer in what experts believe is a sign of climate change. Starting in spring with tree pollen, it runs through summer with grass pollen and into autumn with mugwort and hybrids such as chrysanthemum.

Many sufferers blame the spread of oil seed rape. Are they right?

In April and May, the ubiquitous fields of yellow with their distinctive smell burst into flower. But tests show that only one in 25 people (4 per cent) is allergic to oil seed rape. People who venture close to the fields complain of throat irritation, itchy eyes and a runny nose - symptoms indistinguishable from hay fever. But it is not a true allergy because it is not a reaction to the pollen. It is more likely to be due to the volatile organic chemicals that the crop gives off and that produce its smell.

Is there any help for plant lovers?

Maybe. Doctors have come up with a strategy for the sensitive gardener: the low-allergy garden. The basic recommendation is to avoid wind-pollinated plants, such as trees and grasses, which produce quantities of pollen carried on the breeze. Insect-pollinated plants are the ones to choose - brightly coloured with large petals and showy flowers, or which have the pollen deep inside the flower, or are scented to attract insects. Plants to avoid include ceanothus, evening primrose, scented Japanese honeysuckle, lavender, lilac and the ubiquitous privet hedge. Plants to choose include the delicate columbine, scarlet and orange geum, lobelia, love-in-a-mist, blue periwinkle and the strawberry tree.

How is hay fever treated?

Antihistamines and steroid nasal sprays are the standard medical treatments. Many can be bought over the counter but are expensive. Generic versions of branded medicines can be significantly cheaper. Clarityn, an antihistamine, costs £4.45 for a pack of seven tablets. Superdrug sells a generic version containing the same amount of loratadine [the active ingredient] for half the price. Some people dislike antihistamines because they make them drowsy but newer versions on the market, it is claimed, are non-sedating.

For the worst-affected sufferers, vaccination is an option, but it is a tough choice. It involves weekly injections for eight weeks followed by monthly injections for three years and, rarely, it produces a severe adverse reaction.

What about non-drug treatments?

The herbal remedy butterbur is claimed to be as effective as antihistamines but without the side effects. It is sold as capsules, which should be taken three times a day. The plant is found in hedgerows and has been used in herbal medicine as a treatment for tuberculosis and asthma. However, fresh-picked butterbur contains toxins and should not be used.

Or you could simply try eating more onions. Onions contain quercitin, the antioxidant that is known for its supposed ability to relieve hay fever, as well as eczema, sinusitis and asthma.

Are there homoeopathic remedies?

Yes. A mix of all the common tree, grass, flower and weed pollens is available in a homoeopathic dose, which is claimed to modify the immune response. It is best taken as a preventive course from three weeks before the individual's hay fever normally starts. Different remedies are available for itchy eyes, sore throat or runny nose. It is also said that acupuncture can help hay fever but clinical trials have proved inconclusive.

What about devices to protect against pollen?

Portable air conditioning units for home and car, specially designed for hay fever sufferers, will filter out pollen grains. A nasal air guard that fits directly into the nose performs the same function, ostensibly, for a fraction of the price.

A plug-in vaporiser that releases a blend of eucalyptus, menthol, camphor and other natural oils into the room can help relieve blocked noses and feelings of stuffiness.

If nothing works and the GP can't help?

You can try onsulting one of the UK's dozen NHS full-time allergy clinics - contact them via Allergy UK (01322 619898; www.allergyuk.org).

Dishing the dirt: the role of germs in allergy resistance

Why is hay fever getting worse?

The paradox about hay fever is that while dirty - or polluted - air plays a role, so do clean, bug-free homes where the disinfectant wipe is king. A chief cause of the rise in hay fever - and the worldwide rise in allergies - is thought to be the increasingly sterile environment in which we live. This is known as the hygiene hypothesis.

Cleaner homes and smaller families mean that children are less exposed to bacteria and infections that help to educate their developing immune systems to identify and deal with invading threats. They then over-react to substances that mimic the threat - proteins on pollen grains, house dust mites, cat and dog hairs - which trigger allergic reactions.

What's the evidence for the hygiene hypothesis?

It was first advanced more than a decade ago by David Strachan, now professor of epidemiology at St George's Hospital, Tooting, south London, and has been confirmed by many studies since. He noticed that first-born children were more prone to asthma and hay fever than their later-born siblings. Younger members of the family were more exposed to infections so their immune systems had more to work with. In addition to those they encountered in the outside world they had to contend with those brought home by the older sibling.

Is that it?

No. Children raised on farms with livestock have a third fewer allergies than those who are not. A second factor is thought to be the role of diet in pregnant women and infants. Those who consume low levels of antioxidants - fruit and vegetables - and high levels of fats and proteins are thought to be more allergy-prone.

Can hay fever be avoided?

No, but its effects can be reduced. Here are some ways to do so:

* Keep windows shut in buildings and cars.

* Avoid going out in the evening when pollen counts are highest (the pollen grains rise in the day and fall back to earth in the evening).

* Take a holiday in Wales - western coasts have lower pollen counts because they have on-shore breezes, and are cooler and damper.

* Avoid open grassy spaces. Keep the grass mown so it does not go into flower.

* Buy a Porsche - they come with a pollen filter as standard.

* Dry washing indoors to avoid pollen grains clinging to the fabric and being carried into the house.

* Wear sunglasses - they help to prevent (and disguise) itchy eyes.