With fields left ungrazed after the foot-and-mouth crisis, hay fever is affecting more people than ever, some of whom have never suffered symptoms before.

The thousands of fields of tall, uncut meadow grass gently swaying in the warm breeze may look for all the world like an image of rural bliss, but something is wrong.

The grass should not be that long. At this time of the year it ought to be short, chomped down to the ground by grazing cattle and sheep being fattened for market.

The problem is that the foot-and-mouth crisis, and the loss of more than three million culled animals, has left many fields that are usually grazed either out of bounds because of fears over infection or without any surviving stock to keep the grass down.

And that that is not only bad for the animals and farmers, it's depressing news for Britain's growing number of hay fever sufferers. For lurking in Britain's green and ungrazed foot-and-mouth fields are unpleasant pollen grains by the lorry load just waiting to escape on a passing breeze and invade our nasal passages.

Thanks to foot-and-mouth and the right, or perhaps wrong, weather conditions, hay fever has arrived earlier than usual and is affecting more people, some of whom have never before had symptoms. Already this year a near-record high level of pollen has been reported in one area close to foot-and-mouth outbreaks.

Hay fever, or seasonal allergic rhinitis, is an increasingly common complaint that can significantly affect quality of life, reducing the ability to work effectively, drive safely, and take part in sports, exercise, and many other daily activities.

About 10 to 20 per cent of the UK population suffer from the condition, and hay fever is the most common immune disease, accounting for 2.5 per cent of all visits to GPs and more than £50m worth of medication.

Hay fever is triggered by an allergic reaction to allergens that are released into the air at specific times of year, mainly throughout spring and summer. When pollen comes into contact with the eyes and nasal passages of a sensitised individual it may provoke a range of symptoms including redness and itching of the eyes, sneezing, coughing, runny or stuffy nose, dry mouth, headache, ear problems, and sinus congestion.

Most pollen grains, the vehicles used to transport the male DNA of the plant to the female part, have allergens, but some pose a much bigger threat than others. In the UK, pollen from grass affects about 95 per cent of hay-fever sufferers. Birch trees affect 20 per cent, and other problem plants include oak and plane trees, and nettles.

Wind-pollinated plants like these are responsible for most hay fever because they tend to produce more pollen to ensure that at least some of it will reach the target. Most flowers are insect pollinated, so much pollen less is produced and what is manufactured is delivered by insects rather than on the wind.

The mechanisms behind the development of hay fever are complex. The first exposure to an allergen causes little apparent reaction, but an individual who is prone to allergies will produce and store antibodies specific to that allergen. These antibodies attach themselves to the surface of cells found in the lining membranes of the nose.

On the second and subsequent exposure, the allergen gets into the nose and sticks onto these antibodies, starting a cascade of events that results in the sudden release of histamine and other chemicals. It is the body's defences that orchestrate the classic symptoms rather than the allergens.

The chief mysteries about hay fever is why some people develop an allergy while others do not and why the number of sufferers is increasing.

One theory is that it is all down to pollution, but the idea gaining increasing support is that it is explained by the hygiene hypothesis that the growing immune system is not challenged by the chronic diseases and infections of 100 years ago. As a result, its foot soldiers, such as the antibodies, are lowering their sights and attacking invaders that don't do any real damage to the human immune system.

New research from Germany, Australia and Denmark suggests that the children of farmers get far less hay fever and asthma, and have many fewer allergies than children of non-farmers. Hay fever is also more common in people with a family history of similar complaints or a personal history of eczema and asthma.

"The debate over the hygiene hypothesis or pollution is still going on. There is no doubt that the incidence of hay fever is increasing and estimates in the UK are from 10 to 20 per cent. Some people say it is reaching epidemic proportions," says Professor Ron Eccles, head of the common cold unit at the University of Cardiff.

"I think the explanation is that this is a mix of theories, including the hygiene hypothesis. We are not suffering as many chronic infections like TB [tuberculosis] so the immune system has changed. In the past it would have been sitting there doling out antibodies to deal with this infection, whereas now it is relatively unstimulated and so it can be stimulated by substances that are not potentially dangerous. At the same time, pollution has been shown to contribute to allergies."

He says that the foot-and-mouth crisis has contributed to high pollen counts: "A lot of grass won't be getting eaten so there will more pollen around in some regions. Grass pollen comes from the flowering head so if fields are being grazed not much pollen will be produced. Areas to the west or windward side of outbreaks are probably most at risk because they might get more pollen."

Just what areas would be affected by regional outbreaks of foot-and-mouth is not known because pollen can drift considerable distances and still be effective at causing a reaction in sufferers. Weeds in ungrazed fields are also likely to be a source of more pollen than usual.

Professor Jean Emberlin is the director of the National Pollen Research Unit based at University College, Worcester, which monitors pollen counts at 33 centres. He says that this has been a bad year.

"This year the pollen counts are notably higher than they were last year. Foot-and-mouth is likely to have affected air quality in some areas and will be responsible for some of the increases at a local level. The grass is growing well and while some fields are being cut for hay, others have been left ungrazed. The weather has had a major effect."

Latest pollen count figures show that the figures are edging towards a record level. The highest count – 1,100 grains of pollen per cubic metre – was recorded in Cardiff in 1992, and last week in Derby, a count of 1,024 was recorded. A level of 50 grains is sufficient to cause hay fever symptoms in most people.

And there could be worse to come for hay fever sufferers, even if the count goes no higher. For as the summer goes on, more and more cells in the nose become involved so that symptoms are triggered by lower and lower levels of pollen.

For those whose hay fever does not respond to medication, the only hope is that winter is not all that far away.