The UK has been enjoying some lovely summer weather recently. Brits have been making the most of the sunny spell by flocking to the beach and having barbeques in their gardens.

But there is often an unwelcome accompaniment to the sounds of sizzling sausages on the barbeque and buzzing bees in the flowers. The clamour of sneezing and people blowing their noses makes an unpleasant soundtrack to Britain's best summer weather. What is going on?

It is hay fever, an allergic reaction to pollen from trees, grass and other plants. When the pollens come into contact with the tissue inside the nose, they start an immune reaction that causes sneezing, itching and a runny nose.

Between 15 and 20% of people in the UK are thought to be affected by hay fever, with the numbers being even higher among teenagers.

Allergy specialist Professor Stephen Durham has calculated that the number of sufferers has doubled over the last twenty years. What is causing the rise?

"There's some evidence that pollution exacerbates it," he says. "And you've also got the hygiene hypothesis - that our bodies aren't as strong because we aren't exposed to infections when we are small children that our systems rebel against."

There is interesting evidence from Austria to suggest that hay fever is related to living in an increasingly sanitised environment.

Researchers there found that young children in regular contact with farm animals are less likely to develop allergies later on. Children living on farms were found to be three times less sensitive to hay fever and nearly four times less likely to suffer from asthma than those living in a non-rural environment.

Perhaps the worst news for hay fever sufferers is that their ailment can never be cured completely. So perhaps it's time we got used to a... a... a... achoo!