The number of hay fever sufferers is expected to rise to over 13 million by the end of the decade, including many more middle-aged people who are increasingly likely to be affected, say experts at leading UK institutions.

People with hayfever, also known as allergic rhinitis suffer an allergic reaction to pollen, causing a range of symptoms including sneezing, runny nose, itchy nose, itchy eyes, itchy face and throat, low energy and lethargy.

More and more adults, particularly those aged 40-60 have been found to suffer from the allergy.

Why? One suggestion is that the planting of exotic plant species in Britain could be causing a rise in late-onset allergies. Other possible reasons for the allergy increase include climate change, new infections and a tendency to live in more sterile environments.

According to Datamonitor, there were over 12 million hayfever sufferers in Britain in 2010. Of this number, a total of 2.5 million are aged between 20 and 34 but the majority of 9.5 million hayfever allergy sufferers are middle-aged.

Andrew Williams, a consultant allergy nurse at Homerton University Hospital in East London was reported in the Telegraph saying that the hospital was seeing many more middle-aged hayfever sufferers.

Beverley Adams-Groom, from the National Pollen and Aerobiology Research Unit (NPARU) at the University of Worcester was reported as saying: “People are getting it [hayfever] later in life and at all ages. It’s a trend that we know of but I don’t think anyone knows why.”

Many people are thought to have a genetic predisposition to suffer hayfever, but it might only be triggered later in life. Possible triggers include having a serious infection or moving to a more polluted area.

This news is in contrast to previous studies, which had suggested that hayfever might actually become less virulent as sufferers age.

Some experts believe that increasing hygiene standards mean that people’s immune systems are less able to tolerate irritants, known as the hygiene hypothesis.

The changing global environment might also play a role, as plants that release more pollen into the air start to grow in Britain and as trees and flowers bloom for longer in a warmer climate.