Global warming could lead to greater misery for hay fever sufferers, say experts.

Hay fever is caused by an allergic reaction to pollen released when plants begin to flower.

And experts believe the milder climate in the UK is likely to lead to many plants flowering earlier and for longer periods.

Hay fever is affecting more and more people Professor Jean Emberlin

They also suspect that the modern trend for mowing the lawn all year round may pile on the misery still further, and lead to more people developing the condition.

Although cut grass is usually too short to flower, hay fever can still be triggered by chemicals contained in grass sap, which are released when it is cut.

The Woodland Trust and the National Pollen Research Unit plan to monitor the effects of climate change on the hay fever season.

They are asking people to record the time of the first flowering of early grass species such as Yorkshire fog, cocksfoot, meadow foxtail and Timothy.

Wet and warm conditions

Professor Jean Emberlin, of the National Pollen Research Unit said: "Last year the grass pollen season was exceptionally long because it was wet and warm.

"The season extended into August, instead of ending in July.

"Generally, hay fever is affecting more and more people.

"Numbers have risen since 1965 when between 10 to 12% of the UK population were affected, to today's figure of 15 to 25% in the population as a whole.

"Whilst grass pollen is the commonest trigger, birch pollen is important too. The birch pollen season has been occurring five to ten days earlier per decade over the last 30 years.

"By letting us know when you see trees and grasses beginning to flower, it will give us information about flowering times in advance of pollen release and it will help us to improve forecasting the start of pollen seasons.

"This will help you to take your medication at the right time."

Common allergy

Statistics comparing grass flowering dates between 2002 and 2001 show that for the UK as a whole, cocksfoot flowered nine days earlier; meadow foxtail 13 days earlier and Yorkshire fog nine days earlier.

Hay fever is one of the commonest allergies in UK, affecting about 12 million people.

In general terms, the area with the highest hay fever rate is the English midlands.

It was virtually unknown before 1800 and has become common only during the last century.

The first case to be described medically was in 1819 but the causes of the disease were not identified until 1873.

There are 33 pollen monitoring sites in the UK network.