The sneezing, itchy eyes and runny noses brought on by pollen are just around the corner. Within a matter of weeks, pollen released by agricultural grasses will cover the State, making life a misery for those who are allergic to it.

For more than two decades, The Irish Times has been providing its readers with daily forecasts of how severe the pollen count will be, through the Department of Botany at Trinity College Dublin. Researchers there developed a forecasting model for pollen that is now used right across Britain and the Republic.

The model and the pollen service continue to undergo changes and improvements due to the ongoing efforts of the model's originator, Dr Paul Dowding, senior lecturer in botany. It is much changed from the mid-1970s, when the count was actually a historical record and not a forecast.

"We started producing a pollen count for The Irish Times in 1976," he says. The count is taken each day and a figure provided for the next day's paper. The huge variability in levels, however, made it clear that a method was needed to predict levels rather than just report 24hour-old levels.

A working pollen model had been developed in Britain, he says, but this did not work in the Republic, where cool Atlantic weather keeps the pollen count lower. "I decided we had to go about developing our own."

Grass pollen is the predominant hay fever irritant here, he says. Grasses come into flower and release pollen when soil temperatures rise and air temperatures reach a trigger level. It is usually difficult to say whether the release will begin on June 1st or June 15th because a few weeks of cold, wet weather is enough to hold it back.

The main flowering usually takes place during June, but it can be earlier or later. "The first high day over the past 25 years has ranged from May 30th to June 30th," Dr Dowding says. He also knows that this year's release will come later because of a cool April.

Hay fever occurs when a person becomes allergic to pollen, which in itself is a harmless substance. The body responds as though the pollen were dangerous and it mounts a powerful immune response to it, leading to inflammation and the familiar symptoms so dreaded by those who have hay fever. The response continues until pollen levels fall some time during July.

Dr Dowding's model produces a link between daily weather conditions, grass biology and the way that airborne particles are distributed through the atmosphere. Initial efforts could not deliver a reliable result because the latter aspect was not included. He overcame these early problems by using elements of US models used to forecast pollution events.

The early research work that linked the weather to pollen release was laborious. "We did detailed hourly pollen counts for Dublin, Dartry and Baldonnell," he says. By 1980 he had a working model that could give a 24-hour prediction of what the pollen count would be in Dublin on the basis of what the weather was doing.

Dr Dowding made his model available to researchers in Britain but it did not perform, mainly because of differences in climate. He continued working on the model and it is now used successfully across Britain.

The model is sophisticated enough that it can give a forecast for any part of the Republic. All that is necessary is that a reliable local weather forecast be available, which is why Dr Dowding has formed an alliance with Met Eireann. The Met service funds the forecasting programme and it makes the pollen count forecast available to outlets such as The Irish Times.

Dr Dowding appointed a postgraduate research student who does a daily pollen count to confirm that the forecast model is working properly. The current incumbent is Mr Kevin Carey. It is a labour-intensive task but with practice he can do a full day's count in under an hour, he says.

The device used is called a Burkard Spore-trap. "It is basically like a small dustbin," he explains. It sucks in air at a known rate and directs it towards a slowly revolving wheel, which moves at just two millimetres per hour.

The wheel surface is coated with petroleum jelly and any pollen in the air sticks to it. When Mr Carey does a reading at the end of the day he chemically fixes the pollen and then counts the number of particles stuck to the surface. It gives him a complete hourly pollen level throughout the day.

He is also responsible for running the prediction model on the basis of the weather forecast provided by Met Eireann. "By 3 p.m. each day the Met service has sent the weather details and within 30 minutes I have emailed back the pollen forecast."

This year Dr Dowding obtained financial support from SmithKlineBeecham to set up a website where the pollen forecast and count details will be available to the public. It will start once the pollen season arrives and is available at www.pollenlevels.com