When you hear on the news that the pollen count is "high", each cubic metre of the air you inhale is likely to contain at least 100 pollen grains, and perhaps far more.
The nerve-centre of the European network is in Vienna, where daily counts from across the Continent are fed into the world's most comprehensive pollen database. In Britain alone, 33 stations around the country collect daily counts of grass pollen grains throughout the summer, while a dozen specialist centres also identify pollen from scores of tree and weed species as well as spores from hundreds of species of fungi. It is painstaking work; an experienced eye is needed to recognise the distinctively sculpted pollen grains trapped on a greased microscope slide.
August may be holiday time for most of us, but not for the pollen watchers. These "aerobiologists" are busy filtering the air for the tell-tale signs of the plant kingdom's annual sex orgy. Every summer, flowering plants discharge their reproductive propagules into the atmosphere. Pollen grains are male sex cells, light enough to float on the breeze in search of a receptive female flower part.
Particularly prolific are plants such as grasses and birch trees, which rely on wind rather than bees to spread their pollen. In allergy-prone people - and there are least six million of us in Britain alone - the body's immune system becomes sensitised by repeated, massive exposure to the proteins lurking on the surface of grass pollen. As these foreign proteins make contact with the moist, welcoming tissue inside a human nose, the immune system unleashes a barrage of histamines. These chemical messengers in turn trigger the snuffles, sneezes and itchy, watery eyes that are the miserable symptoms of hay fever.
The higher the concentration of a particular allergenic pollen in the air, the greater the chances that some people will become sensitised to it. So olive trees cause trouble in the Mediterranean, while birch trees are notorious sources of allergy in Scandinavia.
Across North America, a group of small, weedy annuals aptly named ragweeds is a particular scourge. Tim Rich, a botanist at the National Museum of Wales in Cardiff, says that they are probably the most noxious hay fever plants in the world. Their pollen bears a protein that irritates even relatively tolerant immune systems. Ragweed seeds seem to have arrived in Europe via shipments of contaminated American grain, and the plants have now established a stronghold in France, around Lyon. So far, however, ragweeds have failed to gain a root-hold in Britain, because they need a long, hot growing season to set seed. But botanists fear that global warming will soon create climatic conditions in Britain that will be just right for the cosmopolitan ragweeds. By 2050, says Rich, the climate of south-east England is likely to be rather similar to that of present- day Lyon. Hay fever sufferers have reason to be worried.
Signs of global warming are already with us, the aerobiologists report. Long-term records of pollen counts in Cardiff, Derby and London, going back more than 40 years, reveal that birch trees in Britain have begun to release their pollen earlier in March. On average, the birch pollen season has become earlier by about five days over the last decade, compared with the previous 10 years, says Jean Emberlin, of the Pollen Research Unit at University College Worcester.
Much the same is happening across Scandinavia, according to pollen-watchers in Stockholm (Sweden), Trondheim (Norway) and Turku (Finland). Looking at trends over a 14-year period, they report a significant earlier start to the pollen season in not just birch, but alder, hazel, pine and elm as well. Their results match pollen data from Vienna, in the heart of Europe.
Climate change is also likely to alter the grass pollen seasons over much of Europe and Britain. "This will be very important to hay fever sufferers, as about 90 per cent are allergic to grass pollen," Dr Emberlin explains. Regions north of the Alps may soon experience longer grass pollen seasons than they do now, and may come to resemble southern Europe, where the mild climate allows some grasses to flower all the year round.
Olive-growing may spread northwards, too, bringing allergies with it. In Cordoba in Spain, olive pollen is already the main cause of hay fever and is also thought to aggravate asthma. Meanwhile, researchers in Genoa are keeping a close watch on a member of the nettle family, pellitory of the wall, which in Italy is a notorious cause of hay fever. In southern parts of the country, the noxious nettle is responsible for up to 80 per cent of hay fever cases. The plant, which was introduced into Britain by medieval monks, is now largely confined to areas around London and Southampton. But populations could dramatically spread as the weather warms up.
No one quite knows why more and more people seem to be developing allergies to pollen. The steady rise over the last few decades in the incidence of both asthma and hay fever in Britain is particularly surprising given that, in south-east England and the Midlands, at least, concentrations of grass pollen have generally fallen, as farmers plough grasslands or cut grass early to make silage, rather than hay.
Increasingly, researchers in the field suspect that part of the explanation could lie in a malign interaction between pollen and pollution. For a start, says Emberlin, plants that have been stressed by pollution produce more allergenic proteins on their surface. Furthermore, pollen can collect pollutants on its surface during its many hours of travel on the air flow. These pollutants could be toxic in themselves, or they could cause trouble by enhancing the allergenic properties of the pollen's surface proteins.
Even the growing incidence of asthma may turn out to be linked to pollution- laden pollen, or rather, pollen fragments. Whole pollen grains are too large to enter the tiny airways deep inside the lungs and set off asthmatic attacks. But pollen grains can become broken up, during thunderstorms, for instance, releasing starch granules that bear allergy-causing proteins just like the pollen grain itself. These granules are small enough to penetrate the narrowest airways, where they can provoke asthmatic bronchial spasms in susceptible individuals.
This important finding, by aerobiologists at the University of Melbourne in Australia, explains why parts of southern and central England experienced an epidemic of asthmatic attacks soon after thunderstorms struck one evening in late June 1994. Casualty departments were flooded with bewildered people struggling to breathe. The victims were mostly young people who suffered from hay fever but had never had an asthmatic attack before. The culprit eventually proved to be grass pollen, broken up by the storms.
Asthma can also be triggered by tiny fungal spores, and perhaps, too, by pollen fragments that have picked up particulate matter from diesel exhaust fumes, according to research in Japan. More work is urgently needed to discover exactly what is going on each hot summer day when pollution and pollen meet, says Dr Emberlin.
Even the current debate over the environmental perils of genetically modified crops has much to learn from aerobiological research, says Eric Caulton, who heads the Scottish Centre for Pollen Studies at Napier University in Edinburgh.
"To think that that GM crops can be easily contained is a gross misunderstanding of the problem," he says. "Pollen is produced in vast quantities and you need only one pollen grain to contaminate a susceptible GM-free plant."
Attempts to set up cordons sanitaires around GM crops are likely to fail, he says. Recently, the Government's advisers on the release of genetically engineered crops ruled that any field of GM maize is highly unlikely to cross-pollinate with conventional maize in another field 200 metres or more distant. But in a report she produced for the Soil Association, Jean Emberlin calculated that cross-pollination would not be a rare event.
"In conditions of moderate wind speeds the rates of cross-pollination at 200 metres would be in the order of one kernel in 93," she wrote.
"We've long known that pollen has the potential for long-distance travel," says John Mullins, a botanist who has worked at Cardiff's aerobiology unit for more than 30 years.
Pollen grains easily travel hundreds of miles high up in the atmosphere, as research going back to the early Fifties has shown. In those early days of aerobiology, one pioneering researcher, Jim Hirst, of the Rothamsted Experimental Station in Hertfordshire, was trying to track the fungal spores that were spreading potato blight through war-torn Britain. Hirst took a pollen trap up in a Royal Air Force plane and flew out over the North Sea. Miles from land, and several thousand feet up, Hirst picked up bursts of both spores and pollen.
The biggest clumps occurred some 300 miles out to sea, and probably represented spores and pollen liberated from right across England on the day before the flight, he concluded. Aerobiologists now reckon that British pollen regularly arrives on the Contin- ent, and vice versa; the stuff is no respecter of national, let alone field boundaries.
It's highly topical research. Yet Britain's pollen network now runs on a shoestring, scraping along with funds garnered largely from selling pollen forecasts to the media.
Even commercial sponsorship from a leading manufacturer of paper handkerchiefs has dried up. It's not a fashionable field; there's no money or status in studying pollen. Yet perhaps Bob Dylan was more of a prophet than even he supposed. The signs really are growing - we ignore at our peril what's blowing in the wind.