Tree pollens are a major irritant (Image: Jeremy Burgess/SPL)
The distribution and abundance of allergy-inducing pollen is changing, and this could certainly play a role in the hay fever epidemic – perhaps, as a result, more people are becoming sensitised or are seeking treatment for worsening symptoms.
Last September, invasive super-pollens made the news in the UK after they were detected in central England. The offending pollen was , a long-standing scourge for Americans with hay fever that has recently become established in parts of central and southern Europe – which is presumably where the pollen blew in from. Whether ragweed will establish itself further north is up for debate. “It’s a very plastic plant; it can survive at cooler temperatures, but whether it can prosper is another question,” says Roy Kennedy, director of the National Pollen and Aerobiology Research Unit in Worcester, UK.
Advertisement
Climate change could also be a factor, shifting the distribution of allergenic plants such as olive trees, which are a major cause of hay fever in Spain, and subtropical grasses, which are a problem in northern Australia. “With climate change, we are likely to see a spread and change in distribution of the types of grass pollen, with subtropical species that flower predominantly in summer coming in to play in more temperate regions,” says Janet Davies at the University of Queensland in Brisbane. If such plants then flower at different times, this could extend the hay fever season for people who become sensitised to them.
The flowering seasons of individual species could also change; one that the annual length of the ragweed season increased by between 13 and 27 days from 1995 to 2009 in certain regions of North America. Similarly, the UK’s birch flowering season – typically in late March – started five days earlier each decade between the 1960s and 1996 due to milder winters and warmer springs.
Warmer temperatures and increased rainfall can ratchet up the agony for allergy-prone people in other ways too. When pollen gets wet it’s more inclined to burst in the air, making it easier to inhale the allergens. That may be why symptoms are often worse in the morning, when mists and dew are more common. But dry weather also causes pollen grains to shrink and burst, which is why symptoms often peak around midday when humidity levels drop.
“Warmer temperatures and increased rainfall can ratchet up the agony”
Changing weather patterns are another factor. Pollen tends to be released in the early morning, but because it is designed to be blown far and wide by the wind, levels can be high at any time of day. Under certain climatic conditions, pollen, dust and other particles can get swept up into the atmosphere and travel long distances. “Often people will blame their neighbours’ trees for their allergies, and these may have some influence, but you also get pollen clouds blowing across countries,” says Tariq El-Shanawany, a consultant clinical immunologist at Cardiff University, UK.
Pollution can exacerbate hay fever by making the linings of our noses and lungs more permeable to allergens and by physically modifying allergens to increase their potency. Ozone, for example, seems to trigger the production of reactive oxygen species within pollen grains, and exposure to these highly reactive chemicals is more likely to prompt immune cells to respond.
Pollutants may also make pollen more potent by causing allergens to leach out onto the surface of the grains. A that birch pollen collected from trees in Munich, Germany, was more likely to provoke an immune reaction than pollen collected from trees in rural areas.
Read more: “Hay fever: Why it’s not to be sneezed at“
Know your enemy
Hay fever gets its name from its long association with the summer haying season, when farmers cut grasses to store as fodder. The condition was officially recognised in 1819, when John Bostock, a doctor from London, in one of the journals of the Royal Society of Medicine. Bostock described it as an “unusual” condition but by the end of the century it was common across Europe and North America.
Hay fever is not specific to hay or the haying season; almost any pollen from a wind-pollinated plant can cause it, at almost any time of year. The most common troublemakers are trees – especially birch – grasses and weeds (see “Allergy in the UK”). Grass pollen allergy is by far the most common, but there are tens if not hundreds of known allergenic pollens and many people are allergic to two or more.FIG-mg30160801.jpg
It doesn’t take much to trigger a reaction. Symptoms typically kick in once the pollen count reaches 50, which means there are 50 grains of pollen per cubic metre of air. To put that in perspective, a single ragweed plant can release . Graham Lawton
This article appeared in print under the headline “Are allergy seasons getting worse?”
