Aachoo! - why your hay fever is getting worse

Friday, May 6, 2011

The prevalence of pollen allergies in North America has been increasing
over the past few decades – particularly in Ontario

by HENRY HENGEVELD

For most of us, the burst of blossoms that arrives with the onset of spring is a welcome relief from the doldrums of winter.

However, for about 7.5 million Canadians, it also marks the start of a season of misery. They begin to sneeze and wheeze, and often end up with puffy, weepy and itchy eyes. In some cases it can also trigger or significantly exasperate problems with asthma. For these sufferers, the blossoms indicate that allergic rhinitis or "hay fever" season has begun.

Surveys suggest that most victims of hay fever also become less productive in their work, and many end up missing days at school or work.  While net costs to the Canadian economy appear not to be readily available, the figure for the U.S. economy is estimated to be in the range of $21 billion per year. Nothing to "sneeze" at! 

Hay fever occurs when pollens from flowering plants enter the nasal passages of individuals with sensitive immune systems, triggering the production of anti-bodies. These, in turn, cause the nasal passages to become inflamed and other symptoms to appear.

For afflicted people in Ontario, the season for such reactions begins as early as March, when the air becomes laden with pollen from tree blossoms. By early summer, tree pollens give way to grass pollens, then to weed pollens, particularly that from ragweed. For some reason, many sufferers are particularly sensitive to the latter. The pollen allergy season ends when the first frost kills off the last of the season's plant blossoms, usually in October. 

Not surprisingly, the length and intensity of the hay fever season appears to be largely determined by the length of the frost-free season. Thus it is usually more severe in lower latitudes, where frost-free seasons are longer, and less severe than at mid- and high latitudes. The percentage of the population affected also appears to increase with length of season, perhaps due to higher pollen counts and increased likelihood of exceeding critical tolerance thresholds to pollens. Thus, on average, a higher percentage of Ontarians are affected by hay fever than in other parts of Canada. 

The good news is that there are a range of methods available to help relieve many of the symptoms of hay fever. The bad news is that there is now undeniable evidence that the prevalence of pollen allergies among people in North America has been increasing over the past few decades. Until recently, experts were not sure why. However, a new study by a team of American and Canadian experts has provided some convincing answers. 

The study, published in the prestigious Proceedings of the National Academies of Sciences in February of this year, was led by researchers associated with the U.S. Department of Agriculture. Their hypothesis was that the rise in hay fever suffering in North America was due to a lengthening and intensification of the pollen season caused by a rise in air temperatures. The results of the study not only confirmed that is indeed the case, but surprised the investigators with the magnitude of the changes observed in some regions, particularly in Canada. 

To undertake the pollen-climate comparison, the researchers made use of pollen data collected at a network of monitoring stations across North America. They focused on pollen data for 15 different species of ragweed collected at 10 primarily non-urban sites in central North America extending along a south-north transect from Georgetown, Tex. (latitude 30.6°N), to Winnipeg and Saskatoon (52.1°N), a distance of more than 2,000 kilometres. Each of the sites selected had at least 15 years of pollen data. The relevant weather data was obtained from nearby American and Canadian weather stations.  

Ragweed is known as a "'short-day" species, which doesn't begin to bloom – and thus release pollens – until the days start to get shorter after June 21. Its season ends when local minimum temperatures drop to the freezing point, killing off the ragweed blossoms.

Not surprisingly, the beginning of the pollen season thus varies much less from location to location than does the ending. Indeed, the results of the study show that the ragweed pollen season, on average, is longest at the south end of the transect, where killing fall frosts come very late in the year, and shortest at the northern end. In 1995, for example, the ragweed pollen season at Georgetown was 122 days. At Saskatoon, it was only 44 days. 

Over the 15-year period of the study (1995 to 2009), the ragweed pollen season changed very little at sites in the southern parts of the transect located in Texas, Oklahoma and Arizona (although pollen intensity did increase). In these locations, there was also little change in the timing of first fall frosts.

In the central part of the transect (sites from Nebraska to North Dakota), the pollen season increased by 10 to 16 days. However, the researchers were astonished at the magnitude of change in season length at the two Canadian stations. That for the Winnipeg region increased by 25 days, while the season length for the Saskatoon site expanded by almost four weeks – from 44 to 71 days.

While ragweed pollen is only one of the sources of allergens that cause hay fever, the authors of the study suggest that similar results are likely for other pollens. 

Unfortunately, the bad news doesn't end there. In their latest assessment released four years ago, experts involved with the Intergovernmental Panel on Climate Change noted that the length of pollen seasons and the intensity of pollen production during the seasons can be expected to continue to increase as global temperatures rise in the decades to come.

Furthermore, last year, Harvard scientists reported that pollen production of ragweed plants exposed to air containing twice the normal concentrations of carbon dioxide was about 61 per cent higher than that from plants grown under normal conditions. Doubling of carbon dioxide in the global atmosphere, relative to pre-industrial conditions, is expected to occur sometime during the second half of this century. 

In short, both the length and intensity of the hay fever season are likely to continue to increase. Gesundheit! BF
 
Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.