OTTAWA — Scorching temperatures and sparse rain across much of Canada have given respite to some seasonal allergy sufferers this summer.

Grasses have shrivelled and the onset of the late-summer ragweed scourge has been somewhat delayed.

But the hot, dry summer may yet produce more misery than usual for other allergy sufferers during the fall and next spring.

Stressed by the parched summer, trees may drop their leaves sooner in the autumn, potentially producing more leaf mould spores, says Frances Coates, CEO of Aerobiology Research Laboratories, which conducts pollen and fungal spore counts across the country.

And they may well produce more pollen next year.

"Right now, the trees are determining how much pollen is going to be produced on these trees for next year," Coates says.

"If they're under a lot of stress, which they would've been this summer, sometimes their reaction is to create a lot of pollen and that's basically a survival method."

Still, forecasting the severity of allergy seasons is almost as inexact as forecasting the weather. That's especially true in a country as vast as Canada, with myriad and often substantial variations in regional weather conditions.

Coates stresses that winter and spring weather conditions will have a bearing on next year's tree pollen count. For instance, heavy spring rains can help wash pollen out of the air.

Moreover, she points out that different tree species react differently to weather fluctuations. And conditions that may reduce one type of allergen can increase others.

This summer, for instance, Coates says grass pollen was down in many parts of the country but fungal spores, which thrive on humidity, were up.

All those variables make it difficult to predict whether a coming allergy season will be milder or worse than usual.

In any event, the Canadian Society of Allergy and Clinical Immunology says changes in pollen and spore counts from year to year are usually small. Hence, it advises sufferers to disregard forecasts when determining how to cope with their seasonal allergies.

"There's absolutely no doubt that the weather will impact local pollen levels ... but these differences are not generally drastic," says society president Dr. Stuart Carr.

"These make for interesting topics of discussion, but they don't really make a big clinical difference for patients overall."

In his view, the worst thing allergy sufferers can do is delay starting their medication regimen because they've heard the allergy season may be milder than usual.

With the exception of those with very mild hayfever, Carr says: "Patients do better if they start their preventative treatment plan before the start of the season. So, if you're waiting to see how the season plays out, you're asking for trouble. It's always harder to play catch-up after the fact."

However the coming fall and spring turn out, it seems the worst way to gauge the severity of an allergy season is to listen to the complaints of allergy sufferers themselves.

"Every year I hear it from allergists, 'Oh, this is the worst year ever, my patients are complaining,"' says Coates, adding that such assertions are rarely supported by pollen and fungal spore data.

Carr says "lots of patients" believe they're suffering from pollen allergies when they're actually reacting to year-round allergens, like dust mites, which multiply in hot, humid weather.

And some who are convinced they're allergic to ragweed have no allergy at all.

"They get worse during the ragweed season, but that's because that's also when the common cold virus peaks and they may just have problems with frequent colds."