This story is part of a CBC News series entitled In Our Backyard, which looks at the effects climate change is having in Canada, from extreme weather events to how it's reshaping our economy.

A sneezy, wheezy, bloodshot mess. That's what Stephen Spong feels like when he doesn't take his cocktail of seasonal allergy medications.

The 37-year-old librarian lives in downtown Toronto, and says his allergic reaction to grass pollen and ragweed recently started getting worse.

"I actually had to call in sick a couple days at work because my allergies were so bad," Spong says. "You kind of cease to function and want to stay inside and hide, so that's what I did."

And he's not alone. Millions of Canadians suffer from seasonal allergies, and many city dwellers across the country have noticed their symptoms worsening in recent years. Experts in the allergy community believe climate change is largely to blame, with rising temperatures kicking pollen-producing plants into high gear.

And in cities, tree-planting policies and large amounts of concrete are creating allergen traps — leading some researchers to warn urban Canadians from coast to coast should be bracing for more airborne irritants in the years to come.

Pollen levels rising in many Canadian cities

While each allergy season is slightly different, the number of annual allergen samples collected by Aerobiology Research Laboratories — which counts and identifies allergens in the air across Canada for a variety of clients— have been rising overall in many cities over the last decade and a half.

One of those sneeze-inducers, pollen, is among the most common triggers for seasonal allergies. Statistics Canada figures show it impacts around 40 per cent of all sufferers. And it's not surprising: the tiny fertilizer grains are released by various plants throughout every spring, summer and fall.

According to Aerobiology's data, provided to CBC News, an ongoing rise in pollen grains per cubic metre of air each year has been identified in urban centres, including Montreal, Ottawa and Winnipeg.

In Calgary, for instance, there's been one of the biggest spikes — with the annual pollen grains per cubic metre of air jumping from below 10,000 around the year 2000 to about 25,000 by 2015.

It's a similar trend in downtown Toronto, with current rates often far above the pollen levels recorded in the early 2000s.

That seems to tell us that due to climate change … there's a longer growing season, and that longer growing season would bring more ability to have a longer pollen season as well​​​​​​. - Daniel Coates, Aerobiology Research Laboratories spokesperson

"That seems to tell us that due to climate change … there's a longer growing season, and that longer growing season would bring more ability to have a longer pollen season as well," says Daniel Coates, a spokesperson for Aerobiology.

Climate change connection

Research backs up Coates's suspicion that a changing climate could be a key factor in the higher concentrations of pollen and other allergens in city air, both in Canada and beyond.

One 2019 study published in the Lancet Planetary Health journal looked at 17 locations around the world, including Winnipeg and Saskatoon, and found over several decades, more than 70 per cent of the cities studied showed increases in annual pollen rates.

"This study, done across multiple continents, highlights an important link between ongoing global warming and public health — one that could be exacerbated as temperatures continue to increase," the authors say.

An earlier study on seasonal allergies in the mainland United States, led by a research team from Rutgers University, found the pollen seasons of many plants started an average of three days earlier in the 2000s than in the 1990s. The annual total of daily airborne pollen also increased by 46 per cent during the same time frame, the researchers found.

Data from the United States Environmental Protection Agency also shows for ragweed pollen, in particular, there's been a big jump in how long it grows for in various cities in the central U.S. and Canada.

The plant's growing season increased by 25 days in Winnipeg, for instance, and 24 days in Saskatoon, over the two-decade span between 1995 and 2015.

Ragweed, shown here, is growing longer in many cities in the central U.S. and Canada, according to data from the United States Environmental Protection Agency. The plant's growing season increased by 25 days in Winnipeg, for instance, over the two-decade span between 1995 and 2015. (Sarah Leavitt/CBC)

The increase also grows more pronounced from south to north, the data shows, meaning Canadian cities are among those more likely to experience longer allergy seasons and higher pollen counts.

So why are researchers seeing this correlation between higher temperatures and airborne allergens?

It could be thanks to one of the greenhouse gases linked to global warming: carbon dioxide (CO2).

"There's increasing CO2 in the atmosphere, and plants need CO2 as food," explains Ada McVean, a science communicator with McGill University's Office for Science and Society.

"They're able to grow bigger and produce more pollen, and rising temperatures mean they can grow earlier — and later."

But while allergy-inducing plants grow across the country, there are also factors unique to cities that make the concrete jungles an allergen trap.

Urban areas allergen magnets

"We've always thought that if you live in the country, you have a higher chance of experiencing high pollen levels," says Dr. Susan Waserman, an allergist clinical immunologist at McMaster University.

But the Hamilton clinician — who is seeing a rising number of patients with increasingly severe symptoms — says it's not the case.

Urban areas are allergen magnets thanks to the types of plants grown in cities, including homeowners' ornamental grasses known for spreading pollen, she says.

"People are also starting to plant trees that don't have much in the way of flowers or seeds or fruit, only because they make less of a mess and require less care," Waserman adds. "But they're also pollen-producers."

And that difference is due to the trees' sex.

Rising pollen rates have been identified in many urban centres. In Calgary, there's been one of the biggest spikes — with the annual pollen grains per cubic metre of air jumping from below 10,000 around the year 2000 to about 25,000 by 2015. (Helen Pike/ CBC)

More than 90 per cent of trees planted in many Canadian cities — including Edmonton, Halifax, Montreal, Toronto and Vancouver — are male pollen-producers, instead of the messier fruit-dropping female varieties, CBC News previously reported.

"When we have 20 to 30 per cent of the population in Canada having some form of allergy … that's a significant proportion of the population that could be negatively affected through a program that's actually well-intentioned," says Michael Brauer, a professor in the University of British Columbia's faculty of medicine.

"What this means is we need to be a little bit smarter about what we do — which trees we're planting and where they shouldn't be."

According to Toronto allergist Dr. Mark Greenwald, when pollen hits the ground in rural areas, it's also more likely to stay put — and not become airborne again — thanks to the cling of damp soil and vegetation.

"In the city, you have a lot of hard concrete surfaces where the pollen can settle," he continues. "When the winds change, the pollen starts moving."

Greenwald, a board member with the Canadian Allergy, Asthma, and Immunology Foundation, says more research is needed since there's still no unifying, fully-proven theory behind the trends allergists are witnessing.

One thing, he says, is becoming clearer: "There seems to be an increase in allergies and asthma pretty much around the world."