The four seasons of hay fever

The cause of season allergies in Canada is usually tree pollen in April-May, grass pollen in June-July and ragweed pollen from August to October, plus mould spores around mid-July.

Pollen from trees, grass and weeds and mould spores responsible

A microscopic image of various types of plant pollen. Seasonal allergies may be caused by pollen from some trees, grass and weeds, as well as mould spores. ((Dartmouth College/Wikimedia Commons))

Whether it's called hay fever, seasonal allergy or allergic rhinitis, the symptoms are not pleasant.

Each spring, Canadians with allergies to tree pollen are consumed by coughing and sneezing. Soon, though, the tree pollen passes, and those Canadians hand off the tissue box to people who have allergic reactions to grass pollen.

For those with allergies to both trees and grass, the agony drags on. Because the allergic trigger is one or more proteins in pollen, it is not unusual to have reactions to both trees and grass. Those same proteins may also be present in other natural substances, including food.

Seasonal allergies


As a result, "we see cross-reactivity with things in nature that come from fruits and nuts and other things," immunologist Judah Denburg said in an interview with CBC News.

Denburg, who is the CEO of AllerGen, Canada's allergy research network, noted that in recent years scientists have learned a great deal more about the proteins that cause allergies. "Proteins that can provoke an allergic response have been sequenced, cloned and available," he said.

Only some pollens provoke an allergic reaction. Those that do, says biologist Estelle Levetin, who chairs the aerobiology committee of the American Academy of Allergy, Asthma and Immunology, release their protein quickly once in the mucous of the nasal passages. With pollens that are not allergenic, the protein "would not be released quick enough for it to be a trigger," she told CBC News.

April and May worst months for tree allergies

Oak trees release huge amounts of pollen, which causes allergic reactions in some people. The oak pollen is magnified 200 times. ((Left: IStock. Right: Estelle Levetin/University of Tulsa))
In Canada, April and May are the months when tree pollen counts are at their peak. 

For people with tree allergies, the culprits are the varieties that shed airborne pollen. Trees produce great quantities of pollen because they have to rely on the wind to get it to the intended target, and the odds are much of it won't reach its goal.

Among the worst offenders are ash, alder, birch, box-elder, cedar, elm, maple, mulberry, oak and walnut. For species with both male and female trees, like ash and box-elder, it is only the male tree, since they produce the pollen.

"The oaks shed copious amounts of pollen, more than all other plants," according to Aerobiology Research Laboratories in Nepean, Ont.

For people in eastern Canada who are allergic to the tree pollens released around this time, the wet weather they are experiencing ought to be the silver lining in those rain clouds. "Not only will the rain wash out the pollen that's in the air, it's going to inhibit pollen release from the flowers that are ready to release pollen but are waiting for dry weather," Levetin said.

June and July: the season for grass allergies

Grass pollen allergies peak in June and July. Grass pollen is characterized by a single pore, seen on its left, but identification of the type of grass is difficult with just a light microscope. ((Left: iStockphoto. Right: Estelle Levetin/University of Tulsa))
Pollens from grasses take over from trees in June and July. The grasses that most often cause allergic reactions include Bermuda, Johnson, Kentucky bluegrass, timothy, fescue, orchard and sweet vernal.

Lower pollen counts for grasses usually mean the symptoms are not as severe as they would be with trees or ragweed. However, pollination generally lasts longer with grasses, so the symptoms may be present for a longer period.

Itchy, watery eyes are more common for grass allergy sufferers than for those with tree or ragweed allergies.

Mid-summer mould blooms

Alternaria, one of the most common moulds, is found indoors and outside. Its spores may cause an allergic reaction, or asthma. ((Left: Aerobiology Research Laboratories. Right: iStockphoto))
In mid-July, as allergy season for grasses is tapering off, there is "a big bloom of certain types of moulds," Mark Larché of McMaster University said in an interview with CBC News.

Moulds that cause allergic reactions may grow outdoors and indoors. Outdoors, the mould may be growing on rotting vegetation and logs, in soil, in compost and on grasses and grains.

Rather than pollen, moulds reproduce with spores. Some of the proteins in the spores may cause allergic reactions. Airborne spores can end up on inside the mouth, nose, and throat, producing hay fever symptoms similar to those from pollens. In the lungs, the spores may cause asthma.

Only some mould spores cause allergies but two of the most common moulds that do so are alternaria and cladosporium. Both moulds are found indoors and outdoors.

Alternaria is found on carpets and window frames, and outdoors in soil and on plants.

Cladosporium moulds are usually the largest contributor in airborne spore counts. Their colonies usually have a brown or black colour. Indoors, they can be found on moist surfaces.

"If indoor humidity is above 50 per cent, risks of fungus growth rise steeply," according to the Asthma and Allergy Foundation of America.

People are less likely to realize that they are allergic to moulds than if they are allergic to trees or grass, according to Larché. Because mould may be present in lawns, people who suffer allergic reactions after cutting the grass may be allergic to the mould spores stirred up rather than the grass pollen. Or both, possibly.

Ragweed season begins late summer

Ragweed pollen, which causes allergic reactions, usually starting in August, is enlarged about 2000 times by a scanning electron microscope. ((Left: iStockphoto. Right: Estelle Levetin, University of Tulsa))
From about August to October, ragweed is the chief culprit for allergy sufferers. It is not the only weed that causes allergic reactions, but ragweed is the most important.

The plant produces copious amounts of pollen and it also releases its protein quite quickly, according to Levetin.

Ragweed likes to grow in disturbed soil. And there's bad news for city dwellers. Ragweed grows faster and larger and produces much more pollen in urban conditions, mainly because of the slightly warmer micro-climate.

Pollution, climate change and allergies

Although air pollution is not believed to cause allergic reactions, it may be a factor, according to Larche. Pollution irritates the membranes in breathing passages and then the proteins released by pollens are more likely to cause an allergic reaction. Also, pollens can stick to the airborne pollutants, increasing the likelihood they will get inside our bodies.

2011 research paper by Lewis Ziska, et. al., reports that in North America, the ragweed season is getting longer, especially in more northerly latitudes. Between 1995 and 2009, the length of the ragweed season increased by 27 days in Saskatoon and 25 days in Winnipeg.

The researchers say this may be related to climate change, the theory being that warming is happening sooner as we approach the poles, resulting in a longer growing season.

Whether there is a connection between climate change and the steadily increasing number of people with allergies — a number that doubles nearly every decade —  is uncertain. "We don't know enough about climate change to make any of these statements about that being a factor in the rise of allergy," Judah Denburg said.