TORONTO – If you are one of the millions of North Americans who have seasonal allergies, it’s time to start thinking about spring - even if you are currently shoveling snow.

Climate change is not only affecting the amount of pollen in the air; it is also affecting when the various allergy seasons start. The phenomena is called season creep. Spring weather is coming as much as 10 days earlier than it did 30 years ago, according to researchers at Stanford University. As a result, the growing season (and pollinating periods) for many plants are starting sooner and lasting longer.

Some types of allergies will not be affected. For instance, trees can begin pollinating as early as March in most of North America. Different tree species will release their pollen at varying times, leading to a season of tree allergies that usually extends from March through May.

Trees are early pollinators - if you live in the southern US states, you may be facing tree pollination now!

The good news is that most trees pollinate for a limited time period - a number of weeks in general - so while you might be coping with your tree allergies earlier, you won’t face a longer season per se. By May, all the trees on the continent are done.

Those with allergies to other plants, including flowers, grasses and weeds, may not be so lucky. Some types of flowers and many weeds will grow as long as weather permits. As a result, they will pollinate repeatedly.

With lengthened seasons, grass allergies can be the worst. Grasses can pollinate for as many as 11 months of the year, depending on your latitude.

It’s not just the length of pollination period that may adversely affect the allergy sufferer. It is also the quantity of pollen. Research has shown that increased carbon dioxide can lead to increased release of pollen. This can intensify the allergy season by increasing the air’s saturation. Where plant species are wind pollinators (rather than using animals or insects), the air can actually be thick with pollen.

Think you are safe because you are far from farmers’ fields? Think again. City dwellers may be at the highest risk from global warming effects. Carbon dioxide levels are the highest in urban areas. Heat-absorbing buildings, roads and sidewalks pump up temperatures. As a result, city plant life will produce more pollen and for longer periods.

If you are interested in trying immunotherapy for your allergies - either sublingual treatments or traditional allergy shots - the best time to start is now. Most allergy treatments provide improved symptom control when started before you are exposed to your allergen.

Sources: Union of Concerned Scientists, Science Daily