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Childhood Asthma

Asthma Triggers: Allergens and Irritants

Asthma can be triggered by allergens (things people can get allergic to), irritants (things that irritate the airways), and certain other situations (which cause asthma through quite complicated means). This section will tell you more about asthma triggers.

Dust Mites

Dust mites are extremely tiny little animals that live in dust, feeding off bits of shed human skin and other appetizing little morsels. As the insulation in modern homes gets better and better (and air circulation gets potentially worse and worse), housedust, and dust mite, accumulation becomes an increasing problem. Dust mite allergy is probably the most important allergy leading to asthma in Canadian children.  In children with dust mite allergy, exposure to dust can not only lead to asthma attacks, but long-term exposure can increase airway inflammation, increasing the severity of the reaction to other asthma allergens and irritants.

Dust mites grow best in high humidity (as do molds), so, contrary to popular belief, keeping your house reasonably dry (humidity under 50%) is preferable for children with asthma. Humidifiers in the bedroom are sometimes helpful during colds, but should be used for as short a time as possible, to avoid promoting dust mites. Humidifiers should also be kept extremely clean, as mold can easily grow in the humidifier, and then get "blown" around the room.

People spend more time in their bedroom then any other single location, so for people with dust mite allergy, reducing dust in the bedroom is especially important. Dust mites grow especially well in mattresses, pillows, and bedding. Some ways of reducing dust mites in your child's bedroom include:
  • Use a hardwood floor, remove upholstered furniture
  • Enclose the mattress, box spring, and foam pillows with zippered vinyl (or other "dust-proof") covers. Vinyl covers let you remove the bedding and remove dust with a damp sponge. These covers are available in medical-supply stores.
  • Wash sheets and blankets every 2-4 weeks
  • Wash blinds regularly; mop the floor with a damp mop each week.
  • Remove unnecessary toys, books, and stuffed animals — within reason!


Furry animals (and, less commonly, birds) can cause quite serious asthma in people who are allergic to them. Like dust mites, for people who are animal-allergic, not only can exposure lead to asthma attacks, but long-term exposure (to a pet) can increase airway inflammation, increasing the severity of asthma attacks in reaction to other allergens and irritants. If you have a pet and aren't  sure whether your child is allergic to it, you may wish to ask your doctor about allergy tests. If you don't have a pet but your child has animal allergies, you should avoid visiting homes with that animal.

Cats tend to cause more severe allergies than dogs; but dogs, horses, (and other animals) can also cause problems. People who react to animals are actually allergic to the animal's "dandruff;" if the animal has hair, it's going to have dandruff. For this reason, a "hypo-allergenic" dog can cause allergies.

To remove all traces of pet after a pet is removed from the house, it is important to get the heating ducts cleaned, and the carpets and upholstery steam-cleaned. This whole process should be repeated about 4 months later. If your child is pet-allergic and removing the pet is not an option, excluding it from the child's bedroom, and washing the dog or cat weekly might be helpful. The child should obviously minimize contact with the animal.

Plants & Pollens

Outdoor plants and pollens often cause seasonal allergies, and asthma at particular times of the year. In Ontario, trees (such as elm, and poplar) generally cause problems between April and June. Grasses (such as timothy grass and bluegrass) generally cause problems between mid-May and mid-July. Ragweed causes problems between August and October.

Pollen forecasts available on the Internet can give you an idea of what plant allergens are present at high levels in your community. One web resource for pollen forecasts in Canada is: Pollen Forecast Ottawa, ON (as an example).

Pollens can effectively be kept out of your house by keeping your doors and windows closed during pollen season. An air conditioner is helpful to keep your house comfortable while you do this. If your child is grass-allergic, they shouldn't mow the lawn. You might wish to remember that your child probably isn't allergic to washing the dishes!


Outdoor and indoor molds are another important group of allergens. Outdoor molds are a problem, in Ontario, between March and November. Outdoor molds tend to release their spores in damp weather, and the spores travel better on windy days. This is probably the reason why many people's asthma is worse in miserable, damp weather. Spores can also be released in dry weather. Indoor molds grow especially well in damp places like damp bathrooms. They also like damp basements — especially if there are open pools of water. Humidifiers can also be contaminated by molds. Indoor molds can be a problem year-round.

Indoor molds can be reduced by reducing dampness in bathrooms and the basement. A de-humidifier is sometimes helpful. Canada Mortgage and Housing Corporation (CMHC) publishes an excellent book on fixing damp basements called "Investigating, Diagnosing, and Treating Your Damp Basement," and it is available through the CMHC office in your region. Humidifiers should be used sparingly, and cleaned often.

Air pollution

Air pollution is an important irritant, which can worsen asthma and increase the risk of asthma attacks. You can find out about air pollution levels in your community, in Canada, from the Air Quality Health Index (AQHI), which gives real-time information about air quality in your community, and reports it relative to the risk to people’s health. It’s available at:  Environment and Climate Change Canada


Foods are actually a pretty unusual cause of asthma. Unless your child wheezes or has other asthma symptoms shortly after ingesting a food, it probably isn't a problem.

One food you should be especially aware of is peanut, if your child has peanut allergy. Recent studies have shown that children who have asthma and severe, life-threatening reactions to peanut are more likely to die after eating peanut, than children who have severe, life-threatening reactions to peanut alone. If your child has severe, life-threatening reactions to peanut, you should speak to your doctor about getting a Medic-Alert bracelet and having an adrenaline syringe (Epipen® or Ana-Kit®) with you at all times. In addition to watching ingredient lists (as instructed by your doctor), you need to be on the lookout for unexpected things — like the friend who makes a plate of peanut butter sandwiches and a plate of cheese sandwiches, and then uses the same knife to cut all the sandwiches.
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