BMJ Group Medical Reference
This information is for parents of a child who has asthma. It tells you about avoiding allergens, a treatment used to prevent asthma symptoms.
Does it work?
We haven't looked at the research on avoiding allergens in as much detail as we've looked at the research on most of the treatments we cover. (To read more, see Our method.) But we've included some information because you may have heard of this treatment or be interested in it.
What are they?
There may be things in your home and outdoors that can trigger your child's asthma.  These are known as allergens. For example, you may find that being around pollen or pets brings on your child's symptoms. Staying away from these things may help keep your child's asthma under control.
When your child breathes in an allergen, their immune system overreacts because it thinks the allergen is dangerous. It pumps lots of chemicals into the blood. These chemicals make your child's airways swollen and inflamed. The muscles in their airways also tighten, the air passages get narrower, and your child has trouble breathing. It all happens very quickly.
About 30 in 100 to 50 in 100 people with asthma get these symptoms again six to 10 hours after they breathe in an allergen. This is a late reaction. 
Some doctors call all asthma triggers allergens, but others group them into allergens and environmental factors.
The most common allergens are droppings from dust mites, pollen, and flakes of skin or hair from animals.
Environmental factors include tobacco smoke and air pollution.
How to avoid allergens
You can take steps to avoid allergens. They may help keep your child's asthma under control.   
Keep your child away from the things that start their asthma attacks, such as pollen or animals.
Don't smoke. Keep your child away from smoky places.
Make certain your child washes their hands after touching things that trigger their asthma (a friend's dog or cat, for example).
Remember that pet hair is hard to get rid of. It's possible to get asthma symptoms for months after a pet last lived in a house.
Use solid or liquid cleaning products rather than sprays.
Keep strong smells and perfumes out of your home.
Open your windows regularly to air the rooms. But close the windows if there are exhaust fumes or smoke coming in.
Some people advise trying to get rid of dust mites in the home with frequent cleaning. But research shows this is unlikely to help with asthma. To read more about dust mites, see Trying to get rid of dust mites in your home to prevent wheezing in babies and young children.
As well as avoiding allergens, make sure your child takes his or her asthma medications the way your doctor or nurse advises.