Most people with asthma get symptoms when their immune system overreacts to things in the air.  These things are called allergens.
The most common allergens are:
If you are allergic to house dust mites, it isn't the mites themselves that trigger asthma symptoms. It's their droppings.
When you breathe in an allergen, your body thinks the allergen is dangerous. Your immune system overreacts. It pumps lots of chemicals into your blood. These chemicals make the airways swollen and inflamed. The muscles in the airways also tighten, the air passages get narrower and you have trouble breathing. It all happens very quickly.
About 30 in 100 to 50 in 100 people with asthma get these symptoms again, 6 to 10 hours after they breathe in an allergen. This is called a late reaction. Smoke and air pollution
Breathing in tobacco smoke can cause asthma.  And if you already have asthma, smoke can trigger the symptoms (give you an asthma attack).
Tobacco smoke contains many different chemicals. Some of these chemicals can make the tubes in your lungs swollen or narrower. So, some people get asthma symptoms when they go into a smoky room.
Air pollution probably doesn't cause asthma. But it can make your symptoms worse. These are some of the chemicals that can cause asthma symptoms:
Nitrogen dioxide (NO2)
Sulphur dioxide (SO2).
If there are warnings that air pollution is bad in your area, you may want to stay indoors.
Some people get an asthma attack when they breathe in cold air.  This may happen when you leave your home and go outside. Exercise
Many people with asthma find that their symptoms are worse when they exercise. Doctors call this exercise-induced asthma.
It's more likely to happen if you exercise in cold and windy conditions.   So you're more likely to get symptoms if you're skiing or ice skating than if you're swimming in a heated pool. Some people get symptoms when they leave their home on a cold day. 
Your symptoms may start during exercise. But more often they start within five to 15 minutes of stopping exercise. They may start up to 30 minutes after you stop.
Some people can 'run through' their symptoms. As they continue to exercise, their symptoms slowly go away.  And if you use your reliever inhaler (usually blue) before you exercise, it may prevent you getting symptoms. Medicines that may trigger an asthma attackAspirin and NSAIDs
About 1 in 10 people with asthma are allergic to aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen (Nurofen).  There are many different NSAIDs. They are painkillers and they can help with inflammation. If you're allergic to these drugs, there's also a 1 in 10 chance that you'll get asthma symptoms when you eat food or drinks that contain a food colouring called tartrazine (E102). If you need to take painkillers and you've had a bad reaction to an NSAID, it's a good idea to tell your doctor so he or she can advise you about alternatives. Beta-blockers