Though asthma is common, what causes it isn't fully understood. It's likely there is a genetic component that predisposes a person to develop the disease, but typically certain environmental factors must also be in place.

But although experts may not know for sure why one person has asthma and another doesn't, they do understand the changes in the body that lead to asthma symptoms – bronchoconstriction (narrowing of the bronchi, or airways) and excess mucus production that together restrict airflow. There also are a variety of known risk factors for asthma, such as excess weight, as well as many common triggers ranging from allergens like dust mites and mold to exercise and respiratory infections such as a common cold.1

This article describes what's currently known about the risk factors for asthma.

Asthma Triggers

The potential causes of asthma symptoms and asthma attacks in those who have the condition are as variable and unique to each individual as are the factors that put them at risk in the first place. There's a wide variety of such triggers, and many people have more than one.

Indoor Triggers

Your home may harbor any of several common allergens known to bring on asthma symptoms.

Dust mites: Dust mites (Dermatophagoides pteronyssinus) are microscopic insects that exist in every home and feed on tiny flakes of skin and hair found on bedding (mattresses, pillows, bed covers), carpets, upholstered furniture or anything covered in fabric, and stuffed toys.

Mold: Mold is most often found on wet or damp surfaces in bathrooms, kitchens, and basements.

Cockroaches and other

pests: Body parts, urine, and droppings of cockroaches and pests contain proteins that can trigger allergy symptoms.

Pets: Allergens from your pets' dead skin, droppings, urine, and saliva can trigger asthma.

Secondhand smoke: Environmental tobacco smoke contains more than 250 different chemicals, including benzene, vinyl chloride, and arsenic, that may irritate airways and bring on asthma symptoms.

Nitrogen dioxide: Nitrogen dioxide is a gas released by gas stoves, fireplaces, and gas space heaters. It can irritate lungs and lead to shortness of breath.

Outdoor Triggers

During the spring and fall, airborne pollens and molds commonly trigger asthma symptoms, among them:

Pollen: Pollens are small, powdery granules that are essential for plant fertilization. Pollens from many different kinds of grasses, weeds, and trees may trigger allergy and asthma symptoms. The season and weather conditions greatly influence the amount of pollen in the air. Pollen season varies depending on location but typically lasts from February to October.

Mold: Mold growing in soil or on outdoor vegetation can become airborne and trigger asthma symptoms.

Weather: Certain weather conditions can make asthma triggers more problematic. Pollen is particularly plentiful when it's hot, dry, and windy outside, for example. Mold thrives in rainy or humid weather. Dry, cold, or windy weather can also set off asthma episodes.

Respiratory Infections

Any type of respiratory infection – such as a common cold or the flu – can trigger asthma symptoms.13 If you have asthma, it's especially important to take measures to stay well:

Wash your hands frequently

Don't touch your nose or mouth while you're out in public or around someone who's sick

Get a flu shot every year.

Less Common Asthma Triggers

Although these triggers are relatively uncommon, they are potentially serious for people who are sensitive to them.

Medications: A number of different medications are associated with asthma flare-ups, including pain medications (aspirin, ibuprofen, naproxen) and beta-blockers.

Food allergies: Some foods like fish, soy, eggs, wheat, and tree nuts are common food allergens. In some patients with life-threatening food allergies, eating these foods can also trigger asthma attacks, which can be deadly.

Exercise: Wheezing, coughing, and chest pain can occur in response to physical activity in people with asthma. This is known as exercise-induced bronchoconstriction (EIB) and is most common in teens and young adults.