Risk Factors

The causes of asthma are not well understood. Asthma probably starts early in life, as the immune system is developing.1 Certain traits of one person’s immune system might make that person more vulnerable to allergens or viruses. Therefore, coming into contact with some allergens (ex: cockroach or dust-mite) or viruses (ex: RSV) sets the immune system on a track toward allergic disease and asthma.

Since asthma runs in families, some important immune system traits could be genetic.1 However, exactly which genes are linked to asthma is not known.

Before birth

How does smoking during pregnancy affect the baby?
Children have a higher risk of asthma if the mom smoked while she was pregnant.1 At age two years, these children are 40% more likely to wheeze and 85% more likely to have asthma.2 Between ages five and 18, they are 52% more likely to wheezing and 23% more likely to have asthma.2 Children exposed to smoke prenatally have life-long decreases in lung function.1

Infancy and early childhood

How does exposure to allergens affect the risk of asthma?
Allergens are one of the two most important risk factors for asthma.1 Allergies develop after a person comes in contact with an allergen. Common allergens are dust-mites, cockroaches, and mold. The allergen turns on your immune system, which starts to make antibodies. Antibodies are important for fighting harmful infections. However, they also cause allergic reactions to things that are harmless, such as dust-mites or pet dander. Once the allergy develops, continuing to be in contact with the allergen can lead to asthma.1

One study looked at the risk of asthma in six-year-olds who developed an allergy by age three.3 Compared with children without allergies, the results showed that asthma risk was:

  • More than 9 times higher for children with a dog allergy.
  • Almost 4 times higher for children with a cat allergy.
  • Almost 3 times higher for children with a dust-mite or mold allergy.

How do viruses affect the risk of asthma?
Viral infections are the other major risk factor for developing asthma.1 Children who wheeze when they get a cold are more likely than non-wheezers to have asthma when they are six years old.4 Two common cold viruses that are linked with asthma are RSV (respiratory syncytial virus) and HRV (human rhinovirus).

RSV is a common viral infection that causes chest colds.5 The season for RSV infections is October to January, except in Florida where the season goes from July to February.5 Babies with an RSV infection have symptoms that seem like asthma. One-third of children who get an RSV infection develop an ongoing wheeze. This is most likely to happen if one of the parents has asthma.4

HRV often causes the “common cold,” with symptoms that affect the nose, throat, and lungs.4 Children who wheeze when they catch a cold are nearly ten times more likely than non-wheezers to develop asthma.4

How does second-hand smoke affect the risk of asthma?
Children who grow up in homes with smokers are at increased risk wheezing and asthma.1 For two-year-olds whose mom smokes, the risk of wheezing is about 70% higher and the risk of asthma is 20% higher than if mom does not smoke.2

The consequences of second-hand smoke can be seen into adulthood. One study showed that people who were exposed to second-hand smoke as children were nearly twice as likely as others to develop asthma in adulthood.6

What other factors could increase the risk of asthma for children?
Other factors may increase the risk asthma, but the links are not as clear. Examples include:1,7,8

  • Air pollution
  • Poor diet
  • Over-use of antibiotics
  • Exposure to irritants
  • Ongoing stress and hardship


Can smoking cause asthma in adults?
It is hard to study whether smoking causes asthma in adults, but most studies indicate that the two are linked. Over a ten-year period, smoking was a risk factor for developing asthma for adults with hay fever.6 Asthma is more common in heavier smokers than lighter smokers.6 Compared with non-smokers, for a person with a one to ten pack-year smoking history is twice as likely to develop asthma and a person with a 21+ pack-year smoking history has five times as likely.9

Can irritants cause asthma?
Some asthma appears to start in adulthood because of allergens and irritants at work. Cleaners, nurses, textile works, and farmers who are regularly exposed to irritants on the job are more likely to have asthma.10 In other cases, massive exposures to high levels of irritants, such as spills, have led to asthma.10 People living in homes with high levels of irritants, such as volatile organic compounds have a higher risk of asthma.11

Does obesity cause asthma?
The increase in obesity seems to have stopped recently.12 However, for many years, obesity and asthma increased together. How the two are related is not known.1 Obesity can lead to airway changes that causes shortness of breath. Very overweight people may also have more inflammation that leads to problems with airway function.

Protection against asthma

What is the Hygiene Hypothesis?
One theory is that increased cleanliness and smaller family size have decreased our exposure to helpful germs, increasing our risk of allergic disease. This theory is called the “Hygiene Hypothesis.” The idea is that if contact with the “wrong” allergens and viruses sets the immune system on a track toward allergic disease, contact with the “right” viruses, bacteria, and parasites could set the immune system on a protective track.1 A number of factors are linked with lower risk of asthma:1,9,13

  • Having more siblings
  • Starting daycare at a young age
  • Less frequent use of antibiotics
  • Having pets between birth and three months
  • Some types of infection
  • Growing up on a farm

So far, this is just a theory. The specifics of how this might work have not been proven. Not enough is known yet to make practical recommendations.

Can breastfeeding reduce the risk of asthma?
Breastfeeding may protect your child against asthma and other allergic diseases, especially if asthma runs in your family.14 The American Academy of Pediatrics recommends exclusive breastfeeding (no formula or solids) for about six months, and continuing to breastfeed until one year or longer.14

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Written by: Sarah O'Brien | Last Reviewed: May 2016.