Asthma affects different people in different ways, which is one reason why it is so hard to define and diagnose. But research is showing us that the process that leads to asthma starts very early in life – possibly before birth – and involves a complex interaction between genetic and environmental factors.

There is much we still don’t know, especially about the environmental factors that contribute to asthma. Research is finding some clues, particularly about tobacco smoke, allergies, and the possible protective effect of infections.

Tobacco smoke and a baby’s lungs

Exposure to tobacco smoke affects lung development before and after birth. While tobacco smoke does not cause asthma itself, it does increase the risk of transient wheeze, affects airway growth and can induce asthma attacks in children with asthma.

Infections, wheezing and asthma

Infections such as bronchiolitis are the main cause of transient wheezing, and conditions that increase exposure to infections (such as having more brothers and sisters or being in child care) are risk factors for transient wheezing.

On the other hand, these same factors seem to protect children against persistent wheezing.

It is thought that common childhood infections may change children’s immunity so they are less susceptible to allergic responses such as inflammation. More research is needed to explore this further.

Asthma and allergies

Content Updated March 2005