A team at the University Hospital of North Staffordshire has been investigating how environmental pollutants might influence the risk of the illness.

Scientists already know that asthma can be a genetic condition but increases in the rates of asthma in the past 20 years suggest the environment can also be an important factor.

The team at the hospital found a mother's reaction to environmental toxins can influence her unborn child's development of asthma.

If her genetic make up means she reacts badly to toxins in the environment her foetus becomes stressed, which can lead to asthma.

Equally, a change in the mother's genes providing her with protection from environmental toxins also protects her unborn child.

The scientists say further research could lead to a greater understanding about what environmental factors mothers could avoid to protect their child.