Children seem to be most vulnerable to the harmful effects of ambient air pollutants because their defence mechanisms are still evolving and because they inhale a higher volume of air per body weight than adults. Air pollutants can also harm the foetus if the mother is exposed to high levels of air pollution during pregnancy. An increase in respiratory neonatal mortality has been associated with ambient levels of air pollutants. Exposure to fine particles has been shown to increase allergen sensitisation, increase the risk of worsening asthma and decrease lung function.