Most studies suggest that asthma prevalence has beenincreasing in children and young adults over the last fourdecades. This increase is not confined to asthmabut includes atopic diseases, such as hay fever and eczema.It has been suggested that a disease is caused bythe interaction of genetic susceptibility and an adverseenvironment. Environmental differences may explainthe variations in the prevalence of asthma found inwestern countries. Several studies have shown an associationbetween the risk of developing respiratory allergyand the season of birth. Other environmentalfactors, such as respiratory viral infection, tobaccosmoke and outdoor pollutants, have been considered asadjuvants for initial sensitization in childhood. Anadverse environment does not necessarily imply an outdoorenvironment, but could be the workplace where asubject spends a substantial part of his life.