Asthma, eczema, food allergy and allergic rhinitis are common and the cause is still largely unknown, but it is the subject of extensive research. The research results are inconclusive, probably due to individual differences in vulnerability to different exposures and to a highly multifactorial origin. Several environmental factors have been identified as risk factors or as having a potentially protective effect. Smoking during pregnancy or childhood increases the risk of obstructive symptoms and asthma. Breastfeeding reduces the risk of infectious diseases and thereby also the risk of early obstructive symptoms. Even if mothers avoid certain foods during pregnancy or breastfeeding, this does not reduce the risk of allergic disease in the child. The child should be introduced to foods between four and six months of age, without avoiding or delaying any particular food. Vaccinations do not affect the risk of allergic disease. The risk of allergic disease is not increased by exposure to furry animals in the home, if the child is healthy. However, children who are already allergic should at least avoid the animals to which they are allergic.