The increase in asthma prevalence over the past 20 years could be due to modification of exposure to environmental factors (environmental theory) or to the lost of protective factors (hygienist theory). Among environmental factors this paper reviewed the controversial role of exposure to house dust mite (HDM). If exposure to HDM is deleterious in asthmatics known to be sensitised to this allergen, the effect of HDM exposure on asthma incidence has been challenged recently, based on longitudinal studies showing no correlation between level of exposure to HDM and asthma incidence. Exposure to animal dander may have protective effects. This review has shown that, even if some studies have shown potential protective effect of early exposure, the protection seems to be better in families with a low risk of atopy; almost all studies tend to show a deleterious effect of current exposure; exposure to dog dander may be more protective than cat. These studies cannot give definite conclusions to change current advices of early eviction of HDM and animal dander, especially in families with a high risk of atopy. The deleterious effect of passive smoking on asthma prevalence and severity is undisputable and our efforts should concentrate on obtaining effective eviction of passive smoking.