A marked increase in allergic disease has occurred over the past century. For example, between 1992 and 1997, the prevalence of asthma increased by 26% and skin-prick sensitivity to house dust mite (HDM) increased by 63% in Australian children.1 In determining the causes of this increase it is important to distinguish between primary and secondary causes of allergic disease. Primary causes are those considered to induce allergic disease in a non-sensitised person, while secondary causes are those that trigger symptoms in people who are already sensitised. Primary prevention strategies are aimed at reducing sensitisation. In the early 1980s it was considered that a clean environment, avoidance of pets, the provision of synthetic "allergy free" bedding (rather than feather bedding) and prolonged breastfeeding were all important in primary prevention. But recent epidemiological studies have challenged these beliefs.

There is evidence that a clean environment in early life may actually promote rather than inhibit the development of allergy. The "hygiene hypothesis" is based on epidemiological studies comparing the prevalence of allergic disease in "clean" and "dirty" environments. For example, children growing up in East Germany before the fall of the Berlin Wall had a lower prevalence of allergic disease than children in West Germany, despite having more exposure to pollution and infection.1 These results have been confirmed in similar comparative studies. Other relevant studies supporting the "hygiene hypothesis" have demonstrated fewer allergies in children from large families, in younger siblings, in children exposed earlier to day-care centres, and in children growing up on farms in Europe.

Prevention programs for allergic disease have recommended avoidance of pets, particularly cats. However, recent studies showing either less asthma or less sensitisation among children exposed to cats in infancy have challenged this view.2,3 Exposure to cats in infancy does not appear to increase the risk of developing asthma. With regard to sensitisation, the evidence is conflicting, with some studies suggesting decreased sensitisation following cat exposure in infancy and others indicating the reverse. Cat exposure is associated with increased environmental levels of bacterial endotoxin. There is a hypothesis that endotoxin derived from pets may play a role in the prevention of allergy, as endotoxin can induce immune deviation away from "allergic" TH2 responses.

The common belief that feather bedding promotes and synthetic bedding prevents allergic disease is now in doubt. This belief arose because of purported allergy to feathers or accumulation of HDM allergen in feather products. In fact, feather pillows contain up to eightfold lower levels of HDM allergen and accumulate this allergen more slowly than synthetic pillows. Children using a feather quilt are less likely to be sensitised to HDM.4 Prospective studies show that use of feather bedding in early childhood is associated with reduced asthma5 and use of synthetic bedding with increased asthma6 in later childhood. Studies of bedding are potentially complicated by selection bias: children with asthma may preferentially use synthetic rather than feather bedding, because of the widely held belief that synthetic bedding is less harmful.

It is widely believed that breastfeeding should be recommended for primary prevention of allergic disease. Exclusive breastfeeding beyond four months of age reduces the development of atopic disease in early life,7 but the long-term benefits are now in question. One study has suggested that breastfeeding increases both asthma and allergen sensitisation in adult life;8 however, the fact that the breastfeeding was not necessarily exclusive may be a possible confounder. Another study demonstrated a protective effect of breastfeeding in early life but increased asthma in older children.10

A parental history of allergy is the most important risk factor for childhood allergy. What, then, are we to recommend to parents? Firstly, it is not possible to guarantee that any steps taken will prevent allergic disease. It seems reasonable to recommend exclusive breastfeeding for at least four months to increase the chance of reducing allergic disease in early childhood. It is not clear that the benefits extend to later life. Currently, it is not possible to provide firm recommendations on allergen reduction measures. Local environmental factors are important for HDM replication, and the benefits or otherwise of measures to reduce HDM exposure in infancy need to be demonstrated in the local environment. An Australian study of the effect of HDM reduction measures in infancy is in progress and the results are awaited with interest. Feather pillows or Doonas do not need to be avoided and may in fact be more beneficial than synthetic bedding. Once a child is sensitised, there may be a role for effective HDM encasing on any type of bedding, although again not all studies agree on this issue. Avoidance of household pets is not likely to prevent the development of allergic disease and cannot be recommended as a prophylactic measure. Nevertheless, it is advisable for clinically sensitive patients. It is clear that we need to critically re-examine the previous recommendations given to parents.