REBUTTAL FROM DR. VON MUTIUS

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Platts-Mills and colleagues state that at the time of Noon's report in 1911, hay fever was a common condition. Data from Switzerland, however, show that this was not the case, because the prevalence of hay fever was estimated at 0.8% in 1926. Platts-Mills and colleagues, furthermore, state that there was no increase in the prevalence of hay fever after 1960 in the United States. Although this statement may be correct for the United States, it is not for other countries, where abundant evidence exists to suggest that rates of hay fever increased after the 1960s (1). For example, the prevalence of hay fever in England rose from 9.4 to 14.9% from 1973 to 1988 (2). Platts-Mills and coworkers, furthermore, state that "the rise in asthma did not start before 1960." Data from Australia, where in 1963 the prevalence of wheeze was already estimated at 11% (3), suggest that an increase in the prevalence of wheeze and asthma is likely to have taken place before that time. Similar figures of 12.1% were reported from Tecumseh, Alabama in 1962 (4). The argument that "the major changes in `cleanliness' occurred long before the increase in asthma" is thus not based on solid ground. Asthma is a syndrome with multiple genetic and environmental determinants. It is thus not surprising that risk factors other than cleanliness also contribute to the expression of the illness. Statements arguing in favor of obesity, such as those proposed by Platts-Mills and colleagues, are therefore not suited to attack the hygiene hypothesis. Finally, Platts-Mills' statement that the exact mechanisms by which unhygienic contacts reduce the prevalence of asthma have not been conclusively elucidated cannot argue against the epidemiological evidence. In many circumstances epidemiological findings have been confirmed by subsequent experimental studies, although the precise mechanisms were unknown at the time of the field survey. Epidemiological associations are likely to reflect causal relations if the following criteria are fulfilled: consistency among studies, dose-response relation between exposure and outcome, significant size of the effect, and biological plausibility. The findings of the epidemiological studies relating to the hygiene hypothesis are in accordance with these criteria and thus suggest causal relations.

	References

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1. von Mutius E, Martinez F. Epidemiology of childhood asthma. New York: Marcel Dekker; 1999.

2. Burr M, Butland B, King S, Vaughan-Williams E. Changes in asthma prevalence: two surveys 15 years apart. Arch Dis Child 1989; 64: 1452-1456 [Abstract/Free Full Text].

3. Williams H, McNicol K. Prevalence, natural history, and relationship of wheezy bronchitis and asthma in children. An epidemiological study. Br Med J 1969; 4: 321-325 .

4. Broder I, Harlow P, Horton R. Allergy 1962; 33: 513 .