Dales et al. irrespective of parental history of atopy, using airborne rather than dust endotoxin, including indicators of respiratory illness in addition to wheeze, and considering confounding by indoor mold exposure. Nevertheless, even with children not selected based on atopic parents, we also found that wheeze was the symptom with the strongest association with endotoxin, and the effect size became smaller with increased duration of follow-up. Sources of endotoxins. Gram-negative bac- teria are found in water, soil, and outdoor air. Reported indoor sources of gram-negative include contaminated humidiﬁers, pets, stor- age of food waste, and increased amounts of settled dust (Park et al. 2001b). The need for water availability is consistent with our ﬁnding that relative humidity was positively associated with air endotoxin, not previously described. Gehring et al. (2001a) found that dust con- centrations were higher with cats and dogs present. 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Exposure to endotoxin or other bacterial components might protect against the development of atopy. Clin Exp Allergy 30:1230–1234. 614 VOLUME 114 | NUMBER 4 | April 2006 • Environmental Health Perspectives