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Objectives To measure levels of the major Dermatophagoides pteronyssinus allergen (Der p 1) in homes in Wellington, New Zealand, and to examine factors which affect these levels. Methods As part of a study of risk factors for asthma among 474 8—10-year-old children, standard procedures were used to collect reservoir dust and to measure Der p 1 levels on the living room floor and child's bedroom floor and bedding. Der p 1 levels were analysed both as geometric mean μg/g of fine dust and as μg/m². Questionnaires collected information about factors which might influence these levels, and an average relative humidity in the bed and on the bedroom floor was also measured. Results Similar geometric mean levels of Der p 1 were found at each floor site — 25.5 µg/g bedroom floor. The geometric mean level of Der p 1 in the child's bed was 46.6 µg/g (95% CI: 42.3-51.3). After controlling for possible confounders, geometric mean living room and bedroom floor Der p 1 levels were significantly higher in households with older carpet than households with no carpets or newer carpets, and higher in the autumn. Households with three or more children had higher levels of Der p 1 than households with fewer children. Bedding levels were significantly higher in beds with kapok or inner sprung mattresses, or wool underlays and at relative humidities above the mean (51%). Conclusion The very high levels of house dust mite allergen (Der p 1) found in Wellington are likely to be due to a variety of life-style and climatic factors. However, the type and age of floor covering appears to be the single most important factor. [ABSTRACT FROM AUTHOR]