The importance of housing characteristics in determining Der p 1 levels incarpets in New Zealand homes

Background A previous study of homes in Wellington, New Zealand showed that having carpets on floors was the most important determinant of floor Der p 1 levels, but there was much unexplained variability between houses in car pet levels. Objective To determine to what extent housing characteristics might explain this variability in Der p 1 levels between houses. Methods We returned to a selection of houses with carpets and sampled livin g room dust from 1 square metre for 1 min and from the whole floor at 5 m(2 ) per min. Der p 1 levels were estimated by double monoclonal antibody ELIS A and are expressed as geometric mean mug/g and mug/m(2) (95% confidence in tervals). Questionnaires were used to collect information on housing charac teristics. Results Der p 1 levels were significantly higher in the 1 square metre samp le (40.0, 31.9-50.2 mug/g; 53.4, 41.4-68.9 mug/m(2)) than in the whole room (25.8, 21.3-31.1 mug/g; 5.3, 3.8-7.4 mug/m(2)). However, results from the different sampling methods were correlated (r = 0.51, P = 0.001 for mug/g a nd r = 0.58, P = 0.001 for mug/m(2)). After controlling for possible confou nders, houses with insulation or a room or garage below the living room had approximately half the Der p 1 concentration (P = 0.05 for both samples) a nd the amount of Der p I per m2 (P = 0.004 for the 1 square meter sample, P = 0.06 for the whole room sample) than houses without these features. Havi ng more than two children was associated with higher levels of Der p 1 in I square meter, significant (P = 0.05) for mug/m(2). Carpet underlay less th an 8 mm thick was associated with an almost 3-fold increase in ug/m(2) Der p 1 (P = 0.03) and a 1.6-fold increase in ug/g Der p 1 (P = 0.08) in the wh ole room sample, when compared with thicker carpet underlays. Conclusion The presence of insulation is the single most important housing characteristic explaining the between-house variability in Der p 1 levels o n carpeted living room floors.