Background Studies investigating the effect of exposure to indoor allergens and endotoxin on human respiratory health usually take dust samples only at one single point of time and consider them to be representative for the indoor biocontaminant burden during a time period of several years. This singly measured biocontaminant concentration is related to adverse respiratory outcomes such as asthma, wheeze or others.
Objective We analysed two repeated measurements of mite and cat allergens in mattress and living room floor dust as well as endotoxin concentrations in living room floor dust. The repeated samples were taken over a time period of about 6 years. We investigated the repeatability over time of their concentrations by determining correlation coefficients and computing within- and between-home variance components.
Methods Our analysis was based on the population of a study on Indoor Factors and Genetics in Asthma (INGA) being carried out in 1995/96 and followed up in 2000/01. Complete data were available from 152 participants.
Results The measured allergen concentrations were low and a considerable percentage of the values was below the limit of detection. The crude Spearman's rank correlation coefficients between the two measurements taken in 1995/96 and 2000/01 ranged from 0.32 to 0.61 for the dust mites allergens, from 0.21 to 0.44 for cat allergen and from 0.35 to 0.51 for endotoxin. Correlations were higher if measurements were performed on the same floor or the same mattress at both sampling time-points. The within-home variance for all measured biocontaminant concentrations was of about the same order as the between-home variance.
Conclusion For studies like ours with low allergen and endotoxin concentrations, the repeatability of these concentrations over a time period as long as 6 years is low and a single measurement does not accurately reflect the true long-term exposure in the homes.