Use of Free-standing Filters in an Asthma Intervention Study

Abstract

This study characterizes the use of HEPA air filters provided to 89 households participating in an intervention study investigating the respiratory health of children with asthma. Freestanding filters were placed in the child's bedroom and monitored continuously for nearly a year in each household. Filter use was significantly affected by study phase, season and monitoring week. During the "intensive" weeks when a community education worker and a field technician visited the household, the use rate averaged 70±33%. During season-long "non-intensive" periods between seasonal visits, use dropped to 34±30%. Filter use rapidly decreased during the 3 to 4 weeks following each intensive, was slightly higher in spring, summer, and in the evening and at night when the child was likely to be home, although households did not follow consistent diurnal patterns. While participants expressed an understanding of the benefits of filter use and reported good experiences with them, use rates were low, particularly during unobserved non-intensive periods. The provision of freestanding air filters to individuals or households must be considered an active intervention that requires monitoring and evaluation, otherwise unknown and unexpected patterns of filter use may alter and possibly bias results due to exposure misclassification.

Keywords: Air quality; HEPA; behavior; exposure misclassification; indoor environment; intervention.

Figure 1

Filter use over study, showing week-long baseline and seasonal intensive visits 1 and 2, and non-intensive periods 1 to 3. Number of households-weeks (n) and number of households shown as n/N. Plots show median, interquartile range, and 10th and 90th percentile.

Figure 2

Cumulative probability of weekly filter use for intensive and non-intensive periods based on household-weeks.

Figure 3

Filter use by the hour of day for non-intensive periods. A: Use by household-study grouped by tertiles of average use among households with low (6–23%), medium (24–48%) and high (49–94%) use groups after excluding households with little use (≤5%, n=13) or near continuous use (≥95%, n=3). B. Use by household-day grouped by tertiles of average daily use with low (6–33%), medium (34–50%) and high (51–94%) use groups after excluding household-days with little use (≤5%, n=7,685) or near continuous use (≥95%, n=3,543).

Figure 4

Fraction of study households complying with various weekly filter use (average within the week) and consistency criteria (minimum use in each week of the study) for intensive and non-intensive periods.