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Inference for environmental intervention studies using principal stratification.

ABSTRACT: Previous research has found evidence of an association between indoor air pollution and asthma morbidity in children. Environmental intervention studies have been performed to examine the role of household environmental interventions in altering indoor air pollution concentrations and improving health. Previous environmental intervention studies have found only modest effects on health outcomes and it is unclear if the health benefits provided by environmental modification are comparable with those provided by medication. Traditionally, the statistical analysis of environmental intervention studies has involved performing two intention-to-treat analyses that separately estimate the effect of the environmental intervention on health and the effect of the environmental intervention on indoor air pollution concentrations. We propose a principal stratification approach to examine the extent to which an environmental intervention's effect on health outcomes coincides with its effect on indoor air pollution. We apply this approach to data from a randomized air cleaner intervention trial conducted in a population of asthmatic children living in Baltimore, Maryland, USA. We find that among children for whom the air cleaner reduced indoor particulate matter concentrations, the intervention resulted in a meaningful improvement of asthma symptoms with an effect generally larger than previous studies have shown. A key benefit of using principal stratification in environmental intervention studies is that it allows investigators to estimate causal effects of the intervention for sub-groups defined by changes in the indoor air pollution concentration.

SUBMITTER: Hackstadt AJ

PROVIDER: S-EPMC4224995 | biostudies-literature | 2014 Dec

REPOSITORIES: biostudies-literature

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Project description:BackgroundIndoor air quality represents a modifiable exposure to Chronic Obstructive Pulmonary Disease (COPD) health. In a randomized controlled trial (CLEAN AIR study), air cleaner assignment had causal effect in improving COPD outcomes. It is unclear, however, what is the treatment effect among those for whom intervention reduced air pollution and whether it was reduction in fine particulate matter (PM2.5) or nitrogen dioxide (NO2) that contributed to such improvement. Because pollution is a posttreatment variable, treatment effect cannot be assessed while controlling for pollution using intention-to-treat (ITT) analysis.ObjectiveUsing principal stratification method, we assess indoor pollutants as the intermediate variable, and determine the causal effect of reducing indoor air pollution on COPD health.MethodIn randomized controlled trial, former smokers with COPD received either active or placebo HEPA air cleaners and were followed for 6 months. Saint George's Respiratory Questionnaire (SGRQ) was the primary outcome and secondary measures included SGRQ subscales, COPD assessment test (CAT), dyspnea (mMRC), and breathlessness, cough, and sputum scale (BCSS). Indoor PM2.5 and NO2 were measured. Principal stratification analysis was performed to assess the treatment effect while controlling for pollution reduction.ResultsAmong those showing at least 40 % PM2.5 reduction through air cleaners, the intervention showed improvement in respiratory symptoms for the active (vs. placebo), and the size of treatment effect shown for this subgroup was larger than that for the overall sample. In this subgroup, those with active air cleaners (vs. placebo) showed 7.7 points better SGRQ (95%CI: -14.3, -1.1), better CAT (β = -5.5; 95%CI: -9.8, -1.2), mMRC (β = -0.6; 95%CI: -1.1, -0.1), and BCSS (β = -1.8; 95%CI: -3.0, -0.5). Among those showing at least 40 % NO2 reduction through air cleaners, there was no intervention difference in outcomes.ConclusionAir cleaners caused clinically significant improvement in respiratory health for individuals with COPD through reduction in indoor PM2.5.Trial registrationClinicalTrials.gov: NCT02236858.

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Inference for environmental intervention studies using principal stratification.

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