Effect of air quality alerts on human health: a regression discontinuity analysis in Toronto, Canada

Background — Ambient air pollution is a major health risk globally. To reduce adverse health effects on high air pollution days, government agencies worldwide have implemented air quality alert programs. Despite their widespread use, little is known whether these programs produce any observable public health benefits. We evaluated the effectiveness of such programs using a quasi-experimental approach.

Methods — We assembled a population-based cohort comprising all individuals who resided in the City of Toronto (Ontario, Canada) from 2003 to 2012 (~2.6 million). We ascertained seven health outcomes known to be affected by short-term elevation of air pollution, including cardiovascular- and respiratory-related deaths, as well as hospitalizations or emergency-department visits from acute myocardial infarction, heart failure, stroke, asthma, and chronic obstructive pulmonary disease (COPD) using provincial health administrative databases. We applied a regression discontinuity design to assess the effectiveness of an intervention. To quantify the impact of the air quality alert program, we estimated, for each outcome, the absolute rate difference and rate ratio attributable to alert announcements and program eligibility (intention-to-treat analysis), respectively.

Findings — Between 2003 and 2012, there were on average three to 27 daily cardiovascular or respiratory events in Toronto (depending on the outcome). Alert announcements reduced asthma-related emergency-department visits by 4.73 cases (95%CI: 0.55, 9.38) per 1,000,000 persons per day or in relative terms by 25% (95%CI: 1%, 47%). Program eligibility also led to 2.05 (95%CI: 0.07, 4.00) fewer daily asthma emergency-department visits. Furthermore, there was a tendency toward decreased asthma- and COPD-related hospitalizations. However, we did not detect a reduction in any other outcome as a consequence of alert announcements or program eligibility.

Interpretation — In this population-based cohort, air quality alert program was related to some decreases in respiratory morbidity, but not any other outcome examined.