Air Quality Matters -- At Least for Kids With Asthma

William T. Basco, Jr, MD, MS


July 24, 2019

Climate Change Versus Air Quality

Despite many dire indicators that the climate is indeed changing, air quality is getting better—at least since 1990 and in California. So what are the clinical implications of that improvement? Could it drive reductions in asthma incidence?

Using air-quality monitoring data collected over 20 years (1993-2014) in nine communities in southern California, investigators from the Department of Preventive Medicine at the University of Southern California examined the correlation between community-level ambient air quality and asthma incidence.[1] Families participating in the study completed assessments at enrollment that provided demographic and baseline medical condition data. All children were assessed annually thereafter.

Data on environmental exposures to ozone, nitrogen dioxide, and other air pollutants were collected over the study years by monitoring stations in the communities. Additional information collected included:

  • Tobacco smoke exposure, both in utero and secondhand;

  • Family respiratory history;

  • The presence of a gas stove in the home; and

  • Organized or team physical or sport-related activities of the child.

The primary outcome of interest was new cases of asthma diagnosed between annual assessments. Patients with asthma already diagnosed at initial enrollment and those without follow-up assessments were excluded from analyses.

There were over 4000 children in the study; 52.6% were female and 42.2% were of Hispanic ethnicity. Just over 17% had a parent with asthma, 22.5% had secondhand smoke exposure, and 12.3% in utero smoke exposure.

Environmental pollutants declined during the study years in all of the communities included in the analysis. Across the board, asthma incidence decreased, with the largest decreases found in the communities that had the largest pollutant reductions. The only pollutant with consistent statistically significant associations with asthma was nitrogen dioxide; this association persisted even after controlling for family history and other environmental exposures, such as tobacco smoke and gas stoves. Atmospheric pollutant matter (PM) with a diameter of less than 2.5 microns (referred to as PM2.5) was also associated with incident asthma, though the findings were less robust in sensitivity analyses.


The authors point out two important factors to keep in mind when reviewing such studies as this one that, because of their inherent design, can only show association rather than causation:

  • First, is there biological plausibility? That appears to be the case for the pollutants studied and the outcomes assessed.

  • Second, does the observational study identify a "dose-response" relationship between the exposures and the outcomes? That finding strengthens the case that the relationship is potentially cause-and-effect rather than just association.

Applying these findings to daily practice requires a leap that I do not generally espouse. But in this case, the data do indirectly support the idea that patients with bad asthma should be more careful on poor air-quality days. Maybe we need to teach patients to follow air quality reports and use them as their own reminder to make sure they take preventive medications on bad-air days.

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