Asthma and Ozone

by Steven Babin, MD, PhD

Some people wonder why asthma exacerbations are lowest in the summer when tropospheric ozone is highest.  Why the inverse correlation?  Are these two even related?  Does ozone protect you from asthma exacerbations?   Within the answers to these questions lies the distinction between correlation and causality. 

Ozone does indeed result in asthma exacerbations.  But there are other causes as well: pollen, respiratory illness, flu season, etc.  These other causes result in widespread asthma exacerbations that appear as strong signals and tend to obscure the effects of ozone.  To uncover the effects of ozone, one must remove these stronger signals.  The papers referenced below[1][2] provide at least two mathematical techniques for doing this.

Some people have commented that high temperatures and relative humidity cause asthma exacerbations.  In these cases, they are correlated but not causes.  Tropospheric ozone comes from several sources, but the main anthropogenic one is its production by photochemical reactions involving nitrogen oxides, carbon monoxide, and volatile organic compounds (produced primarily from fuel combustion and evaporation).  Because sunlight is needed, the more sunlight and the longer it is available, the more ozone can be produced.  But sunlight also raises the air temperature; hence, the correlation.  Relative humidity is a measure of atmospheric moisture but it includes temperature, so it tends to go up when temperature goes up.  These correlations are not causes.  In fact, cold dry air may cause asthma exacerbations especially during exercise, but that is a story for another time.