One exception is ozone, a substance

One exception is ozone, a substance found naturally in the atmosphere at
altitudes where most commercial aircraft fly. It enters the cabin with outside air
that is used for cabin ventilation. Ozone is a respiratory system irritant and can
cause chest tightness, coughing, and shortness of breath if exposure occurs at
high enough concentrations. In general, low levels have been found in aircraft
cabins although, in several instances, levels were measured slightly exceeding
regulatory standards. Most aircraft flying at altitudes and latitudes where high
ozone concentrations are encountered now have ozone converters which break
down the ozone before it reaches the cabin.