The real world is a complicated place. It is often exceedingly difficult to disentangle the causes of phenomena, such as the increasing rate of childhood asthma, from the spaghetti-like mass of possible contributors. Was a new pollutant or irritant added to our air? Did something disappear from our air? Are children today exposed to more allergens at an earlier age than their parents were? Or less exposed? Do these exposures affect susceptibility to asthma?
For years, scientists speculated that there was a link between increasing asthma rates and the pollution generated by burning gasoline in cars and trucks.