Comment on "Invited Perspective: The NO2 and Mortality Dilemma Solved? Almost There!"

In their “Invited Perspective” commentary on “The and Mortality Dilemma Solved? Almost There!”[1] Forastiere and Peters suggest that the important question as to whether nitrogen oxides are toxic per se or are markers of more complex and toxic emissions has been “almost” solved; the suggestion is explicit in their title and somewhat more nuanced in their text.

The study by Qian et al.[2] that Forastiere and Peters examined appeared in the same issue of Environmental Health Perspectives as their perspective. As Forastiere and Peters noted,[1] this retrospective registry-based study in the southeastern United States showed a direct link between atmospheric nitrogen dioxide () and mortality, and they noted also that had an independent effect after controlling for fine particulate matter (PM) and ozone. Qian et al.[2] also compared results from studies conducted in different countries, all of which showed various adverse effects of . Forastiere and Peters argued that because “combustion-related air pollution mixtures were quite different between study populations … even if other important copollutants of were not assessed … it is unlikely that the observed associations of and mortality are solely attributable to one of those copollutants.”[1] The implication seems to be that acts by itself and is not a marker for a more complex exposure.

However, the situation is more complex than that. First, as Forastiere and Peters point out, “there is [only] limited experimental evidence … for an independent biological response that might result in the observed mortality impacts.”[1]

More important, in a meta-analysis of long-term studies, Faustini et al.[3] estimated that risks associated with were almost twice as high in Europe as those in North America. In Europe, over 50% of circulating vehicles are powered by diesel fuel,[4] whereas in the United States approximately 95% of passenger cars and light-duty trucks are powered by gasoline.[5] Furthermore, although comes from various sources, road traffic—and primarily diesel vehicles—is the most important source in urban settings, where most people live. The higher estimated mortality risks of in Europe could potentially be attributed to the greater proportion of coming from diesel engines vs. other sources. Given the multiple noxious substances emitted by diesel engines,[6] one might infer that alone is not responsible for these effects but is a marker of diesel exhaust.

That inference raises an important question: Would it be enough to install capture technology to diesel engines, leaving other combustion products untreated? This approach would be unwise because “the and mortality dilemma” is far for being solved. It would be more prudent to limit the access of diesel cars and light-duty vehicles to urban environments, even though manufacturers may soon be able to reduce emissions by trapping technology.