Comment on “Invited Perspective: The ${\mathrm{NO}}_2$ and Mortality Dilemma Solved? Almost There!”

In their “Invited Perspective” commentary on “The ${\mathrm{NO}}_2$ 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 (${\mathrm{NO}}_2$) and mortality, and they noted also that ${\mathrm{NO}}_2$ 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 ${\mathrm{NO}}_2$. Forastiere and Peters argued that because “combustion-related air pollution mixtures were quite different between study populations … even if other important copollutants of ${\mathrm{NO}}_2$ were not assessed … it is unlikely that the observed associations of ${\mathrm{NO}}_2$ and mortality are solely attributable to one of those copollutants.”¹ The implication seems to be that ${\mathrm{NO}}_2$ 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 ${\mathrm{NO}}_2$ 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 ${\mathrm{NO}}_2$ 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 ${\mathrm{NO}}_2$ comes from various sources, road traffic—and primarily diesel vehicles—is the most important ${\mathrm{NO}}_2$ source in urban settings, where most people live. The higher estimated mortality risks of ${\mathrm{NO}}_2$ in Europe could potentially be attributed to the greater proportion of ${\mathrm{NO}}_2$ coming from diesel engines vs. other sources. Given the multiple noxious substances emitted by diesel engines,⁶ one might infer that ${\mathrm{NO}}_2$ 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 ${\mathrm{NO}}_2$ capture technology to diesel engines, leaving other combustion products untreated? This approach would be unwise because “the ${\mathrm{NO}}_2$ 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 ${\mathrm{NO}}_2$ emissions by trapping technology.

Refers to 10.1289/EHP10286