Urban air pollution received considerable attention in the late 20th century as a dominant health threat in many urban areas.  Motor vehicles spew out a witches brew of obnoxious and toxic compounds that adversely affect human health including volatile organic compounds (VOCs), nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM) and other byproducts from burning gasoline and diesel fuel in conventional cars. VOCs and NOx combine in the atmosphere in the presence of sunlight to create low level ozone, a major constituent of unhealthy smog.

While the threats of climate change and wars over oil have dominated the public dialogue in recent years, urban air pollution remains a serious and growing health threat in many urban centers around the globe.  As with GHGs and oil consumption, society must find an alternative to burning gasoline and diesel fuel in the internal combustion engine.