Diesel exhaust contains nitrogen oxides and volatile organic compounds that combine to create ground-level ozone, a major component of smog. Diesel smoke also contains toxic compounds and fine particles that lodge in human lungs and have been linked to several health problems.

By contrast, natural gas-burning engines emit almost no toxic chemicals and lower levels of nitrogen oxides, though they do produce higher levels of carbon dioxide and carbon monoxide.

Smith [diesel supporter] said the environmental differences between the two are negligible and the damaging effects of compressed natural gas buses, also known as CNG, aren’t subject to the same scrutiny by the Environmental Protection Agency.

“The interesting thing about CNG is that they emit a bevy of chemical pollutants not cited by the EPA and not regulated,” said Smith, who added that Metro’s 1,422 buses cause so little of the region’s pollution that it doesn’t make sense to buy more expensive models.

While I think it’s great that we’re having debates over which type of transit vehicle to buy from an environmental point of view, I don’t have a strong sense that a great deal of analysis went into this argument.