What ails Modesto is what ails Bakersfield (No. 1), Hanford (No. 3), Fresno (No. 4) and other small Central Valley cities (the Valley accounts for all the California cites on the list aside from Los Angeles): heavy doses of nitrogen oxide and hydrocarbons stemming from gasoline, chemical processing, paints and other industrial byproducts that harm the ozone and release particle pollution into the air.

FORBES: The Worst Cities In America For Your Lungs

The Anthropocene's equivalent of iridium is nitrogen - although there will be other candidates (such as heavy metals, and carbon isotope ratios characteristic of fossil fuel emissions) for geochemists of the far future to identify as the chemical signature of the new epoch's dawn.

BBC: Leaving our mark: Fossils of the future