Most experts warn against ozone, which results from the sun heating hydrocarbons and nitrogen oxides in vehicle and industrial emissions.

CNN: Research finds smog could deal a blow to children's health

In the 1980s nitrogen controls were placed on industrial plant and vehicles, This has led to a cut in emissions of 30%, despite an increase in traffic and economic activity.

BBC: Nitrogen pollution 'costs EU up to ?280bn a year'

The fact that people have used industrial chemistry to short-circuit the nitrogen cycle, by making fertilisers out of nitrogen in the air at a rate which greatly exceeds what natural systems can manage, has real environmental effects.

ECONOMIST: The global environment

The subsequent industrial production of billions of tonnes of synthetic nitrogen fertilisers - particular after the 1950s - have flooded terrestrial and aquatic ecosystems with reactive nitrogen, and supported the century's human population explosion.

BBC: Leaving our mark: Fossils of the future

Air Products sells gasses like nitrogen, hydrogen, oxygen and helium used in industrial and medical capacities, while Airgas sells gasses as well as wielding products and safety equipment.

FORBES

The residents, for instance, of Kafue, an industrial town that once bustled with workers at its textile and nitrogen factories, now talk of unemployment, the cost of sending children to school and stinking garbage.

ECONOMIST: A victory for rigging and a split opposition

When teachers guide students in gaining awareness that industrial agriculture writ large, for example, results in chemical runoff from nitrogen-based fertiliers in the Mississippi River that in turn creates a massive dead zone in the Gulf of Mexico so large that it is visible to the human eye through satellite distant imaging, students are able to grasp the basis for sustainable agriculture.

FORBES: Daniel Goleman on the Importance of Ecological Intelligence

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