Nitrogen oxides are emitted from high temperature combustion sources such as autos, trucks, aircraft and from boilers used to provide heat, steam or electricity.

Since air is made up of almost 80 percent nitrogen, when high temperature burning occurs, some of the nitrogen in the air is burned to release nitric oxide or nitrogen dioxide gas. These gases can form a reddish-brown haze over urban areas or areas near large emitters.

Airborne nitrates reduce visibility, contribute to acid rain, play a major role in the formation of ozone smog or react with other chemicals to form particulate matter. These particles can fall to earth in rain or snow to increase nitrogen levels in soils and water bodies. Nitrates deposited into water contribute to algae blooms that can cause depleted oxygen. For example, a significant portion of nitrogen that enters the Chesapeake Bay comes not from surface runoff or water discharges, but from airborne nitrates. What percent of nitrates in Iowa water originated from airborne deposition is currently unknown.

Nitrogen oxides have gone unmonitored in Iowa since the 1980s. The state renewed monitoring in 2000.