Nitrogen Dioxide

Houston Levels of Nitrogen Dioxide Remain Well Below EPA’s Health Based Standard

What is Nitrogen Dioxide?

Nitrogen dioxide (NO2) is one of a group of reactive gasses known as “oxides of nitrogen,” or “nitrogen oxides (NOx).”   Other nitrogen oxides include nitrous acid and nitric acid, but the U.S. EPA has established health-based standards for only NO2. NO2 is also significant because it contributes to the formation of ground-level ozone and fine particle pollution. NO2 is linked with a number of negative health effects on the respiratory system, including airway inflammation in healthy people and increased respiratory symptoms in people with asthma. NO2 levels can be higher on or near streets and highways, so people who live and work near major roadways may be at more risk from NO2’s health effects.

Where is it Found?

NO2 is a byproduct of combustion. Some of the major sources of NO2 are cars, trucks and buses, power plants, and off-road engines such as construction equipment and aircraft. Wildfires and outdoor burning also produce NO2.

EPA first set standards for NO2 in 1971, at 53 parts per billion (ppb), averaged annually. In January 2010, EPA established an additional one-hour average standard of 100 ppb. Houston meets both of these standards, as shown in the chart below.