Nitrogen oxides (NOx) are a group of gases made up of oxygen and nitrogen molecules produced from combustion processes. One of the most common nitrogen oxides is nitrogen dioxide (NO2) which has an unpleasant smell and is poisonous in high concentrations.
Nitrogen dioxide at higher temperatures is a reddish-brown and is an air pollutant that contributes to the formation of photochemical smog, which can have a significant impact on human health. The smog results predominantly from the burning of fossil fuels such as coal, oil and gas and as a result of exhaust fumes from cars, trucks, buses, powerplants, and off-road equipment. Indoors, exposure could arise from cigarette smoke, butane, and kerosene heaters and stoves.
Nitrogen dioxide is an important atmospheric trace gas, not only because of its human health effects but also because as it absorbs visible solar radiation and contributes to impaired atmospheric visibility; as an absorber of visible radiation it could have a potential direct role in global climate change if its concentrations were to become high enough.
The main effect of breathing in raised levels of nitrogen dioxide is the increased likelihood of respiratory problems. Nitrogen dioxide inflames the lining of the lungs, and it can reduce immunity to lung infections. This can cause wheezing, coughing, colds, flu and bronchitis. Increased levels can have significant impact on people, specifically children, with asthma, as well as older people with heart disease.
When side effects do occur, they often happen as a result of inhaling too much of the gas or inhaling too fast. Common short-term effects include sweating, shivering, nausea, vomiting, dizziness and fatigue.
The Clean Air Act required the EPA to set national ambient air quality standards for nitrogen oxides as one of the six criteria pollutants. The National Ambient Air Quality Standard (NAAQS) for nitrogen oxides are currently set using nitrogen dioxide as the indicator of the large group.
The law also required the EPA to frequently review the standard and revise them if appropriate to ensure that they provide the requisite amount of health and environmental protection and to update those standards as necessary.