NOx is a term used to refer to nitrogen oxides. The most common NOx related to air pollution are nitrogen oxides (NO) and nitrogen dioxides (NO2). NOx formation occurs when nitrogen and oxygen react with each other during combustion of fuels, oil, gas and organic matter. NOx can cause serious damage to human and animal health particularly through respiratory diseases. NOx is responsible for smog and poor air quality in cities and form ozone when reacting with volatile organic compounds (VOCs) in the presence of light resulting in damage to ecosystems, animals and plant life.
Nitrogen dioxide (NO2) at high concentration causes inflammation of the airways resulting in respiratory disease, coughing, flu and bronchitis. Asthma sufferers are prone to have most intense attacks. Prolonged exposure can cause irreversible damage to the respiratory system.
Although nitric oxide (NO) is not considered to be hazardous to health at ambient condition, excess NO may cause respiratory ailments, hematologic side effects, metabolic disorders and other health issues. Furthermore, NO is oxidised by ozone in the atmosphere to give NO2. Ozone is formed in the ground-level by VOCs through the burning of fossil fuels and released from vegetation. Air containing ozone in cities mixes with NO forming NO2 (secondary).