Nitrogen dioxide is a colourless and odourless gas produced mainly via combustion, sources include vehicle exhausts, gas appliances, industrial heating and heat treatment and power stations. Emissions arise in the form of NOx, a mixture of nitric oxide (NO) and nitrogen dioxide (NO2). NO2 is also formed in the atmosphere as a result of chemical reactions, influenced by sunlight. Nitrogen dioxide levels also influence the increase in ground level ozone formation.
Healthy people will not be troubled by nitrogen dioxide, even during times where levels are high. For persons with breathing difficulties exposure to NO2 can affect lung function and lead to the airway being more sensitive. Asthmatics are at a small risk of being affected if they exercise during times of high pollution levels. However, some research has found no change in lung function even at high concentrations. It is extremely unlikely that outdoor concentrations normally experienced in South Yorkshire will reach such high concentrations (575 micrograms of nitrogen dioxide per cubic meter of air). Some reports also state that asthmatics could be more sensitive to irritant chemicals and allergens, such as house dust mite and pollen, after being exposed to NO2.