Pollutants of concern
Nitrogen dioxide
Nitrogen dioxide in York is the result of NOx emissions from a variety of different sources, the main ones being:
- Transport related emissions, arising mainly from road transport but also including a small contribution from rail. This is the major threat to clean air in York.
- Emissions from large industrial chimney stacks.
- Emissions from domestic and commercial space heating and any other local source emissions.
Nitrogen dioxide (NO2) is one of two gases which together are
referred to as nitrogen oxides (NOx). 
The other member of the
group is nitric oxide (NO). Oxides of nitrogen are formed whenever combustion
takes place. During the combustion process the majority of the oxides of nitrogen
(NOx) released are in the form of nitric oxide (NO). Once released into the atmosphere,
nitric oxide (NO) can react with oxygen (O2) and ozone (O3) to produce nitrogen dioxide (NO2).
As the majority of nitrogen dioxide in the atmosphere has been formed from nitric oxide it is
often referred to as a 'secondary' pollutant.
Nitrogen dioxide concentrations are usually measured in parts per billion (ppb) or mg/m3 (micro-grammes per metre cubed). 1 ppb is equivalent to 1.91 ug/m3. 1 mg = 10-9 grammes.
Nitrogen dioxide is a brown gas which acts as an irritant to the eyes, nose, throat, and respiratory tract. Nitrogen dioxide can have both short term 'acute' effects and long term 'chronic' effects. The short term 'acute' effects of nitrogen dioxide involve irritation of the eyes, nose and throat and an increase the symptoms of existing respiratory conditions such as asthma, bronchitis or emphysema. Based on current medical evidence the short term concentrations of nitrogen dioxide found in York are unlikely to give rise to acute health impacts, even in the most vulnerable members of society.
The long term 'chronic' effects of nitrogen dioxide are associated with a gradual deterioration in the health of people who are already suffering from lung diseases, and an increased susceptibility to respiratory infections within the general population. In York the annual average nitrogen dioxide objective has been found to be currently exceeded at a number of locations around the inner ring road. There are also a number of locations along the radial routes into the city where concentrations are approaching the annual average objective level.