In this post, we discuss everything you need to know about NOx air pollution – what it is and what you can do about it.
Nitrogen is an inert gas making up 79% of the atmosphere. It can react with oxygen to form several different oxides, of which the most abundant are nitric oxide (NO) and nitrogen dioxide (NO2). Together, these are referred to as NOx when talking about air pollution.
The main source of NOx is combustion - i.e. burning - in electric power plants, motor vehicles (both petrol and diesel), the manufacturing industry, central gas heating and gas cookers.
A recent report from the London Assembly Environment Committee showed that central gas heating is a leading cause of NOx emissions in London, contributing 16% to the total. Buses in the centre of the city also contributed 16% of NOx emissions, while diesel cars accounted for 11% and petrol cars 7%.
NO formed by the combination of nitrogen and oxygen during combustion is not harmful in itself, however, when it combines with more oxygen to form NO2 it is.
NO2 is not only harmful in itself, but it reacts, in the presence of air and ultraviolet light in sunlight, to form ozone and NO. This so-called ‘tropospheric ozone’ is, in itself, a pollutant with significant negative health impacts and the primary constituent of the photochemical smog formed under certain weather conditions. Meanwhile, the NO thus formed reacts further in the presence of volatile organic compounds (VOCs) to form more NO2, thereby increasing pollutant levels still further. The reaction continues until the VOCs have been reduced to molecules that are no longer photoreactive.
Limits on NO2 (taken as a ‘marker’ of NOx pollution) set by the European Union, and applicable in the UK, are as follows:
• A one-hour mean value of 200µg/m3 must not be exceeded on more than 18 occasions in one year.
• The annual mean limit of NOx is set at 40µg/m3.
You can check daily levels of NO2 by looking at the Daily Air Quality Index (DAQI) published by the Department of Food, Environment and Rural Affairs.
The reason why the upper limit - which has been breached on many occasions by the UK - is set, is that NO2 has been associated with adverse health effects. These include increased hospital admission for various diagnoses, reduced lung function, increases in respiratory symptoms, response to allergens, airway irritation, asthma, cancer, adverse birth outcomes, and mortality. However, it is not clear whether these effects are caused by exposure to NO2 itself or to some accompanying air pollutant, such as particulate matter.
The Committee on the Medical Effects of Air Pollution recently reviewed the evidence for the adverse health effects of NOx and concluded that:
• Short-term exposure to NO2 likely causes respiratory ill-health; may cause an increase in hospital admissions for heart problems; may cause an increase in overall mortality.
• Long-term exposure to NO2 may cause an increase in all-cause, respiratory and cardiovascular mortality, children's respiratory symptoms and decreased lung function.
In conclusion, the evidence for exposure to NOx and a range of adverse health effects has increased in previous years, although hard evidence on causality is still lacking.
Diesel and petrol cars do contribute a significant proportion of NOx pollution, and NOx can enter an indoor environment from outdoors through windows, doors, cracks and so on. Furthermore, there can be active sources of NOx pollution in the domestic or office environment. Gas cookers, gas central heating, paraffin heaters, and tobacco smoke can create NOx - particularly if they are unvented, inefficient or poorly maintained.
What is often misunderstood, however, is that NOx air pollution is primarily an outdoor air pollution problem. In indoor environments that do not contain active sources of NOx, levels of NOx will be significantly less than level outdoors and might not be found at all. The reason for this is that NOx oxidizes - straight away – with a variety of different chemicals. It oxidizes when VOCs are present indoors, formaldehyde breaks it down, N0, N2O5 and many other oxidants. When a window is opened, NOx comes into your home again of course, but over a period of 10 or 15 minutes, NOx will most likely dissipate.
Outdoor NOX Pollution.
• Check the DAQI and plan activities accordingly.
• Lobby policymakers on traffic congestion control at the local/national level by joining a campaign group.
For Indoor NOX Pollution.
• Consider switching from a gas to an electric cooker.
• Install an exhaust fan over a gas cooker to vent emissions to the outside.
• If you have central gas heating, make sure it is properly installed, inspected and properly maintained, to prevent leaks.
• To take NOX out of the air indoors, use a high-quality air purifier that uses lots of activated granular carbon; such as the IQAir GC MultiGas or the IQAir HealthPro 250.