Nitrogen Oxides

Nitrogen Oxides are the pollutants most widely monitored across Kent and Medway. Nitric oxide (NO) is mostly produced by road transport emissions and other combustion processes such as the energy supply. NO is not considered to be harmful to health. However, once released to the atmosphere, NO is usually very rapidly oxidised to nitrogen dioxide (NO2), which is harmful to health. NO2 and NO are both oxides of nitrogen and together are referred to as nitrogen oxides (NOx).

Nitrogen dioxide can irritate the lungs and lower resistance to respiratory infections. Continued or frequent exposure to concentrations that are typically much higher than those normally found in the ambient air may cause increased incidence of acute respiratory illness in children.

Fine Particles

Particulate matter consists of a mixture of particles small enough to be suspended in the air. These airborne particles are measured in a number of different size fractions according to their effective size (referred to as their “median aerodynamic diameter”). Most air quality monitoring measures PM10 and/or PM2.5 (particles with median aerodynamic diameters of 10 or 2.5 microns, or less respectively). A micron is one-thousandth of a millimetre.

Particulate matter is composed of a wide range of materials, natural and human-made, including:

  • combustion sources (mainly road traffic)
  • Non-exhaust emissions from road traffic, e.g. from tyre and break wear, and suspended road dust.
  • secondary particles, mainly sulphate and nitrate formed by chemical reactions in the atmosphere, and often transported from far across Europe
  • coarse particles, suspended soils and dusts (e.g. from the Sahara), sea salt, biological particles such as pollen and particles from construction work

Fine particles in the PM10 fraction are small enough to be inhaled and can travel into our airways. There, they can cause inflammation, and worsen the condition of people with heart and lung diseases. The smaller PM2.5 particles are considered even more harmful, as they can be carried deep into the lungs. These ultrafine particles may carry surface-absorbed toxic, or carcinogenic, compounds into the body.


Ozone (O3) is not emitted directly from any man-made source in any significant quantities. In the lower atmosphere, O3 is primarily formed by a complicated series of chemical reactions initiated by sunlight. These reactions can be summarised as the sunlight-initiated oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides (NOx).

The chemical reactions do not take place instantaneously, but can take hours or days, therefore ozone measured at a particular location may have arisen from VOC and NOx emissions many hundreds or even thousands of miles away. Maximum concentrations, therefore, generally occur downwind of the source areas of the precursor pollutant emissions. Ozone irritates the airways of the lungs, increasing the symptoms of those suffering from asthma and lung diseases.

Volatile Organic Compounds - VOCs

The sources of VOCs are similar to those described for NOx above, but also include other activities such as solvent use, and petrol distribution and handling.

The two most widely monitored compounds are Benzene and 1,3-butadiene. Benzene is a VOC which is a minor constituent of petrol. The main sources of benzene in the atmosphere in Europe are the distribution and combustion of petrol. Of these, combustion by petrol vehicles is the single biggest source (70% of total emissions). 1,3-butadiene, like benzene, is a VOC emitted into the atmosphere principally from fuel combustion of petrol and diesel vehicles. It is also an important chemical in certain industrial processes.

Possible chronic health effects of VOCs include cancer, central nervous system disorders, liver and kidney damage, reproductive disorders, and birth defects. They are also a key contributor to the formation of Ozone.

Sulphur Dioxide

Sulphur dioxide (SO2) is produced when a fuel or material containing sulphur is burned. Globally, much of the sulphur dioxide in the atmosphere comes from natural sources such as volcanos, but in the UK the predominant source is power stations burning fossil fuels. Widespread domestic use of coal can also lead to high local concentrations of SO2. SO2 is also damaging to the environment; it contributes to acid rain and can react with other pollutants to form fine particulate matter.

Even moderate concentrations may result in a fall in lung function in asthmatics. Tightness in the chest and coughing occur at high levels, and lung function of asthmatics may be impaired to the extent that medical help is required. Sulphur dioxide pollution is considered more harmful when particulate and other pollution concentrations are high.

Carbon Monoxide

Carbon monoxide (CO) is a colourless, odourless poisonous gas, produced when a fuel containing carbon is burned, without sufficient oxygen to complete the combustion process and turn it into carbon dioxide (CO2). This can happen in many situations, including petrol vehicle engines, industrial processes, and domestic heating systems.

The role of petrol engines as a source has decreased in recent decades as they have become more efficient, the most important source is now residential fuel use. Which is why you are likely to have a CO detector in your home. The gas prevents the normal transport of oxygen by the blood. In high enough concentrations, this can lead to a significant reduction in the supply of oxygen to the heart, particularly in people suffering from heart disease.

Carbon Dioxide

Carbon dioxide (CO2) is produced when a material, or fuel, containing carbon is burned. Globally, much of the carbon dioxide in the atmosphere comes from natural sources, but increasingly humans are adding carbon dioxide to the atmosphere through the combustion of fossil fuels for energy.

CO2 displaces oxygen in the air and in confined spaces can lead to shortness of breath and headaches. CO2 is also an important greenhouse gas, contributing to climate change. It can also cause acid rain, when it interacts with moisture in the atmosphere to form a carbonic acid.


Ammonia is a gas that is mostly released into the air by farm activities. These include the use of fertilisers and farm animals’ waste. Around 85-90% of the UK’s ammonia emissions come from agriculture. Ammonia contributes to air pollution because it can react with other pollutants (the oxides of nitrogen and sulphur) to produce fine particles of ammonium nitrate and ammonium sulphate. On some days, especially in spring and summer, a substantial proportion of the particulate pollution (PM10 and PM2.5) is made up of this type of particulate matter.

Toxic Organic Micro-Pollutants - TOMPS:

TOMPs are produced by the incomplete combustion of fuels. They comprise a complex range of chemicals some of which, although they are emitted in very small quantities, are highly toxic or carcinogenic. Compounds in this category include:

  • PAHs (PolyAromatic Hydrocarbons)
  • PCBs (PolyChlorinated Biphenyls)
  • Dioxins
  • Furans

TOMPS can causing a wide range of effects, from cancer to reduced immunity to nervous system disorders and interfere with child development. There is no "threshold" dose - the tiniest amount can cause damage.

Lead and Heavy Metals

Since the introduction of unleaded petrol in the UK there has been a significant reduction in urban lead levels. In recent years industry, particularly secondary non-ferrous metal smelters, have become the most significant contributors to emissions of lead. The highest concentrations of lead and heavy metals are now therefore found around these installations in industrial areas.

Even small amounts of lead can be harmful, especially to infants and young children. In addition, lead taken in by the mother can interfere with the health of the unborn child. Exposure has also been linked to impaired mental function, visual-motor performance and neurological damage in children, and memory and attention span.