Advice – The environment – Pollutants

Learn about your emissions

Most vehicles use a combustion engine to provide motive power. In use, within a combustion engine, chemical reactions take place between the hydrocarbons of the fossil fuel, the fuel additives and the gases that naturally occur in the atmosphere. These processes include oxidation of the fuel, which produces carbon dioxide (CO2) and carbon monoxide (CO). Nitrogen from the air is also oxidised to nitrogen oxides (NOx). Partially burnt and unburned fuel is present in the exhaust gases forming a complex cocktail of hydrocarbons (HCs) such as methane (CH4) and other volatile organic compounds (VOCs) including benzene and 1,3-butadiene. Particulate matter (PM) is also produced and is especially prevalent in diesel exhaust. Some pollutants are also produced away from the vehicle – for example, ground-level (tropospheric) ozone (O3) is formed by the chemical action of sunlight on emitted VOCs.

Carbon Dioxide (CO2) – While carbon dioxide is non-toxic, its main environmental effect is as a greenhouse gas. Each year an estimated 30 billion tonnes of carbon dioxide are emitted due to human activity, 2% of which originates from the United Kingdom.

To illustrate the scale of the impact of these emissions as a result of human activities, the atmospheric concentration of carbon dioxide (from all sources) has increased by 31% since 1750. The present concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The current rate of increase is unprecedented during at least the past 20,000 years. Over the last two decades, about three-quarters of the anthropogenic emissions of carbon dioxide have been a result of burning of fossil fuels, the rest being predominantly due to land-use change (eg deforestation).

By enhancing the greenhouse effect, greenhouse gas emissions are leading to increases of the Earth’s atmospheric, land and sea temperatures. During the 20th century the global average surface temperature (the average of near surface air temperature over land and sea surface temperature) increased by 0.6 (+/-0.2)°C. This temperature is predicted to increase by 1.4-5.8°C by 2100 (1990 baseline). Based on palaeo-climate data, the projected rate of warming is very likely to be without precedent during at least the last 10,000 years. The concomitant rises in sea levels and resulting climatic change will be of great (and as yet unknown) significance to all patterns of life on Earth.

Carbon Monoxide (CO) – Produced during the incomplete combustion of carbon compounds such as fossil fuels, this gas is known to be deleterious to human health. During respiration it readily combines with haemoglobin in the blood thus hindering the body’s ability to take up oxygen. It is thought therefore to aggravate respiratory and heart disease.

Carbon monoxide also contributes to global warming to a small degree. This it does indirectly after first taking part in chemical reactions within the atmosphere. One such reaction would be with oxygen, forming carbon dioxide and thus contributing to the enhanced greenhouse effect.

Nitrogen Oxides (NOx) – As a result of the high temperatures occurring during combustion, nitrogen combines with oxygen from the air forming oxides of nitrogen (NO, NO2, N2O etc.). These gases are known to be responsible for acid deposition via the formation of nitric acid. Nitrogen dioxide (NO2) is toxic even in small concentrations and is known to cause and aggravate human respiratory diseases. Nitrous oxide (N2O) also contributes directly to global warming and is responsible for around 7% of the enhanced greenhouse effect.

Particulates (PMs) – Particulates, commonly known as ‘black smoke’, are fine particles produced by incomplete combustion, the burning of lubrication oil and by the presence of impurities within the fuel. Typically with a dimension of the order of 10 microns or less (known as ‘PM10’), they are known to cause and aggravate human respiratory diseases and are thought to be carcinogenic. The World Health Organisation has issued a report stating that there are no concentrations of airborne micro-sized particulate matter that are not hazardous to human health.

Volatile Organic Compounds (VOCs) – Volatile organic compounds consist of a number of different chemicals including hydrocarbons (eg methane), which are released during the production, refining, storage and combustion of fossil fuels. The largest environmental risks of VOCs are due to the presence of benzene and 1,3-butadiene, which are both carcinogens and are easily inhaled due to their volatile nature. Other chemicals in this category are responsible for the production of tropospheric ozone, which is toxic even in low concentrations.

Methane is a significant greenhouse gas and is released during the drilling for oil and gas and during the combustion of petroleum products. Around 5% of methane emissions are due to the production and use of fuels used for road transport.

Tropospheric Ozone (O3) – In the stratosphere, ozone absorbs ultraviolet light, therefore reducing the number of harmful rays reaching living organisms at the Earth’s surface. However, at ground level (the troposphere), ozone is toxic to animals and plants. Ozone is thought to be responsible for aggravating human respiratory disease and is known to reduce crop yields.

While the concentration of stratospheric ozone is being depleted by the action of chlorofluorocarbons and other chemicals, exhaust emissions from road vehicles are increasing the concentration of ozone at ground level. Although there are a number of sources of man-made tropospheric ozone, transport is known to be a major contributor of emissions through the action of sunlight on emitted VOCs.

Lead (Pb) – Lead is known to affect the mental development of young children and is known to be toxic. It was originally introduced into petroleum products as an ?anti-knock? additive to improve combustion in a spark-ignition (petrol) engine. At its peak, road transport was responsible for three quarters of airborne lead in the UK. However, due to the introduction of unleaded petrol and the elimination of leaded fuels in Europe in 2000, the amount of lead emitted has fallen by over 80%.

References: Motor Vehicle Pollution, Reduction Strategies Beyond 2010. Organisation for Economic Co-operation and Development, Paris; Holgate, S. Quantification of the Effects of Air Pollution on Health in the United Kingdom. The Stationery Office, London, 1998; One in three child deaths in Europe due to environment. World Health Organisation, June 2004. Available online: http://www.euro.who.int/mediacentre/PR/2004/20040617_1; Environmental Impacts of Road Vehicles in Use. Cleaner Vehicles Task Force, Department of the Environment, Transport and the Regions, 1999, The Stationery Office, London; Climate Change UK Draft Programme, 2000. Department of the Environment, Transport and the Regions. The Stationery Office, London.