Cars and Air Pollution

Aside from Carbon Dioxide (CO2) the other pollutants from petrol, diesel and alternative fuel engines are mainly Carbon Monoxide, Oxides of Nitrogen, un-burnt Hydrocarbons and fine particles.  The first three are gases and are invisible. Fine particles are usually invisible although in certain operating conditions diesel engines may produce visible particles, appearing as smoke. Petrol engines will also produce visible particles if they are burning engine oil or running rich, for example, following a cold start. Unlike CO2 , emissions of these pollutants are not directly linked to fuel consumption. Pollutant levels are more dependent on vehicle technology and the state of maintenance of the vehicle. Other factors, such as driving style, driving conditions and ambient temperature also affect emission of pollutants. However, as a starting point new passenger cars must meet minimum EU emissions standards. Cars which are subject to the NCT must also meet prescribed emission levels.

The main exhaust gas pollutants and their effects are described in more detail below:


Carbon Dioxide is the most important of the greenhouse gases which are contributing to Climate Change. Compared to improvements in the emissions of toxic pollutants, there has been less progress on reducing CO2 from cars. For a given type of fuel the CO2 emissions of a car are directly proportional to the quantity of fuel consumed. There is no easy technical way to deal with CO2. The best way to reduce it and the other emissions is to use the car only when it is necessary and to walk or use public transport where possible.


Carbon Monoxide reduces the blood’s Oxygen carrying capacity which can reduce availability of Oxygen to key organs. Extreme levels of exposure, such as might occur due to blocked flues in domestic boilers, can be fatal. At lower concentrations CO may pose a health risk, particularly to those suffering from heart disease.


Oxides of Nitrogen react in the atmosphere to form Nitrogen Dioxide (NO2 ) which can have adverse effects on health, particularly among people with respiratory illness. High levels of exposure have been linked with increased hospital admissions due to respiratory problems, while long term exposure may affect lung function and increase the response to allergens in sensitive people. NOx also contributes to smog formation, acid rain, can damage vegetation, contributes to ground level Ozone formation and can react in the atmosphere to form fine particles (‘secondary particles’).


Fine particles can have an adverse effect on human health, particularly among those with existing respiratory disorders. Particles have been associated with increased hospital admissions due to respiratory and cardiovascular problems, bringing forward the deaths of those suffering from respiratory illnesses and a reduction in life expectancy.


Hydrocarbons, contribute to ground level Ozone formation leading to risk of damage to the human respiratory system. In addition, some kinds of HCs are carcinogenic and they are also indirect greenhouse gases.

Emissions of the above pollutants are being reduced by improving the quality of fuels and by setting increasingly stringent emission limits for new vehicles. As an example it would take 50 new cars to produce the same emissions per kilometre as a vehicle made in 1970. For the last twenty years emission limits have been set at a European level and are quoted in grams of pollutant per kilometre travelled.

Fuel use facts:

  • One litre of petrol used results in 2.36kg of CO2 emitted to the atmosphere
  • One litre of diesel used results in 2.68kg of CO2 emitted to the atmosphere
  • One litre of E85 blend Bioethanol used results in approx 1.04kg of CO2 emitted to the atmosphere (feedstock dependent)

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