The principal air-quality pollutant emissions from petrol, diesel and alternative fuel engines are carbon monoxide, oxides of nitrogen, un-burnt hydrocarbons and particulate matter.  It is emissions of these pollutants that are regulated by the Euro emissions standards.  Modern cars, if kept in good condition, produce only quite small quantities of the air quality pollutants, but the emissions from large numbers of cars add to a significant air quality problem.  

Carbon monoxide, oxides of nitrogen, and un-burnt hydrocarbons are gases, and are generally invisible.  Particulate matter is usually invisible although under certain operating conditions diesel engines will 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. Fine particles can also be produced by tyre and brake wear. Unlike emissions of CO₂, emissions of the air quality pollutants are not directly linked to fuel consumption. Pollutant emission levels depend more on vehicle technology and the state of maintenance of the vehicle. Other factors, such as driving style, driving conditions and ambient temperature also affect them. However, as a starting point, all new passenger cars must meet minimum EU emissions standards.

The effects of these exhaust gases are described in more detail below:

CO - Carbon monoxide reduces the blood?s oxygen carrying capacity which can reduce the 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.

NOx - 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?).

Particulate matter (PM) - Fine particles have an adverse effect on human health, particularly among those with existing respiratory disorders. Particulate matter is 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.

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

The European Union Ambient Air Quality Directive sets maximum permissible levels for atmospheric concentrations of pollutants thought to be harmful to human health and the Government is committed to working towards full compliance with these standards. Achieving the air quality standards for nitrogen dioxide and fine particles presents the greatest challenge, especially in urban areas and close to busy roads.

Emissions of these air quality pollutants from road vehicles 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 quantity of air quality pollutant emissions per kilometre as a vehicle made in 1970. Over the last twenty years increasingly stringent emission limits have been set at a European level, starting with the "Euro1" limits in 1993. All new cars currently have to meet the Euro 5 standard and all models sold have had to meet that standard from 1st January 2011.