Here is an explanation of some of the terms used on this site.


Biofuels are an alternative to traditional 'fossil fuels'. They're produced from renewable resources and are becoming available at the fuel pump. Check with the manufacturer to see whether your car is compatible.

Carbon dioxide (CO2)

It's widely believed that CO2 is the main greenhouse gas contributing to climate change. Fuel combustion in the engine of your car emits this gas. The amount of gas emitted is directly related to the amount of fuel used. So the more fuel-efficient your car is, the less CO2 it will produce.

Carbon monoxide (CO)

Carbon monoxide is a colourless, tasteless and odourless gas that is highly toxic to humans and animals in high quantities. Cars create carbon monoxide through the incomplete combustion of 'fossil' fuels.


Combustion means burning. In the engine of a car, fuel is burned to gain power. Unfortunately, air pollutants are produced at the same time as a by-product of this process.

Electric cars

These cars run on electricity from an on-board battery pack. They can be charged at home or at a limited number of special charging stations in large cities. Although they have limited range at present, it is plenty for day to day use in urban areas.

Fossil fuels

The term 'fossil fuels' refers to traditional fuel types such as coal, oil and gas; when we mention fossil fuels on this site, we specifically refer to petrol or diesel. These types of fuels are called 'fossil' fuels because it's believed they've formed from the organic remains of prehistoric animals and plants. These fuels are also mixtures of hydrocarbons.


A 'hybrid' is a car that can switch between electric battery power and conventional 'fossil fuels'.


Most cars run on hydrocarbon based fuels such as petrol and diesel. Air pollution occurs when unburned or partially burned fuel is emitted from the exhaust. It also occurs when fuel evaporates into the atmosphere. If hydrocarbons react with nitrous oxides in sunlight, they create ground level ozone, a key contributor to smog.

Nitrous oxides (NOx)

Nitrous oxides form as a direct result of nitrogen and oxygen gases reacting together in the air during combustion. These air pollutants are found in vehicle exhausts but seem to be more prevalent in the emissions of vehicles that run on diesel.

Particulate matter (PM10)

Particulate matter refers to tiny solid or liquid particles in the air - some of which contribute to air pollution. PM10 specifically refers to particles less than 10 microns in diameter (this is smaller than the diameter of a strand of human hair). But while they're tiny in size, they can be harmful to humans; PM10 has been linked to serious health effects. Vehicle exhaust emissions can be a significant source of PM10.