Air quality, climate change and health effects

Although transport related pollutant emissions and greenhouse gases in Europe have substantially reduced over the past 30 years. Air quality and climate change are still key in EU and beyond.

Air quality in specific has improved largely due to the introduction of catalytic convertors and filters.

What causes transport related air pollution and greenhouse gases?

If we could burn gasoline or diesel perfectly in pure oxygen it would produce only carbon dioxide (CO2), water vapour, and energy.
However, in reality there are always some emissions of unburned and partially burned fuel, producing:


carbon monoxide (CO)


hydrocarbons (HC)


Particulate matter (PM)


nitrogen oxides (NOx) formed from nitrogen present in the air.


Other potential pollutants:

Ammonia (NH3)


Other species:



As well as greenhouse gases:

Carbon dioxide (CO2)
Methane (CH4)
Nitrous oxide (N2O)

Motor vehicle contribution to urban air quality and consequent health effects due to these emissions can be reduced by the implementation of exhaust gas aftertreatment technology in road transport. Although kerbside, ‘street canyon’ (streets surrounded by high buildings) or local emissions are of particular concern because the concentration of pollutants is likely to be highest in these situations, effects can also occur away from city or town centres as the pollutants react with each other and are transported by air movement.

Health and climate risks from vehicle pollution


Particulate matter (PM) is mainly soot particulates with volatile hydrocarbons and some sulphate and metallic residues from the fuel and engine lubricant. Particulates are found in the air in a range of sizes.

Old diesel engines not fitted with a particulate filter are responsible for the majority of ultra-fine particulates (less than one micron in diameter or PM1). Particulate emissions from modern Direct Injection gasoline engines are also characterised by a higher number of smaller particulates than traditional port fuel injection systems.

These small particulates (mostly below 100 nanometres diameter) are present in large numbers in untreated exhaust, but amount to only a tiny fraction of the weight of particulate matter. There is evidence that fine and ultra-fine particulates are linked to increased rates of premature death from causes such as cardiovascular and lung disease.

Also, the World Health Organization (WHO) International Agency for Research on Cancer (IARC) has classified untreated Diesel exhaust as carcinogenic to humans. In addition, diesel PM is primarily made of Black Carbon, a short-lived climate pollutant with a high Global Warming potential. In addition, gasoline particulates are also made of carbon.


Carbon monoxide (CO) is a poisonous gas that displaces oxygen from the red blood cells. At high concentrations it is fatal; at lower concentrations, it can exacerbate heart problems.


Hydrocarbons (HC) help form photochemical smog in the atmosphere. Some HC, such as benzene, are known carcinogens.


Nitrogen oxides (NOx) comprise nitric oxide (NO) and nitrogen dioxide (NO2). They react with hydrocarbons in sunlight to form harmful ozone and photochemical smog. NOx, particularly in the form of NO2, can increase respiratory illnesses and are a contributor to acid rain. Ozone causes breathing difficulties and damages plants.


Ammonia is a colourless, reactive gas that is lighter than air. It has a strong smell, similar to urine, which can be detected by most people even in small amounts. Breathing in low levels of ammonia may cause irritation to the eyes, nose and throat. High levels of ammonia may cause burns and swelling in the airways, lung damage and can be fatal.


Carbon dioxide (CO2) is the final product of all combustion processes and the major contributor to the ‘greenhouse’ effect. Catalysts do not affect overall CO2 emissions from cars because all the carbon burnt in the engine eventually ends up as CO2, so CO2 emissions can only be limited by reducing fuel consumption or switching to renewable fuels.

Use of particulate filters or NOx adsorbers can give a small (typically 1 to 2%) increase in CO2 because a small amount of extra fuel is used to regenerate them from time to time. On the other hand, the Selective Catalytic Reduction (SCR) process can reduce fuel consumption and hence CO2 by up to 5% by allowing engine developers to use more fuel-efficient conditions instead of trading fuel consumption for a reduction in combustion NOx emissions.


Methane (CH4) is a potent greenhouse gas (GHG) with a 100-year global warming potential (GWP100) 34 times greater than CO2. Natural gas fuel has generally been considered to produce lower GHG emissions than liquid fossil fuels, because of the lower relative carbon content in the CH4 molecule compared to other fossil fuels. However, the comparison of overall GHG emissions from natural gas and from other fuels depends on the scale of methane losses from natural gas extraction, distribution and utilisation.


Even though chemically it is an oxide of nitrogen, nitrous oxide (N2O) has been excluded from the regulated nitrogen oxides (NOx) emissions (see above) as it has a different type of impact. As a greenhouse gas, N2O has a GWP100 298 times that of CO2.


Lead was, in the past, added to gasoline to boost the octane number. Health concerns focussed on the effect that low levels of ambient lead can have on the educational and behavioural development of children. Lead poisons catalytic converters and, since 2000, sales of leaded gasoline have been banned in the European Union. For the remaining non-catalyst engines (now well over 20 years old) that rely on lead to prevent valve recession, other additives have been introduced.