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In theory, burning any fossil fuel (petrol, diesel, gas, etc.) with air in an engine should produce just carbon dioxide (CO2) and water (H2O). The rest of the exhaust would be the nitrogen (N2) that came in with the air. Unfortunately, the fuels we burn are made up of hundreds of different hydrocarbons that burn in different ways and at different rates. In practice this means that the vehicle exhaust contains some that were partially burned, some that reacted with others and some that reacted with the nitrogen. Many of the by-products of the combustion of fuel produced are toxic, and hence harmful for human health and the environment. Legislation in the EU and elsewhere has put limits to the amounts of pollutants that each car is allowed to emit. This affects the following pollutants: |
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Particulate matter (PM)Particulate matter is a product of incomplete combustion and a complex mixture of both primary and secondary[1]PM. ‘Primary’ PM is the fraction of PM that is emitted directly from the vehicle tailpipe[2] into the atmosphere, whereas ‘secondary’ PM forms in the atmosphere following the release of precursor gases, mainly sulphur dioxide (SO2), nitrogen oxides (NOx), ammonia (NH3) and some volatile organic compounds (VOCs). In terms of its potential to harm human health, PM is one of the most serious pollutants, as it penetrates sensitive regions of the respiratory system and can cause or aggravate cardiovascular and lung diseases, as well as cancer. In addition to exhaust PM, PM is also emitted from tyre and brake wear and tear, as well as road abrasion. In modern vehicles the emissions from these sources can exceed exhaust PM emissions. Currently, these non-exhaust PM emissions are not regulated in the EU. Nitrogen oxides (NOx)Nitrogen oxides constitute a group of different chemicals that are all formed by the reaction of nitrogen – the most abundant gas in air – with oxygen. NOx comprises colourless nitric oxide (NO) and the reddish-brown, very toxic and reactive nitrogen dioxide (NO2). NOx emissions also lead to the subsequent formation of ‘secondary’ PM and ground-level ozone in the atmosphere, and cause harm to the environment by contributing to the acidification and eutrophication of waters and soils. Hydrocarbons (HCs)Hydrocarbons are produced from incomplete combustion and are toxic to human health. HCs, and particularly volatile organic compounds (VOCs), contribute to the formation of ground-level ozone and photochemical smog in the atmosphere. Ozone irritates the eyes, damages the lungs and aggravates respiratory problems. Carbon monoxide (CO)Carbon monoxide is a product of incomplete combustion which occurs when the carbon in the fuel is only partially oxidized, forming CO and not CO2. It is colourless and odourless, but highly toxic. Direct exposure to CO reduces the flow of oxygen in the bloodstream and is particularly dangerous to people with heart disease. Like HCs, CO also contributes to the formation of ground-level ozone and smog. Apart from the above regulated pollutants, there are also pollutants emitted by vehicles that are currently not regulated in the EU. These include certain acidifying pollutants, such as ammonia[3] (NH3) and sulphur dioxide (SO2), although emissions of the latter are indirectly addressed via fuel quality legislation, which limits the amount of sulphur content in fuels. Other non-regulated pollutants include certain carcinogenic and toxic organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), persistent organic pollutants (POPs), dioxins and furans, and heavy metals, such as lead, arsenic, cadmium, copper, chromium, mercury, nickel, selenium and zinc. In addition to the above air pollutants, carbon dioxide, the most important greenhouse gas, is also regulated: Carbon dioxide (CO2) and other GHGs | Anchor |
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Carbon dioxide is the main product of fuel combustion, along with water. CO2 is not an air pollutant because it is not toxic and hence does not directly affect human health. However, it has a significant impact on climate change, hence posing an indirect threat to public health and the environment. Other strong GHGs (currently non-regulated) are methane (CH4) and nitrous oxide (N2O), which both contribute to global warming. In fact, their global warming potential is much higher than CO2, i.e. 25 times higher for CH4 and 265 times higher for N2O (IPCC, 2014). CH4 is of concern when natural gas, biomethane, or synthetic methane are used as fuels, since direct slip is possible from gas operating engines. On the other hand, N2O may be produced by some emissions control devices (Guan et al., 2014; Nevalainen et al., 2018).
[1] Primary pollutants are emitted directly from a source, while secondary pollution is formed as a result of the reaction of primary pollutants in the atmosphere, e.g. in the presence of sunlight, combination with oxygen in the air or even with water. [2] Apart from the vehicle exhaust, PM is also emitted from tyre and brake wear, as well as road abrasion. [3] The inclusion of NH3, as well as the main non-CO2 greenhouse gases (N2O and CH4), is currently considered in the framework of the post Euro 6 regulation process.
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The impact of air pollution to the environment| Column |
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Air pollution can be defined as the presence of pollutants in the atmosphere at levels that harm human health, the environment and cultural heritage (e.g. by damaging buildings, monuments and materials). Identifying the relationship between air pollutant emissions, their concentrations in the air and their subsequent impacts is complex. The quality of the air that each of us breathes depends on many factors, including the mix of emission sources in a given area, the local landscape and meteorology, all of which can affect the formation and the dispersion of the pollutants (EEA, 2016). |
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According to the World Health Organization (WHO), air pollution is one of the biggest environmental health risks in Europe, since there is a clear and increasing evidence of the health impacts that exposure to a whole range of air pollutants can have (WHO, 2021). For example, even low concentrations of particulate matter in the air can lead to health damage, such as reduced lung function, respiratory problems, cardiovascular diseases, resulting in chronic disease and even premature death. Similar problems are caused by inhaling nitrogen oxides. Estimates of the health impacts attributable to exposure to air pollution indicate that PM2.5 concentrations in 2015 were responsible for about 422,000 premature deaths originating from long-term exposure in Europe (over 41 countries). The estimated impacts on the population in these 41 countries of exposure to NO2 in 2015 were around 79,000 premature deaths per year (EEA, 2018). |
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The contribution of road transport to global emissionsGHG emissionsWhile GHG emissions from all other main sectors of the economy have fallen in recent decades, those from transport have increased. Road transport GHGs in 2016 were around 23% above the levels in 1990. As emissions from other sources have decreased, the contribution that road transport makes to total EU CO2 emissions has increased significantly (almost doubled) from 1990, reaching 25% in 2016. In contrast to CO2, the contribution of road transport to other GHGs, i.e., CH4 and N2O is very small (less than 0.2%) (EEA, 2018). Air pollutantsIn contrast to GHG emissions, emissions of the main air pollutants from transport have generally declined over the past two decades. However, the air quality assessment published by the European Environment Agency reveals that a significant percentage of the European urban population was exposed to air pollution levels exceeding EU air quality standards over recent years. For example, the EU annual limit value for NO2, the harmful component of NOx, has been widely exceeded across Europe, mainly at roadside locations. Similarly, a number of Member States reported levels of PM higher than the respective EU air quality standards (EEA, 2018; EEA, 2019). | Section |
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Another important fact is that emission reductions of main air pollutants from road transport have been lower than originally anticipated in some cases. This is partly because transport has grown more than expected and partly because growth in diesel vehicles has been greater than expected. Furthermore, it is widely accepted that ‘real-world emissions’ of NOx, particularly from diesel passenger cars and vans, generally exceeded the permitted European emission (Euro) standards in previous years. As a response, the EC has undertaken a series of actions to address this problem, including the introduction of the mandatory RDE testing procedure, together with the various steps of the Euro 6 emission standard, and other measures (EC, 2019). |
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The percentage contribution of road transport to EU emissions from main source sectors in 2016 was 39% for NOx, 11% for PM2.5, 20% for CO and 9% for NMVOC (EEA, 2018). Emissions are not only released when a vehicle is operated, but throughout its whole life, from the manufacturing of raw material to the vehicle scrapyard. They occur during the extraction of raw materials, the production of vehicle components, the vehicle manufacturing process, the vehicle use, and finally during vehicle dismantling and material recycling. Emissions are also produced during the extraction, processing and transportation of fuels, commonly known as Well-to-Tank (WtT) emissions, as opposed to Tank-to-Wheel (TtW) emissions produced during vehicle use. Life Cycle Assessment (LCA) is a very useful technique for estimating emissions over the entire lifetime of a vehicle (Pero et al., 2018). |
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Types of vehicle emissions
Emissions from a vehicle originate from three main sources, as illustrated in the following picture. These are exhaust emissions, abrasion emissions and evaporative emissions. Tailpipe exhaust emissionsExhaust emissions are produced primarily from the combustion of different petroleum products such as petrol, diesel, natural gas and liquefied petroleum gas, and biofuels. The amount of each pollutant emitted depends on several parameters, such as the type of fuel used, engine technology, emissions control, driving conditions, etc. Tyre wear, brake wear, and road abrasion emissionsTyre wear, brake wear and road abrasion emissions are produced from the mechanical abrasion and corrosion of vehicle parts during the operation of the vehicle. They are mainly relevant for PM and some heavy metals. Fuel evaporationEvaporative emissions are the result of petrol vapours escaping from the vehicle’s fuel system during vehicle operation, refuelling, and parking. They are relevant only for VOCs and are higher at high ambient temperatures. Petrol fuel vapor contains a variety of different hydrocarbon species, which can be emitted any time there is fuel in the tank, even when the vehicle is parked with its engine turned off. 
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Inside the vehicle cabin
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While driving, people inside the vehicle cabin are exposed to significant levels of pollutants[1]. Vehicle cabins accumulate particulate matters and gases from other circulating vehicles, which enter the cabin through the ventilation system and other openings. In addition, carbon dioxide is exhaled by the passengers of the vehicle. Finally, the interiors of vehicles themselves can emit VOCs. These emissions can originate from the manufacturing process, and include emissions from plastics, rubber and other polymers found in tires, carpets, seats, paints, adhesives, etc. |
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Various factors have an impact on the air quality inside the cabin, i.e. the number of passengers, ventilation fan speed, vehicle speed, traffic conditions and levels of pollutants outside. There is no automatic system that keeps the vehicle cabin’s air clean, apart from cabin filters which can only remove relatively large particles. Poor cabin air quality may have an impact on the behaviour of the driver, make passengers sleepy or cause other health problems (Pham et al., 2019). |
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