Meghan Hodges
Cars are a major contributor to air pollution, with over 1.45 billion petrol and diesel vehicles on the road globally. Motor vehicles generate about 20% of the European Union's man-made CO2 emissions, and passenger cars contribute about 12%. Car exhaust contains pollutants such as carbon dioxide, carbon monoxide, nitrogen dioxide, benzene, particulate matter, and volatile organic compounds. These pollutants have harmful effects on human health and the environment, ranging from allergies and skin irritation to heart disease, respiratory problems, and cancer. The impact of car exhaust pollutants is particularly pronounced in densely populated urban areas, with marginalized communities experiencing higher exposure to harmful particulate matter air pollution. To mitigate these issues, advancements in technology and stricter emission standards are being implemented, and the adoption of electric vehicles is gaining traction.
| Characteristics | Values |
|---|---|
| Pollutants | Carbon dioxide (CO2), Carbon monoxide (CO), Nitrogen dioxide (NO2), Nitrogen oxides (NOx), Particulate matter (PM), Volatile organic compounds (VOCs), Benzene (C6H6), Hydrocarbons (HC), Sulphur dioxide (SO2) |
| Harmful Effects | Global warming, Acidification of oceans, Respiratory problems, Skin irritation, Allergies, Heart disease, Lung cancer, Eye irritation, Asthma, Birth defects, Premature death |
| Causes | Burning gasoline, Incomplete combustion of fuel, Fossil fuel use, Industrial sources, Power generation, Transportation |
| Solutions | Catalytic converters, Diesel particulate filters (DPFs), Electric vehicles, Improved engine design, Exhaust system design, Emission standards, Clean air zones, Reduced distance travelled |
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What You'll Learn
Carbon monoxide (CO)
CO is invisible and odorless, making it difficult to detect without special instruments. Exposure to this gas can lead to carbon monoxide poisoning, causing various adverse health effects. Symptoms of acute CO poisoning include headaches, neurological issues, and cardiac events. Chronic exposure to smaller amounts of CO over time can also result in long-term effects and permanent damage.
The combustion process in vehicle engines generates high amounts of CO, typically over 30,000 parts per million (ppm). Catalytic converters are crucial in mitigating this issue by combining oxygen with CO to form carbon dioxide (CO2), significantly reducing the amount of CO released into the atmosphere.
However, defects in the exhaust system, such as leaks from rusted components or a damaged catalytic converter, can interrupt this process. As a result, harmful levels of CO may enter the vehicle's cabin, posing a severe risk to occupants. It is important to maintain the exhaust system properly and be vigilant for signs of CO poisoning to prevent adverse health consequences.
Furthermore, highways and densely populated urban areas tend to have higher CO levels due to the high volume of vehicle traffic. Certain groups, such as children and individuals with heart disease, are particularly vulnerable to the harmful effects of CO. Advances in technology and the implementation of emission standards have helped reduce CO emissions from vehicles in recent years.
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Nitrogen dioxide (NO2)
NO2 is a significant contributor to air pollution, especially in cities. The use of diesel engines, in particular, has increased the toxicity of NOx discharges from exhaust pipes. When diesel is burned in an engine instead of petrol, more NOx is produced overall, and 70% of the NOx produced is NO2, compared with only 10-15% when petrol is burned. The Euro 6 standard, which came into force in 2014, helped to reduce the permitted level of NOx for diesel vehicles. From July 2025, the UK is expected to align with the EU's latest Euro 7 standard, which will further reduce NOx emissions.
The direct emission of NO2 from road vehicle exhaust has significantly contributed to near-road ambient NO2 concentrations in many European cities. Diesel vehicles and their use of emission control technologies such as Diesel Oxidation Catalysts have dominated the emission of NO2 from road vehicles. However, it is important to note that the amount of NO2 emitted by most Euro 6 vehicles has decreased substantially, and absolute emissions of NO2 have been reducing since around 2007. Additionally, the amount of NO2 decreases as vehicle mileage increases, leading to substantial reductions in NO2 emissions from light-duty diesel vehicles in recent years.
NO2 contributes to the production of ozone and fine particulate matter less than 2.5 micrometres in size, called PM2.5. These are two of the most notorious pollutants that can impact human health. When hydrocarbons and NOx combine in sunlight, they produce ozone. While a layer of ozone in the upper atmosphere protects us from the sun's ultraviolet rays, holes in the ozone layer allow ozone to descend, contributing to smog and causing respiratory problems.
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Particulate matter (PM)
PM is classified by size, with diameters of 10.0, 2.5, and 0.1 µM corresponding to PM10, PM2.5, and PM0.1, respectively. The larger particles, such as PM10, are trapped in the airways, while smaller ultra-fine particles (UFP) can pass through barriers and distribute throughout the body, causing inflammation and oxidative stress in tissues. Diesel exhaust is the primary component of traffic-related PM, contributing significantly to the smaller and more harmful particles.
The health effects of PM are a growing concern, particularly in highly populated areas where exposure is more likely. Medical research has linked traffic-related PM to impaired cognitive functions and an increased risk of neurodegenerative diseases such as Alzheimer's. Inhaled components of PM can invade the brain through the olfactory route or peripheral system responses, leading to inflammation and oxidative stress in the brain. While the exact mechanisms are still being studied, the potential harm to human health is evident.
To address the issue of PM emissions, modern cars are now equipped with diesel particulate filters (DPFs) to reduce the number of harmful particles released into the atmosphere. These filters have significantly lowered PM emissions from diesel engines, making them comparable to PFI gasoline engines. Additionally, catalytic converters and particulate filters are now standard on all new petrol and diesel cars, further reducing the release of harmful pollutants.
Various regions have also implemented clean air zones and emission standards to discourage the most polluting vehicles from entering densely populated areas. These measures aim to mitigate the health risks associated with exhaust emissions, particularly in urban centres where pollution levels tend to be higher.
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Volatile organic compounds (VOCs)
A study in Wuhan, China, analysed 102 types of VOCs emitted from gasoline and diesel vehicles, including acetylene, alkanes, olefins, aromatic hydrocarbons, halogenated hydrocarbons, and oxygenated volatile organic compounds (OVOCs). The average emission concentrations of total VOCs in gasoline and diesel vehicles were 5.9 ± 2.4 mg/m3 and 6.8 ± 3.0 mg/m3, respectively.
The health risks associated with VOCs are significant, especially for people living near busy roads. VOCs can cause various health problems, including allergies, skin irritation, asthma, heart disease, and respiratory issues.
To reduce the impact of VOCs on the environment and human health, emission standards have been implemented to limit the pollutants contained in exhaust gases. These standards have facilitated the reduction of organic gas emissions from gasoline vehicles, with VOCs being the most effectively controlled subgroup.
Additionally, advancements in technology and the design of engines and exhaust systems have helped decrease emissions of VOCs and other pollutants. Catalytic converters, for example, are now standard in all new petrol and diesel cars, playing a crucial role in breaking down the pollution of exhaust gases.
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Hydrocarbons (HC)
At high combustion temperatures, nitrogen oxides (NOx) are formed. Hydrocarbons and NOx can combine in sunlight to produce ozone. While a layer of ozone in the upper atmosphere protects us from the sun's ultraviolet rays, holes in the ozone layer bring ozone closer to Earth, contributing to smog and causing respiratory problems.
The presence of HC in car exhaust fumes indicates that combustion is incomplete and the engine is not running efficiently. This can occur when a cold engine is started, as the fuel does not vaporise completely, creating higher emissions of HC and CO. The duration of this start-up phase has been reduced by advances in technology, including computer-controlled fuel injection, shorter intake lengths, and pre-heating of fuel and/or inducted air.
Catalytic converters are used to treat exhaust gases and break down pollution. In petrol-fuelled cars, a three-way catalytic converter is used to convert unburnt hydrocarbons, CO, and NOx into CO2, H2O, and N2. However, the oxygen in the exhaust gas of diesel vehicles makes the conversion of NOx inefficient, so diesel engines are equipped with a two-way catalytic converter.
Overall, HC emissions from car exhausts contribute to air pollution and can have significant health and environmental impacts, particularly in densely populated urban areas.
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Frequently asked questions
The major pollutants from car exhaust are carbon dioxide (CO2), carbon monoxide (CO), nitrogen dioxide (NO2), volatile organic compounds (VOCs), and particulate matter (PM).
Carbon monoxide is a toxic gas that is produced by the incomplete combustion of fuel. Inhaling large amounts of CO can cause headaches, respiratory problems, and even death.
Particulate matter refers to the soot and fine particles of black soot and metal released from diesel engines. These particles are less than one-tenth the diameter of a human hair and can penetrate deep into the lungs, causing serious health issues.
Vehicle exhaust releases large amounts of carbon dioxide (CO2), a greenhouse gas that contributes to the greenhouse effect and climate change. Additionally, exhaust emissions contain other pollutants that trigger global warming, such as nitrogen oxides (NOx) and volatile organic compounds (VOCs).
Exposure to car exhaust pollutants has been linked to various health problems, including allergies, skin irritation, heart disease, respiratory issues such as asthma, and an increased risk of lung cancer, especially from long-term exposure to diesel exhaust fumes.
