Vehicle exhaust emissions: what comes out of your car’s exhaust?

The vast number of petrol and diesel vehicles on the road - estimated to be over 1.45 billion globally - have had widespread effects on the environment. They're a major contributor to air pollution in towns and cities, as well as global greenhouse gas emissions.

Older diesel vehicles, in particular, are in the spotlight for the harmful particulate emissions they produce. As a result, drivers of ageing diesels are being discouraged from entering towns and cities by a growing network of low emission zones.

In this comprehensive guide to car emissions, we look at what exactly comes out of your vehicle, how it affects you and what is being done to tackle exhaust fumes.

Jump to:

• What are vehicle exhaust emissions? 
• Vehicle exhaust gases and pollutants
• The effects of breathing exhaust fumes
• What's being done to reduce exhaust emissions?
• What are the tests for exhaust pollutants? 
• How else does my car cause pollution?
• How toxic is my car?

What are vehicle exhaust emissions?

Exhaust emissions are a mixture of different gases and particles emitted by vehicles when the engine is running. 

These emissions can reduce the quality of the air around us, particularly in big cities that are congested with cars. 

The combination of exhaust fumes and other particulates released into the atmosphere are a major factor of global warming. According to the the UK Government's Transport and Environment Statistics 2024, domestic transport is the largest contributor to our total greenhouse gas emissions, at 28% in 2022.

These emissions also include harmless chemicals such as oxygen, nitrogen and water.

Carbon dioxide (CO2) is the one we’ve all heard of, but there are other less well-known substances coming from our cars that are damaging.

Vehicle exhaust gases and pollutants

Cars emit a potent cocktail of exhaust gases, many of which have harmful effects. They include:

• Carbon dioxide (CO2) – CO2 is a greenhouse gas, thought to be a major contributing factor to climate change. Although technically non-toxic, excessive volumes contribute towards ocean acidification.
• Carbon monoxide (CO) – This invisible gas is the result of incomplete combustion of fuel and is very toxic to humans. Most modern engines only produce tiny amounts of it thanks to efficient combustion processes, but older engines are bigger offenders.
• Nitrogen oxides (NOx) – Nitrogen oxides are produced in any combustion process. They are highly reactive and can contribute to smog when they come into contact with other airborne chemicals. Some manufacturers famously cheated NOx tests.
• Sulfur dioxide (SO2) – This is a colourless gas that smells like burnt matches and occurs naturally in crude oil used to refine petrol and diesel. It forms acids when burned, leading to engine corrosion and smog. 
• Hydrocarbons (HC) – HCs escape from exhausts as unburnt fuel due to incomplete combustion. They also evaporate from the fuel tank and nozzle when you fill up at the petrol station. 
• Benzene (C6H6) – This occurs naturally in petrol and diesel in very small quantities and is also emitted from vehicle exhausts as unburnt fuel. Benzene is a carcinogenic substance and high levels of inhalation can severely harm human health. 
• Particulates – Diesel engines emit airborne particles of black soot and metal, known as particulate matter. Modern cars are fitted with diesel particulate filters (DPFs) to reduce the number of harmful particles being pumped out into the atmosphere.

The effects of breathing exhaust fumes

Pollutants from cars are linked to a range of health problems, from allergies and skin irritation to heart disease and respiratory problems such as asthma. Long-term repeated exposure to diesel exhaust fumes may also increase the risk of lung cancer, according to the World Health Organisation.

A blockage in your exhaust can lead to carbon monoxide poisoning. Breathing in CO can cause headaches, respiratory problems and even death if inhaled in large quantities. It’s particularly dangerous to children and people suffering from heart disease. 

The amount of exhaust fumes people are exposed to varies greatly, but those living in densely-populated urban areas are most at risk of developing health problems linked to pollution.

What's being done to reduce exhaust fumes? 

Thanks to advances in technology and policies designed to tackle climate change, emissions of CO2 and other car pollutants have been significantly reduced in recent years. 

Car manufacturers are reducing exhaust emissions through improved engine and exhaust system design, while catalytic converters and particulate filters are now standard on all new petrol and diesel cars.  

Many cities around the world have introduced clean air zones to discourage the most polluting vehicles from entering them. There are several Clean Air Zones (CAZ) in place in cities across the UK, the most famous of which is London's Ultra Low Emission Zone. Many European cities have the same, for example the Crit'Air clean air sticker system used in France's busiest cities. 

• 11 ways to cut your driving emissions
• Electric and hybrid car reviews
• Euro 1 to Euro 6 guide – find out your vehicle's emissions standard
• Exhaust repair

What are the tests for exhaust pollutants? 

Since 1993, car manufacturers have had to comply with a series of European emissions standards that set maximum limits for harmful substances emitted by new cars.

The standards, which are different for petrol and diesel cars, have become increasingly stricter over time. The Euro 1 standard saw the introduction of catalytic converters and unleaded petrol for all cars. We’re now on Euro 6, which came into force in 2014 and slashes the permitted level of NOx for diesels.

From July 2025 the UK is expected to align with the EU's latest Euro 7 standard, further reducing NOx emissions as well as also setting maximum limits on particulate emissions from brakes and tyres, for example. 

Most vehicles also have an exhaust emissions test as part of their MOT and will fail if they’re found to be emitting too many harmful gases. When the MOT rules were updated in 2018, stricter guidelines were brought in for diesel cars, making it harder for them to pass.

How else does my car cause pollution? 

It’s not only exhaust fumes that pollute the atmosphere. Brakes and tyres also contribute to harmful emissions.

Every time you drive a car, tiny fragments of particulate matter such as dust are released into the air from brake and tyre wear, as well as from the road surface. These particles enter the airstream and can have a damaging effect on people’s health. Plastic particles from tyres can also harm marine wildlife if they get deposited into water through sewers.

Reports from the Air Quality Expert Group said dust from car brakes and tyres will still pollute the air, even when cars are all-electric. The upcoming Euro 7 emission standards will address this with officially measured brake and tyre emission limits. 

Is my car bad for the environment?

Out of non-commercial vehicles, gas-guzzling sports cars and older large cars or SUVs are reportedly the biggest offenders when it comes to climate change. Owners of these vehicles usually pay more road tax due to their motors having a larger engine, and emitting higher levels of CO2. 

Their vehicles can even be banned from certain cities in Europe if they're driving particularly old and polluting models at times, so make sure to find out your vehicle's emissions standard and whether you'll be affected before you travel. At the other end of the spectrum, there’s no VED to pay on fully electric cars with zero emissions - until 2025, at least. 

One of the best ways to reduce emissions is to improve your fuel efficiency. That’s because the amount of CO2 produced by a vehicle is directly linked to how much fuel it consumes. 

It makes sense to choose a car that is most efficient for your individual driving needs. For example, if you mainly drive short distances around town with the occasional longer distance, a hybrid or electric car is ideal, whereas if you undertake a range of different journeys, a petrol car might still be your best option.

Ultimately, switching to a more environmentally-friendly vehicle can save you hundreds of pounds a year in VED and fuel. And the newer your car is, the better it is for the planet.

• How to save fuel - the ultimate guide
• Top nine alternative fuels for cars
• What is a hybrid car and should I buy one?