Low emission cars
What is a low emission car? Before the 2015 VW emissions scandal, also known as dieselgate, a low emission car simply produced a small amount of CO2 (carbon dioxide).
But now we are more aware of pollutants like NOx (oxides of nitrogen), PM (particulate matter) and CO (carbon monoxide) and how poor air quality contributes to tens of thousands of premature deaths here in the UK.
The good news is that in our rigorous lab tests, we've found that, on average, the cars that meet the latest emission regulations (Euro 6d-temp and Euro 6d, explained in more detail below), are producing a fraction of NOx and CO than the cars they replaced.
But to make things more complicated, our tests show that CO2 emissions from these very same cars are actually going up, not down.
In a nutshell, we may have saved our lungs at the expense of our planet's health. It also makes buying a low emission car a lot more complicated.
It's not all doom and gloom though. Not only do we expect cars to become more fuel efficient over the next few years as technology is refined, there are some cars available today that strike a balance of low CO2 and low air pollutants such as NOx and CO. Our free low emissions checker, below, reveals CO, NOX and CO2 for hundreds of cars we've tested, so you can make an informed decision on your next purchase.
NOX, CO and CO2 car emissions checker
Our free low emissions checker shows all emissions we’ve measured from our independent, tough tests for every car we've tested since 2012. This tool shows:
- the car model and all engines we've tested in that car
- the Euro emission legislation the car meets in official tests
- the amount of NOx and CO each engine produced in Which? independent tests since 2012
- the amount of CO2 we measured from the tailpipe for all cars Which? has tested since 2017 - this will be updated to include all cars back to 2012 at a later point.
CO2 figures are given in g/km, while NOx and CO emissions are rated from 'very low (trace)' to 'extremely high'. Scroll below the tool to find out more about what we consider to be ‘high’ and the consequences for that car in our tests.
Car emissions checker - check by make and model
Using our low emission checker: high and low car emissions explained
New cars must meet the current emission regulation in official tests, otherwise they cannot be sold.
Our tests are tougher than the official ones, but we use the official Euro limits as benchmarks against our own test results to give them context.
Euro limits are explained in more detail below. But, in brief, official emission regulations limit the amount of emissions like NOx and CO in official tests.
(We updated our tests in 2017. Figures from our 2012-2016 programme are not directly comparable to figures from the current test programme.)
High and low car emissions in Which? tests explained
| Which? car emission levels explained
|Extremely low (trace)||Less than 0.015g/km||Less than 0.015g/km||NOx emissions are at least five times less than the current Euro 6 diesel limit. Or nearly seven times less than than Euro 6 petrol CO limit.|
|Very low||Less than 0.08g/km||Less than 0.05g/km||Produces less NOx than the current Euro 6 diesel limit|
|Low||Less than 0.18g/km||Less than 0.5g/km||Produces less NOx than the Euro 5 diesel limit|
|Medium||Less than 0.5g/km||Less than 1g/km||Produces less NOx than the Euro 4 diesel limit, or less CO than the Euro 6 petrol limit (CO petrol limit has not changed since Euro 4).|
|High||Less than 0.97g/km||Less than 2.72g/km||Produces more NOx than the Euro 3 diesel limit, or more CO than the Euro 3 petrol limit.|
|Extremely high||More than 0.97g/km||More than 2.72g/km||Produces more NOx than the Euro 1 diesel limit, or more CO than the Euro 1 petrol limit.|
The five lowest emission cars
It is still possible to buy a low emission car without going full electric. When we analysed our results to look at CO2, CO and NOx, our results show the car maker at the forefront of low emission vehicles is .
In our tests, the five cars below all have the lowest all-round emissions we've tested on our current test programme. All have small or trace amounts of NOx and CO, and reassuringly low CO2. This means they are great for our lungs, better for our planet than most and will be kind to your fuel bills.
A Toyota spokesperson told us: 'Our full hybrid electric technology provides customers with excellent overall environmental performance at an affordable price – allowing them access to electrification in a convenient and practical way and have a positive impact on both CO2 and air quality.'
Toyota Prius Plug-In Hybrid (2017 to present)
- Engine: 1.8 Plug-in hybrid (Euro 6b), tested in 2017
- Measured emissions: NOx (0.004g/km), CO (0.025g/km), CO2 (58g/km)
When looking across all emissions we measure - CO, NOx and CO2 - the plug-in version of the Prius is the cleanest car we've tested to date. The only problem is that this is the Euro 6b version of the car and so cannot be bought new anymore - which means you'll need to look for it on the used car market.
Aside from the Hyundai Ioniq Plug-in Hybrid - which itself had worryingly high CO emissions in our tests - this Prius has the lowest CO2 value we've measured from a car (with the exception of zero-emissions electric and hydrogen cars), which is also going to make it amongst the most affordable, traditional-fuel cars to run.
Toyota Prius (2016 to present)
- Engine: 1.8 hybrid (Euro 6b), tested in late 2016 (on 2017 test programme)
- Measure emissions: NOx (0.005g/km), CO (0.040g/km), CO2 (96g/km)
The regular version of the Prius is a common sight on our roads. Like the plug-in version of the Prius, it mixes a 1.8-litre petrol engine with an electric motor. It may not have the same massive battery pack as the plug-in Prius, but we found it still has low levels of NOx and CO.
Like the Euro 6b engine above, however, you'll need to look for it in the used car market. However, scroll down just a few cars and you'll find the same car with the newer Euro 6d-temp engine.
Pro tip for buying a used version of this car - if the model you're looking at was registered to its first owner before April 2017, it should remain tax-free for life under current rules. See our guide to for more.
Toyota Corolla Hybrid (2019 to present)
- Engine: 1.8 hybrid (Euro 6d-temp), tested in 2019.
- Measure emissions: NOx (0.0005g/km), CO (0.063g/km), CO2 (106g/km)
The newest car in this list, the Corolla is a medium-sized hatchback and the replacement for the Toyota Auris. It resurrects the nameplate of the most sold car around the world (the older Toyota Corolla), which Toyota undoubtedly hopes will boost sales.
The 1.8-litre in the Corolla is from the Prius. The Corolla also launched with a 2.0-litre version of the petrol-hybrid engine - in our tests we found the 2.0-litre outputs more CO under full throttle, so is unlikely to make this low emission cars list anytime soon.
Even for a petrol car, the NOx levels we measured from the 1.8-litre engine are staggeringly low. To put '0.0005g/km' (0.000462 actual) in context, that's 129 times less than the current Euro 6 limit of 0.06g/km.
Toyota Yaris Hybrid (2012 to present)
- Engine: 1.5 Plug-in hybrid (Euro 6b), tested in 2018.
- Measure emissions: NOx (0.003g/km), CO (0.07g/km), CO2 (111g/km)
The little Yaris hatchback might be getting a bit long in the tooth, but the 1.5-litre engine we tested in 2018 is a modern, clean engine that won't cost you the earth to run.
Toyota Prius (2016 to present)
- Engine: 1.8 hybrid (Euro 6d-temp), tested in 2019
- Measure emissions: NOx (0.002g/km), CO (0.065g/km), CO2 (113g/km)
Yes, you are seeing double. This is the same car and generation as the Toyota Prius above, but this version has the newer, Euro 6d-temp engine.
The pattern we've seen in most of our tests of cars meeting one of the latest emission regulations (either Euro 6d-temp or Euro 6d) is that they're cleaner in terms of NOx and CO, yet emit more CO2. In our tests, this version has similarly low levels of NOx and CO to the older engine, and slightly higher CO2. However, its CO2 output in our tests, which are tougher than the official tests, is still minimal compared to other cars we've seen in our lab.
The five highest emission cars
At the other end of the spectrum are the five highest emitting cars we've tested on our current test programme. Though they have all been certified by the relevant EU emission regulation, they fared poorly in our own stringent tests.
In much the same way that Toyota makes the cleanest cars we've tested, it's Subaru that has made the three dirtiest we've seen. We approached Subaru for comment, but Subaru was not comfortable commenting on the results as our tests differ from the official ones.
Subaru Forester (2013 to present)
- Engine: 2.0-litre Lineartronic diesel (Euro 6b)
- Emissions: NOx (2.022g/km), CO (0.022g/km), CO2 (189g/km)
While the 2.0-litre diesel boxer engine under the hood of the Forester has been discontinued, you'll still be able to find it on our roads and on the used car market.
In our tests, which are tougher than the official tests, this car produced an alarming amount of NOx - 25 times as much as the Euro 6 limit (which it met in official tests). Combine that with our tests revealing it has high CO2 emissions and you get the highest emitting car we've tested since 2017.
High emitting cars like this one will never be recommended as a Best Buy.
Subaru Levorg Sport Tourer (2015 to present)
- Engine: 2.0-litre Lineartronic petrol (Euro 6d-temp)
- Emissions: NOx (0.008g/km), CO (8.417g/km), CO2 (180g/km)
The 2.0-litre petrol engine in the Subaru's Levorg Sport Tourer is proof that although a car meets the latest emissions limits, that doesn't automatically mean it's clean, based on our tests.
Being a petrol engine, NOx is reassuringly low. But in our tests, the amount of CO it produces is off the charts, at nearly 8.5 times the Euro 6 limit (this figure is given for a comparison - in official tests, this car meets the Euro 6d-temp limit).
CO2 is also very high in our tests, which is why this car makes the number two spot on our list.
Subaru Outback (2015 to present)
- Engine: 2.5-litre Lineartronic petrol (Euro 6d-temp)
- Emissions: NOx (0.010g/km), CO (7.609g/km), CO2 (186g/km)
Another Subaru and another Euro 6d-temp car that, in theory, should be clean (and was in official tests). But it came up as one of the most polluting in our own, independent tests, which are tougher than the official ones.
The 2.5-litre petrol engine in the Outback is similar to 2.0-litre petrol engine in the Levorg in terms of the colossal amount of CO it outputs, along with a a high figure of CO2.
Chevrolet Camaro (2018 to present)
- Engine: 6.2-litre V8 petrol (Euro 6b)
- Emissions: NOx (0.009g/km), CO (4.015g/km), CO2 (284g/km)
It may not come as a surprise that the highest level of CO2 we've recorded in our tests since 2017 has come out of a V8 American muscle car. But producing over four times the amount of CO in our tests than the Euro 6 limit, in addition to a bucket-load of CO2, is still eyebrow raising.
Honda CR-V (2018 to present)
- Engine: 1.5-litre Turbo petrol (Euro 6d-temp)
- Emissions: NOx (0.002g/km), CO (5.915g/km), CO2 (189g/km)
It's one of the most popular cars on which.co.uk, and sadly the 1.5 petrol engine is also one of the highest emitters, largely thanks to it's massively high CO emissions in our independent tests.
A Honda spokesperson told us that the engine we tested was optimised for power and efficiency, that it has a legal obligation to apply the official tests – which Which? tests are different to.
It's not all bad though, there's a hybrid version of this car and, while it's not the cleanest car we've tested, it's significantly less dirty than the regular petrol version.
Car emissions explained: how exhaust emissions affect us
Car emissions we measure in our tests can be divided into two main groups:
NOx (oxides of nitrogen)
NOx is comprised of two gasses: nitric oxide (NO) and nitrogen dioxide (NO2). Of the two, NO2 is the gas that causes us the most harm. It’s an irritant that can cause inflammation of our airways, and can affect immune cells in the lungs.
Over a prolonged period of time, it is thought NO2 can affect how our lungs work.
CO (carbon monoxide)
CO has always been lethal in an enclosed space. But Defra now warns that excessive CO places people with existing diseases that affect delivery of oxygen to the heart or brain, such as angina, at risk.
PM (particulate matter)
These are tiny particles of solid and liquid matter. From cars, PM comes from the exhaust as well as from tyres and brakes as they wear down.
Particles are measured in nanometres, and some are small enough to pass through the pores in our lungs. PM has been linked to cardiovascular and respiratory disease. We hope to add it to our reviews and the tool above later in 2020.
NOx and PM
There are a number of difficulties in trying to get an accurate figure of how many premature deaths that air pollution from NOx and PM cause, and it's impossible to separate the effects of each emission. But a 2018 report by COMEAP puts the figure as an equivalent of 28,000 to 36,000 premature deaths from NO2 from NOx and PM combined, or 328,000 to 416,000 life years.
CO2 (carbon dioxide)
CO2 is a greenhouse gas linked to climate change and rising global temperatures. In our tests, we’ve found that CO2 from cars is rising, not falling.
Euro emissions explained
The Euro emission standards to date are explained underneath.
Each Euro standard has two introduction dates - an early one for new type approvals, and a second one for all remaining new cars on sale.
A new type approval is typically a new generation of car. For instance the 7th generation (or mk7) version of the popular VW Golf (2013-2020) was replaced by a new generation of VW Golf (2020-) in 2020.