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Car emissions

Low emission cars

By Adrian Porter

Article 1 of 2

Want to buy a low emission car? Our tests show that death-causing NOx emission levels are plummeting from the latest cars, but planet-harming CO2 from the same cars is actually on the rise. It's confusing, but there are still some genuinely low emission cars on the UK market - our free car emissions tool will help you find them.

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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 car emissions tool, below, reveals CO, NOX and CO2 for hundreds of cars we've tested, so you can make an informed decision on your next purchase.

This guide contains:

NOX, CO and CO2 car emissions tool 

This free tool shows all emissions we’ve measured from our independent, tough tests (which are tougher than the official ones) 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.

Every car we review is driven by our experts and put through our independent lab tests. As our lab tests are objective, you can trust our results. See our expert pick of the best cars.

Using our tool: 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.

Any engine that has a rating of ‘high’ produced more pollutants in our tests than the Euro 3 limit from 2001. So we withhold Which? Best Buy car status.

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
  NOx CO Equivalent to
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.

Table notes
1 Euro 1-6 limits are used for comparative purposes only, to place our own, independent test results in context. 
2 Which? tests are tougher than the official ones. Go to our guide on how Which? tests mpg and emissions to find out more.

Our tough lab tests reveal which electric cars you should buy, and which you should avoid - see best electric cars.

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 Toyota

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.

This car 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 the most affordable, traditional-fuel car to run.

Read our Toyota Prius Plug-In Hybrid review to find out what this car's like to drive and what fuel economy figures you can expect.

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 car tax for more.

Read our Toyota Prius review to find out if it's the right car for you.

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.

Read our Toyota Corolla Hybrid review to see if it's worth becoming a new Corolla owner.

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.

Small hatchback hybrids are rare and this Yaris is due to be replaced in 2020 by an all-new version.

Read our Toyota Yaris Hybrid review to see if it's worth investing in one before it vanishes from dealerships.

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.

In case you missed it the first time, here's our Toyota Prius review - where you can get more details on the differences between the Euro 6b and Euro 6d-temp engines.

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. 

To find out more about this car, including its mpg in our stringent tests and what it's like to drive, see our Subaru Forester review.

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.

Here's our expert, independent Subaru Levorg Sport Tourer review.

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.

Find out more about this rugged estate-come-SUV by reading our Subaru Outback review.

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.

Not enough to transform your opinion on the Chevvy? Read our Chevrolet Camaro review to find out how it drives. 

Honda CR-V (2018 to present)

  • Engine: 1.5-litre Turbo petrol (Euro 6d-temp)
  • Emissions: NOx (2.022g/km), CO (0.022g/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.

Read more about the popular Honda CR-V and the cleaner Honda CR-V Hybrid.

Car emissions explained: how exhaust emissions affect us 

Car emissions we measure in our tests can be divided into two main groups:

  • Air pollutants: NOx, PM, CO and HC (hydrocarbons) are all air pollutants that are hazardous to human health. All of these emissions are limited by European emission legislation (such as Euro 6d - see Euro emission regulations explained for more detail).
  • Greenhouse gas: CO2 is directly linked to fuel economy. It’s not controlled by the same European emission legislation. Instead there are average CO2 levels manufacturers need to adhere to across their entire fleets, or face potentially massive fines.

Air pollutants

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.

Greenhouse gas

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. 

See our guide to CO2 from cars for more information.

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.

  • Applies to new generations/type approvals: January 2020
  • Applies to all remaining new cars: January 2021

Euro 6d cars are nearly the same as 6d-temp. The difference is that the 2.1 conformity factor has been removed from the RDE tests, and replaced with a margin of error of 1.43. This now means, for example, a car can produce 0.1144g/km of NOx in the RDE section of tests and still be approved.

Diesel limits

  • NOx in lab tests: 0.08g/km
  • NOx in RDE tests: 0.1144g/km (0.08g/km with a margin of error of 1.43 in RDE tests)
  • NOx and HC combined: 0.17g/km
  • CO: 0.5g/km
  • PM: 0.005g/km

Petrol limits

  • NOx: 0.06g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • PM: 0.005g/km (only applies to petrol engines that use direct-injection fuel delivery)
  • Applies to new generations/type approvals: September 2017
  • Applies to all remaining new cars: September 2019

The big difference between cars that are certified as Euro 6d-temp and all those that came before is the introduction of RDE (Real-world Driving Emissions) tests. Now after cars are lab tested, they are taken out onto real roads and have their emissions measured by a portable emission measuring system (PEMS).

Euro 6d-temp cars can produce 2.1 times the amount of emissions as in lab tests. So in terms of NOx, Euro 6d-temp cars can produce up to 0.168g/km of NOx in RDE tests and still pass.

Diesel limits

  • NOx in lab test: 0.08g/km
  • NOx in RDE test: 0.168g/km (0.08g/km with a conformity factor of 2.1 in RDE tests)
  • NOx and HC combined: 0.17g/km
  • CO: 0.5g/km
  • PM: 0.005g/km

Petrol limits

  • NOx: 0.06g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • PM: 0.005g/km (only applies to petrol engines that use direct-injection fuel delivery)
  • Applies to new generations/type approvals: September 2017
  • Applies to all remaining new cars: September 2018

Euro 6c is significant as cars certified as Euro 6c have been through a much tougher, more realistic lab test to determine fuel consumption and emissions compared to all cars that were certified before this.

These tougher tests use the WLTP (Worldwide-Harmonised Light vehicles Test Procedure) cycle rather than the older NEDC (New European Driving Cycle). Read why WLTP is so important, and how Which? tests are tougher still, by heading to our guide on how we test mpg and emissions

Among the various improvements brought about by the WLTP, it takes account of the varying equipment levels on the same car. As a result, official mpg figures are often presented as a range.

Diesel limits

  • NOx: 0.08g/km
  • NOx and HC combined: 0.17g/km
  • CO: 0.5g/km
  • PM: 0.005g/km

Petrol limits

  • NOx: 0.06g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • PM: 0.005g/km (only applies to petrol engines that use direct-injection fuel delivery)
  • Applies to new generations/type approvals: September 2014
  • Applies to all remaining new cars: September 2015

Euro 6b brought about the toughest emission limits to date. Since Euro 6b, the actual limits have not changed, but the methods behind how car emissions (and mpg) are measured have changed significantly.

After Euro 6b, things got a lot tougher for manufacturers - see Euro 6c and Euro 6d-temp/6d for more.

Diesel limits

  • NOx: 0.08g/km
  • NOx and HC combined: 0.17g/km
  • CO: 0.5g/km
  • PM: 0.005g/km

Petrol limits

  • NOx: 0.06g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • PM: 0.005g/km (only applies to petrol engines that use direct-injection fuel delivery)
  • Applies to new generations/type approvals: September 2009
  • Applies to all remaining new cars: January 2011

Euro 5 introduced a limit on particulate matter from petrol cars; but only applies to engines that use direct-injection fuel delivery.

Cars that use direct-injection inject the fuel directly into the combustion chamber, as opposed to first being mixed with air first inside the air intake manifold. Direct injection engines are more fuel efficient.

Diesel limits

  • NOx: 0.18g/km
  • NOx and HC combined: 0.23g/km
  • CO: 0.5g/km
  • PM: 0.005g/km

Petrol limits

  • NOx: 0.06g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • PM: 0.005g/km (only applies to petrol engines that use direct-injection fuel delivery)
  • Applies to new generations/type approvals: January 2005
  • Applies to all remaining new cars: January 2006

Diesel limits

  • NOx: 0.25g/km
  • NOx and HC combined: 0.3g/km
  • CO: 0.5g/km
  • PM: 0.025g/km

Petrol limits

  • NOx: 0.08g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • Applies to new generations/type approvals: September 2000
  • Applies to all remaining new cars: September 2001

Euro 3 was introduced at the turn of the millennia, and added bespoke NOx limits for the first time.

If a car produces more emissions in Which? tests than would have been allowed under Euro 3 in official tests, we remove Best Buy status.

You might think that it wouldn't be a problem for the very latest cars - but we've caught out some Euro 6d-temp cars with this rule.

Unusually, the Euro 3 limit for CO is actually slightly more lenient than the Euro 2 limit.

Diesel limits

  • NOx: 0.5g/km
  • NOx and HC combined: 0.56g/km
  • CO: 0.64g/km
  • PM: 0.05g/km

Petrol limits

  • NOx: 0.15g/km
  • HC: 0.2g/km
  • CO: 2.3g/km
  • Applies to new generations/type approvals: January 1996
  • Applies to all remaining new cars: January 1997

Euro 2 brought about separate limits for petrol and diesel cars.

Though no particulate matter limit was to be introduced for petrol cars until Euro 5 in 2009.

Diesel limits

  • NOx: 0.08g/km
  • NOx and HC combined: 0.17g/km
  • CO: 0.5g/km
  • PM: 0.005g/km

Petrol limits

  • NOx: 0.06g/km
  • HC: 0.1g/km
  • CO: 1g/km
  • PM: 0.005g/km (only applies to petrol engines that use direct-injection fuel delivery)
  • Applies to new generations/type approvals: January 1992
  • Applies to all remaining new cars: January 1993

The first iteration of the Euro emission limits we know today was very simple. The same limits applied to petrol and diesel cars, apart from the PM limit which only applied to diesel cars. There was no separate limit for NOx.

Compared to the latest standard, Euro 6, diesel cars were allowed to emit over 12 times the amount of NOx as they are now.

Petrol and diesel limits

  • NOx and HC combined: 0.97g/km
  • CO: 2.72g/km

Diesel only

  • PM: 0.14g/km
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