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Air pollution and car emissions

Cars that produce the most NOx

By Adrian Porter

Article 2 of 3

Find out which cars produce the highest amount of harmful NOx (and NO2) emissions in our realistic car-emission and air-pollution tests.

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NOx (or specifically NO2, one of the gasses within NOx) is perhaps the most talked about car emission, and with good reason – it's one of the most harmful.

Both petrol and diesel vehicles produce NOx, but diesel-powered cars produce it in much higher quantities than petrol equivalents – 11.5 times as much on average, in our tests.

Keep reading to discover which cars produced the most (and least) NOx in our tests.

Our tests are more stringent than the official ones but, we believe, are more true to life. If a car can't keep its NOx levels down below the Euro 3 limit (passed in 2001) during tough Which? emission tests, it can't be a Best Buy. That means the best cars we recommend are not the biggest NOx offenders.

In a nutshell: why our car-emission findings matter

  • Acute NOx exposure can trigger asthma attacks and worsen respiratory diseases.
  • In our tests, a quarter of diesel cars emit so much NOx that we couldn't make them Best Buys.
  • We found there is no strong link between MPG and NO2/NOx – just because you have low fuel bills, it doesn't mean you have a clean car.
  • It's not just about diesel cars and NOx. Petrol cars also produce large amounts of CO. We have a separate guide just for petrol cars that produce the most and least CO.

Latest diesel car emission tests: the diesel cars that produce the most and least amount of NOx

It seems there is some hope on the horizon for diesel cars: our latest tests has found one that produces less NOx than the average petrol car.

All the cars featured on this page would have met the official limit in official tests, so no car is actually breaking the law. 

However, our tests are more challenging than the official ones, and include a unique motorway cycle. We believe them to be more realistic, and therefore a better reflection of what emissions your car actually produces.

We updated our cars test programme from the start of 2017, making our tests tougher and introducing PEMS testing. This means the results from cars tested from 2017 are not directly comparable with the results from our older tests. 

PEMS testing – a Portable Emissions Measuring System lets us test emissions on a car while it's being driven on real roads. We then compare these values with our lab-based tests to make sure no manufacturer is cheating our lab tests.

For more information about our tests, including how they differ from the official ones, see how we test mpg and emissions.

Low-emission diesel cars in our tests

The cleanest diesel engine we've ever tested is in the updated version of the BMW X1 (2015-) – it's the 150hp 'sDrive 18d Steptronic' diesel engine.

It amazed our experts by keeping emission levels low throughout our extensive assessments, producing an incredibly low average of 0.007g/km. Even in our motorway test, which features heavy acceleration up to motorway speeds, the 18d engine kept emissions at bay.

Our experts said that the emissions were, at times, 'too low to be traceable'.

To put the result in perspective, 0.007g/km is significantly less than the average level of NOx from petrol cars tested under the same test programme (0.022g/km). It is a remarkably clean diesel car.

Close behind the BMW X1 is the Ford Ecosport (2014-). Its 102hp 1.5-litre EcoBlue diesel engine produced 0.009g/km. It's the second car to ever produce a NOx level that's less than 0.01g/km in our tests.

Taking third place is the Mercedes C-Class (2014-) and its powerful C220d 9G-Tronic, 194hp engine. Emitting just 0.013g/km of NOx in our tests, it shows that powerful diesel cars can be clean, too.

To make sure all the results of the cleaner cars were genuine, and not somehow cheating our lab tests, we also used PEMS testing. This means we took the car out of the lab, drove it on real roads, and checked the emission level via our Portable Emission Measuring System (PEMS). This system should catch any lab-based cheating.

All figures recorded in our PEMS indicate that the figures we recorded in the lab are genuine.

The engine in the BMW X1, Ford Ecosport and Mercedes C-Class all officially comply with the latest emissions standards for new generations of cars, Euro 6d-temp, which all cars need to adhere to from September 2019.

To find out more about the official tests, and how our tests are different, head to our guide on how we test mpg and emissions.

The dirtiest diesel in our tests

The worst so far in our tests is the Subaru Forester (2013-). Its 2.0-litre, 150hp engine is compliant with Euro 6, but our own tests found that it produced a staggering 2.02g/km of NOx  – that's more than 25 times the Euro 6 limit.

We will keep this page updated with the cars that produce the most and least amount of NOx from our latest test programme.

Does SCR/AdBlue make a difference?

In a word, yes. SCR stands for Selective Catalytic Reduction. It’s an active emissions system that injects urea, most commonly AdBlue, into the exhaust system to reduce harmful emissions.

And it makes a big difference – we’ve found that diesel cars without an SCR system, on average, produces nearly four times as much NOx as cars with an SCR system.

Tested between 2012-2016: the 10 diesel cars that produce the most NOx

These cars are the models that produced the most NOx on our older test programme.

Many of the engines tested will either be unavailable as new, or be phased out by September 2019. However, they will still be available to buy on the used-car market.

All of them officially met their respective official Euro 5 or Euro 6 requirements. But in our more realistic tests, the worst car produced 15 times the limit of NOx than Euro 5 limits allow.

  • Euro 5 limit: 0.18g/km
  • Euro 6 limit: 0.08g/km

Jeep Grand Cherokee, 3.0-litre (2011-): emits 2.7g/km of NOx.
By far the biggest NOx creator in our tests, it shovels 15 times the amount of NOx into the atmosphere than its Euro 5 engine is allowed to.

Subaru Forester, 2.0-litre (2013-): emits 1.19g/km of NOx
Its Euro 5 engine pumps out more than six and a half times as much NOx as its engine should permit.

Nissan X-Trail, 1.6-litre (2014-): emits 1.05g/km of NOx
The Nissan X-Trail is the only official Euro 6 engine we’ve tested that fails to meet even Euro 1 standards from 1993. In our tests, it emitted 13 times as much NOx as the Euro 6 limit.

Nissan Qashqai, 1.6-litre (2014-): emits 0.99g/km of NOx
A top-selling car, it chugs out so much NOx that it also wouldn’t meet Euro 1 standards.

Subaru Outback, 2.0-litre (2015-): emits 0.94g/km
Its Euro 6 engine should mean it's cleaner than most, but we found it to be the fifth-biggest producer of NOx.

Kia Sportage, 2.0-litre (2010-2016): emits 0.92g/km of NOx
The four-wheel-drive version of this Kia only just meets Euro 1 criteria. That's OK - if you happen to do your driving in 1993.

Hyundai Santa Fe, 2.2-litre (2012-2018): emits 0.89g/km of NOx
Look beyond the exotic name and the bonnet, and you'll find a Euro 5 engine that chucks out large amounts of NOx.

Kia Sorento, 2.2-litre (2010-2015): emits 0.89g/km of NOx
The second entry from Kia in this top 10, the Sorento isn't far behind the Sportage in terms of NOx production.

Dacia Duster, 1.5-litre diesel (2013-2018): emits 0.89g/km of NOx
The price of the Duster may draw you in, but it's far from a squeaky-clean purchase despite having a Euro 6 engine.

Land Rover Range Rover Sport, 3.0-litre (2005-2013): emits 0.87g/km of NOx
This Range Rover produces nearly five times the Euro 5 allowed amount of NOx.

Highest NOx-emitting diesel-hybrid

Peugeot 508 RXH, diesel-electric hybrid (2012-2017): emits 0.53g/km
While it's not in the top 10, this Peugeot is worth mentioning. Although it’s a hybrid, it still pushes out three times the Euro 5 NOx limit in our more stringent tests.

Older tests: the five petrol cars that produced the most amount of NOx

  • The three worst petrol cars listed here produced so much NOx under our older test programme, which is more stringent than the official tests, that they would fail the Euro 3 standards.

Petrol engines produce NOx – most in such small quantities that it’s not of consequence. However, there are some that are worth avoiding:

Euro 5 and Euro 6 limit: 0.06g/km

Mercedes-Benz SL, 3.5-litre (2012-): emits 0.2g/km of NOx
This sporty car from Mercedes created three times as much NOx in our tough tests as petrol limits allow. To put that in perspective, it’s the only petrol car we've seen that would also fail diesel-equivalent Euro 5 limits.

Mercedes-Benz E-Class, 2.0-litre (2009-2016): emits 0.17g/km of NOx
In terms of cars that produce the most amount of NOx from their petrol engines, Mercedes also takes the second spot. The E-Class is almost as bad as the SL, despite the fact it has what should be a cleaner Euro 6 engine.

Mini Cooper, 5-door, 1.5-litre (2014-): emits 0.16g/km of NOx
It might be fun to drive, but its Euro 6 engine pushes out nearly three times the amount of NOx that it’s supposed to.

Nissan Pulsar, 1.2-litre (2014-): emits 0.15g/km of NOx
The small-family sized hatchback may not be the biggest car about, but it creates a big stink in our tests. It only just complies with Euro 3 emission levels.

Nissan Qashqai, 1.2-litre (2014-): emits 0.12g/km of NOx
Just like the diesel version, the petrol version of the Qashqai also exceeds NOx limits. It's not as bad as the diesel, but it's still the fifth highest creator of NOx of all petrol cars we've tested so far.

Best Buys removed

We’ve found that of the diesel cars we’ve tested, nearly a quarter would exceed Euro 3 limits (passed in 2001) according to our more realistic tests. This means these cars don't pass the earliest emission regulations from this century.

Some of these cars had previously performed well enough in other areas of our testing to be named as Best Buys – but considering how much they pollute, we've removed our Best Buy recommendation.

Looking something clean? Find your perfect new car among our Best cars.

What you need to know about car emissions

The Environment Research Group at King’s College London completed a study in 2015, estimating that Londoners lost up to 88,113 life years to NO2 during 2011 – the equivalent of nearly 5,900 deaths. But that assumed a 30% crossover with PM (particulate matter), and is a figure that the report itself advises should be used with caution.

Its effects are more pronounced on those who are vulnerable, such as elderly people and those who have an existing lung condition.

Unlike CO, NO2 levels haven’t reduced across urban areas as much as was hoped in the past 20 years after the introduction of emissions laws, and are currently contributing to the widespread public health issue of poor air quality, alongside PM emissions.