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Air pollution and car emissions

Cars that produce the most NOx

By Adrian Porter

Article 2 of 3

Which? reveals the cars that produce the most 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 of the emissions and with good reason - it's one of the most harmful gasses that cars produce.

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

Keep reading to discover which cars produced the most 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 doesn't mean you have a clean car.

Latest 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 of 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 better reflect 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. The results are not directly comparable to the results from our older tests. 

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

The cleanest diesel cars in our tests

When we tested the 218d diesel engine, as found in the BMW 2 Series Active Tourer MPV, it recorded the lowest level of NOx emissions in our tests to date - at 0.014g/km.

This is even less than the average NOx emission we've measured from petrol cars (0.027g/km). It is a remarkably clean diesel car.

The engine officially complies with the latest emissions standards for new generations of cars, Euro 6d-temp, which will apply to all remaining new cars sold as of September 2019.

The saloon version of the Mercedes E-Class (2016-) produced just 0.024g/km of NOx in our tests, which is much less than the Euro 6 limit it officially adheres to of 0.08g/km.

The estate version of the same car, running the same four-cylinder diesel engine, also produced astoundingly low levels of NOx.

To make sure the results were genuine, and the cars were somehow not cheating in our lab tests, 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. 

We found the real-life value recorded on the PEMS just about matched the lab figure. The emission levels seem genuine.

A spokesperson for Mercedes said: 'For premium vehicles, from our perspective, diesel definitely has a future.

'Therefore, Daimler AG [owner of Mercedes] has invested about 3bn euros into the development and production of a completely new diesel engine family.' 

Mercedes also added: 'Our new engines are both highly efficient and produce low levels of NOx. It's a fact that it's worth improving the modern diesel instead of banning it.'

But while the BMW and Mercedes results are positive, they're also fairly exceptional. Four out of five diesel cars produced more NOx than the Euro 6 limit they officially comply with, during our tougher, more stringent testing.

The dirtiest diesel in our tests

The worst so far in our tests is the Subaru Forester. Its 2.0-litre, 150hp, Euro 6 compliant engine produced a staggering 2.02g/km of NOx in our own tests - 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, produce about three 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. 

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. 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 its cleaner than most, but it still claims the fifth-biggest producer of NOx.

Kia Sportage, 2.0-litre (2010-) - 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-): 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-): 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-): 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-): 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) when faced with 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, they are no longer Best Buys.

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 the elderly 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.