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're 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 in the UK.
The good news is that our tests have 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 the NOx and CO of 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 humanity's 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 car 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.
Our free low-emissions checker shows all emissions that our independent tests have measured for every car we've tested since 2012. This tool shows:
CO2 figures are given in g/km, while NOx and CO emissions are rated from 'very low (trace)' to 'extremely high'. Scroll down to below the tool to find out more about what we consider to be ‘high’ and the consequences for those cars in our tests.
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 such as NOx and CO in official tests.
(We updated our tests in 2017. Figures from our 2012-2016 test programme aren't directly comparable with figures from the current test programme.)
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.
Less than 0.08g/km
Less than 0.05g/km
Produces less NOx than the current Euro 6 diesel limit
Less than 0.18g/km
Less than 0.5g/km
Produces less NOx than the Euro 5 diesel limit
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 unchanged since Euro 4).
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.
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.
It's still possible to buy a low-emission car without going fully electric.
Taking CO2, CO and NOx measured from our tests into account, it's 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.
The Toyota Plug-In Hybrid has been at the top of the list since we first put it together. A 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.'
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 the model we tested is the Euro 6b version of the car and can't be bought new any more, so you'll need to look for it on the used car market.
Aside from the Hyundai Ioniq Plug-in Hybrid – which had worryingly high CO emissions in our tests, so can't feature in our top five despite its strong CO2 performance – this Prius has the lowest CO2 value we've measured from a car (with the exception of zero-emissions electric and hydrogen cars). This is also going to make it among the most affordable traditional-fuel cars to run.
We've not tested a newer Euro 6d engine in the plug-in hybrid as yet, but we have lab tested the Euro 6d-temp in the regular Prius. While it just falls short of appearing on this list, it is another remarkably clean engine – and so we have high hopes for the current engine in the Toyota plug-in hybrid.
That's right, the second-cleanest car we've tested is a 2.7-tonne SUV with a 2.0-litre diesel engine under the bonnet (plus a 31.2kWh battery pack).
Starting with a full battery, we averaged a remarkable fuel economy in our independent tests. Unfortunately, and like every plug-in hybrid, that fuel economy will fall once the battery has depleted. In this case, it will fall drastically.
Although it has a diesel engine, its NOx emissions are very low, and CO is nearly non-existent. Coupled with the low CO2 output, this is a very clean SUV – provided you keep the battery topped up.
The second Mercedes on the list isn't an SUV, but a people carrier (or MPV). For those unfamiliar with the B-Class, this car is based on the ever-popular , but offers much more in terms of space and everyday practicality.
Combined with low fuel costs and emissions, it's a car that is both environmentally and family friendly. But don't fall into the 'well it must be boring' trap. It might be a conservatively styled MPV, but the 1.3-litre four-cylinder petrol engine combines with the electric motor to deliver 218hp of power and 450Nm of torque (pulling power). In short, it's nippy.
The first car from Skoda to feature any electrification is a marvel for how clean it is. Badged as the 'iV', its particularly low NOx and CO2 emissions see this large estate shoot straight into this list of the five cleanest cars we've tested to date.
It combines a 1.4-litre petrol engine with an electric motor, delivering a healthy (indeed quite potent for this type of car) power output of 218hp. It's also spacious and the boot is a decent size.
Currently the only brand to have different models feature in both the five lowest-emitting cars and five highest-emitting cars (see below), Renault's Mégane Sports Tourer is a medium-sized estate that mixes a 1.6-litre petrol engine with two electric motors and a 9.8kWh battery to great effect.
Reasonably low CO2 and just a trace of NOx emissions make this a clean car. The battery pack isn't huge, so you'll need to top it up regularly to keep its impact on the environment to a minimum.
It's not perfect though. The space required for the battery pack does takes a 95-litre chunk out of the boot space compared with the non-hybrid version. While some boot space is often sacrificed to accommodate battery packs, many modern plug-in hybrids are able to mitigate that impact far better than this Renault does.
At the other end of the spectrum are the five highest-emitting cars we've tested on our current test programme. Although they've 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 it wasn't comfortable commenting on the results as our tests differ from the official ones.
While the 2.0-litre diesel boxer engine under the hood of the Forester has been discontinued, and the only engine type you can buy new is a hybrid, you'll still be able to find the diesel version 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.
The 2.0-litre petrol engine in the Subaru's Levorg Sport Tourer is proof that although a car meets stringent emission 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.
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 high figure for CO2.
In our tests, this car produced the highest amount of CO2 compared with any other car in this list, as well as a large amount of NOx.
This SUV may draw people in with its low purchase price, loads of space and reasonable interior, but its high emissions and other flaws may just put them off again.
The seven-seat version of the regular Scénic MPV is the fifth most-polluting car we've tested since 2017.
Just like the regular Scénic, the Grand Scénic has been discontinued, although you'll still find it on the used car market, and probably at an appealingly low price.
But if one of your buying consideration is low emissions, ignore the price tag, as we found this car's high NOx and CO2 emissions make it a highly polluting modern car.
Car emissions we measure in our tests can be divided into two main groups:
NOx (oxides of nitrogen)
NOx is comprised of two gases: 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's thought that 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 increases the risk for people with existing diseases that affect delivery of oxygen to the heart or brain, such as angina.
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 PM measurements 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 the Committee on the Medical Effects of Air Pollutants (COMEAP) puts the UK 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.
The Euro emission standards to date are explained below – click on each drop-down arrow to read the details.
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.
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.
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.
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.
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.
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.
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.
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.
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.