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ABC Science
Topic:Air Pollution
NOx pollution is linked to the premature death of hundreds of people every year, but most drivers have little idea how much their car emits. (Getty: Feodora Chiosea)
Vehicles that burn petrol or diesel emit a type of pollution called NOx, which is linked to most of the hundreds of annual deaths attributed to air pollution from vehicles.
Real-world testing shows many popular vehicles, banned in Europe but allowed in Australia, emit several times more NOx than lab-test rules allow.
NOx standards are being updated for the first time in nine years, more than halving how much of the toxin new diesel models can emit.
Imagine you're stopped at the lights and two cars pull up beside you. One is a compact SUV, the other a beefy four-wheel drive that weighs twice as much.
The off-roader has a petrol engine, while the SUV has the low idling growl of a diesel.
You'd probably guess the big car is polluting more. 
And you're right, but also wrong.
The smaller car emits less carbon dioxide (CO2) but it also produces more of another form of car pollution — one that has a much greater immediate public health impact than the CO2.
Nitrogen dioxide (NO2) and nitric oxide (NO) are collectively known as the oxides of nitrogen, or "NOx".
Mostly invisible and regularly ignored, NOx is linked to hundreds of premature deaths each year from conditions such as heart and lung diseases.  
Despite this health toll, drivers are generally unable to find out exactly how much NOx their car emits. 
Now, new publicly available data is exposing which popular models pump out more NOx than others.
It comes as a long-anticipated standards update goes into force today that more than halves how much NOx new diesel models sold in Australia may emit.
Here's how these changes may affect your next car — and the air you breathe. 
Unlike with CO2, you won't find a point-of-sale windshield sticker or a vehicle emissions guide that lists NOx pollution.
Cars sold in Australia must emit less NOx than mandated maximums, under rules last updated nine years ago.
These rules are enforced by laboratory testing of new models. Tyres are spun on a giant treadmill while exhaust gases are funnelled, split, and measured.
The AAA's test also showed many cars have worse fuel consumption than advertised. (Supplied: Australian Automobile Association)
But lab-testing doesn't necessarily show how much cars pollute on the road.
The Australian Automobile Association (AAA) recently strapped emissions measurement systems onto more than 100 of the most popular cars, utes and vans. They drove these modified vehicles around Geelong in real traffic conditions, following strict protocols for speed and acceleration.
Their testing revealed many cars consumed more fuel and emitted more pollution than lab tests showed.
And some of the biggest discrepancies were for NOx emissions.
The petrol cars were generally under the limit. Two of the 90 petrol cars tested emitted more NOx than the mandated maximum of 60 milligrams per kilometre.
But most of the diesel cars flunked the test.
Seventeen of the 24 diesels tested emitted more than the cut-off limit for commercial or passenger models, which is 280 milligrams per kilometre and 180mg/km respectively (two to three times higher than than the limit for petrol cars).
On average, diesel vehicles exceeded their mandated maximums by almost 40 per cent.
And this wasn't for a new emissions standard. The maximum had been in place since 2016.
You can see if your car is among the 114 petrol and diesel models tested by the AAA, listed in the searchable table below.
Dots on a graph may not look too dangerous, but incremental changes in NOx emissions have a measurable impact on public health, when multiplied by the millions of cars.
If you stand beside a busy road and count the passing cars, about one in every 10 vehicles burn diesel.
Diesel vehicles are less common than petrol ones but emit far more NOx (based on the AAA's data, about 20 times more).
NOx exposure is associated with the majority of deaths from traffic pollution, so the relatively few diesel cars have a disproportionately high public health impact.
Research led by the University of Tasmania's Centre for Safe Air estimates particulate and NOx pollution from traffic leads to more than 1,800 earlier than expected deaths a year.
"Motor vehicles are one of the most important sources of air pollution in Australia," the Centre's director Fay Johnston said.
NO2 is a reddish-brownish gas with an acrid odour, while NO is colourless and odourless.
A near-invisible haze of NOx spreads outwards from highways and suburban streets. Levels drop at about 100 metres to 200m from the source, but remain high in busy traffic areas.
Breathed in, long-term exposure to NOx can damage the lungs and lead to diseases like bronchitis and asthma. It generally inflames and stresses cells in the body, which is linked with heart disease.
"We can measure the increased rates of deaths when NOx pollution is higher," Professor Johnston said.
Despite this high public health impact, Australia has delayed tightening vehicle NOx pollution regulations and is now about a decade behind Europe, the UK and the US.
The AAA's results show a 2024 Toyota LandCruiser Prado (GXL) diesel and a 2024 Mitsubishi Pajero Sport (GSR) diesel have very similar fuel consumption and CO2 emissions.
But the Pajero's NOx emissions are about 17 times higher than the Land Cruiser's.
The reason for this is "after treatment", or the process of cleaning exhaust gases after they come out of the engine.
The most effective method is to inject a nitrogen-rich additive, like AdBlue, into the hot exhaust gases, which converts the NOx into nitrogen gas (N2), water (H2O) and CO2. This is known as selective catalytic reduction (SCR).
AdBlue is stocked at petrol stations around Australia. (Getty: John Keeble)
The 2022 Toyota Land Cruiser Prado uses SCR, while the 2024 Mitsubishi Pajero Sport does not.
SCR is so effective that it can make a diesel emit as much NOx as a petrol car — and even less than some petrol cars.
Essentially all diesels now sold in Europe use SCR to comply with emissions standards, Shawn Kook, director of the UNSW engine research laboratory, said.
Europe's had these standards since 2015. 
"The technology and products are there," Professor Kook said.
"It is just that [adding this technology] will add costs to car makers."
From today, December 1, the respective limits for NOx emissions for new diesel utes, vans and SUV models supplied to Australia has been cut by more than half.
It effectively means that new diesel models without SCR will no longer be sold in Australia.
The limit has dropped from 180mg/km to 80mg/km for diesel passenger vehicles (generally SUVs and sedans) and from 280mg/km to 125mg/km for commercial diesels (generally work utes and vans).
Models already being supplied to Australia have until mid-2028 to comply.
The new standard also changes the laboratory testing procedure so that it more closely matches real-world driving behaviour.
The NOx cut-off limit for petrol cars isn't changing.
Most popular diesels emit more NOx than allowed under the new rules (at least based on real-world testing results), which means many car makers must upgrade their models to keep selling them. 
The peak representative body for car makers, the Federal Chamber of Automotive Industries, told the ABC the most popular diesel models would continue to be available, as car makers have had time to prepare for the new standards.
"These regulatory changes are not expected to have a significant effect on model availability," a spokesperson said.
Nic Surawski, an expert in environmental engineering at UTS, said car makers would have to supply Australia with better cars.
"More than anything, the new standards mean manufacturers have to send us the best available technology and that's not what’s happening at present."
The European Union is now preparing to adopt even tighter emissions rules, known as Euro 7.
Dr Surawski predicted Euro 7 would be the last emissions standard for light vehicles, before combustion-engine vehicles were banned outright.
Some cities in Europe are already banning older diesel cars that, under the outgoing standards, could have been sold in Australia.
"We've had Euro 5 for far too long," Dr Surawski said.
"We've really fallen behind Europe. We've really dropped the ball."
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