Pollution from many popular diesel cars is much worse when it is colder than 18C outside, new research suggests.
Car exhaust pollution (Image: Wikipedia)
Testing company Emissions Analytics told the BBC it has measured a significant rise in poisonous gas emissions from a wide range of models as the temperature drops.
It found the problem is worst among the Euro 5 category of cars, which became mandatory in 2011.
The firm tested 213 models across 31 manufacturers.
The finding means millions of vehicles could be driving around much of the time with their pollution controls partly turned off.
But it seems many cars are deliberately designed that way and it is all perfectly legal.
Taking advantage
European rules allow manufacturers to cut back on pollution controls as long as it is to protect the engine.
Engineers agree that hot and cold weather can damage components.
But some suggest car companies are taking advantage of the rule to switch things off, even in mild weather, because it improves the miles per gallon of the car.
“I would say from the Euro 5 generation of cars, it’s very widespread, from our data. Below that 18 degrees [Celsius], many have higher emissions… the suspicion is, to give the car better fuel economy,” Emissions Analytics CEO Nick Molden told the BBC.
“If we were talking about higher emissions below zero, that would be more understandable and there are reasons why the engine needs to be protected. But what we’ve got is this odd situation where the [temperature] threshold has been set far too high, and that is a surprise”.
Read more: BBC