A new study has discovered that diesel-engined vehicles release much higher levels of emissions in colder weather than previously thought.
The vehicle emissions data service Emissions Analytics carried out the research on 213 models from 31 manufacturers and found a significant rise in poisonous gas emissions from vehicles as temperatures decrease.
This rise is caused by manufacturer-fitted engine management systems that are designed to partially switch off in the cold, in order to protect the engine against the extreme temperature and boost its mpg.
While this is not an illegal process – European rules allow manufacturers to reduce pollution controls to protect the engine against temperature extremes – some companies are thought to be taking advantage and programming the controls to switch off in mild weather.
Cars made since 2011, which fit into the Euro 5 category – of which there are 5.1 million on Britain's roads – were found to be the worst offenders, Emissions Analytics CEO Nick Molden told the BBC.
"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," he commented.
Emissions Analytics found the average Euro 5 compliant-model to be 3.6 times over the Nitrogen Oxides (NOx) limit at above 18 degrees. This increased to 4.6 times the limit at lower temperatures.
Meanwhile, Euro 6 cars, which went on sale in September 2015, averaged 2.9 times the limit above 18C, rising to 4.2 times the limit at lower temperatures. However, three particularly bad performers, who Molden declined to name, reportedly lowered these results.