Safety tests on 11 types of car have shown huge variation the the ability to purify incoming air, exposing those inside to millions of poisonous particles with each breath.
The Toyota C-HR performed worst, blocking out just 1 per cent of pollutants, while the VW Polo managed only 35 per cent and the Ford Fiesta 40 per cent.
However, the Mercedes E-Class was able to purify 90 per cent of the incoming air, demonstrating that the technology exists to substantially protect passengers.
Emission Analytics, the laboratory which carried out the tests, said the disparity was due to an absence of Government standards regulating air filtration systems.
Nick Molden, its chief executive, told The Sunday Times: “Drivers can be exposed to high pollution levels while believing themselves to be protected by the air filtration and ventilation system.”
The Society of Motor Manufacturers and Traders (SMMT) confirmed that the lack of regulation means car firms can use whatever specification of air filter they want, but said the industry is “working with policy makers” to decide if new rules are required.
The hazardous pollution comes mainly from the exhausts of other vehicles, which is full of “particulates” which measure a few millions of a millimetre.
They are dangerous because their size enables them to enter the bloodstream via the lungs.
Pollution is particularly hazardous for asthmatics, and also contributes to heart disease, stroke and some cancers.
Emissions Analytics discovered up to 57,000 particles per cubic centimeter in some roadside air samples.
Because people typically inhale around 55cc of air, that means pedestrians are inhaling 28 million particles per breath.
Even in cars whose filters remove 40 per cent of particles, such as the Ford Fiesta, those inside would still inhale nearly 13 million particles with each breath.
Tests were also carried out on the new £40,500 Jaguar E-Pace, which stopped 43 per cent of pollution, the VW Touran, which stopped 59 per cent, and the Vauxhall Astra, which blocked 83 per cent.
“There is little data to tell consumers what they are buying,” said Mr Molden.
“So if you have kids with asthma or other conditions you cannot tell if the car you are buying will protect them.”
He added the lack of protection may leave the employers of those driving as part of their work open to being sued.
“Their vehicles are their workplace so they may be subject to health and safety laws,” he said.
“Our research suggests many vehicles are a risk to their drivers’ health”.
While the majority of cars have a “recirculation” switch which blocks external air, this has been shown to risk causing drowsiness because it increases carbon dioxide levels caused by the occupants’ breathing.
Mike Hawes, SMMT chief executive, said: “Manufacturers have developed technology, like automatic climate control and active carbon filters, in response to consumer demand rather than legislative requirement.
“Given the absence of a regulatory standard, specifications vary”.
VW said its vehicles were fitted with multiple filters to remove particles and it intended to attach a particulate sensor to future models.
Meanwhile Toyota UK said it was not aware of pollution problems found Emission Analytics results “surprising”.