It is well known that cars produce dangerous levels of toxic fumes for passing pedestrians and cyclists. That threat increases substantially when vehicles sit idly in heavy traffic.

However, researchers have recently discovered that the fumes inhaled from sitting inside a car can be more deadly than being out of it.

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That is because ventilation systems meant to purify the incoming air are still letting in high volumes of toxic particles, researchers have said, meaning car passengers are experiencing them in more confined spaces than pavements.

Scientists at Emissions Analytics have even found that the levels of toxins inside a car can reach 10 million particles in every breath – more than 10 times the comparable amount when outside the car.

Nick Molden of Emissions Analytics told the Times: "Drivers and their passengers can be getting very large doses of particulates if they are in a car with poor ventilation.

"The particles are so tiny – as small as 23 millionths of a millimetre – that they are invisible but long-term exposure is very bad for drivers' health."

Emissions Analytics have so far tested six cars and documented how much pollution builds up inside each vehicle. The firm said it would publish its findings when it has tested enough cars to give consumers better choices.

Of the cars it has already tested, the best-performing vehicle recorded an average 75 particles in each cubic centimetre (pcc) and a maximum of 2,400pcc. The worst-performing car recorded nearly 20,000pcc and a high of 435,000pcc.

On average, adults inhale 500cc of air per breath meaning that someone inside the worst-performing car would breathe in 10 million particles.

There are no regulations on air quality inside vehicles. However, the United Nations is in the process of overseeing talks to develop a new global standard.