Air pollution could be responsible for hundreds of car accidents a year, according to the London School of Economics.
A study looking a five years of data showed that when levels of nitrogen dioxide (NO2) rise just one microgramme per cubic metre, the number of collisions rises by two per cent.
Although it might seem that effect could be explained by more traffic on the roads, and therefore more pollution and more accidents, the researchers found that the increase remained even when adjusting for the extra trafficInstead, they believe that the toxic air impairs driver fitness, while watery eyes and an itchy nose could also be distracting for motorists.
A recent study found that air pollution inside a car can be more than double that on the outside because the NO2 builds up in a small space.
Lead researcher Lutz Sager of the Grantham Research Institute on Climate Change and the Environment at the LSE said: “Although it has already been shown that air pollution adversely affects human health and the ability to carry out mental tasks, this is the first published study that assesses the impact on road safety.
“The analysis identifies a causal effect of air pollution on road accidents, but I can only speculate about the cause of the link.
“My main theory is that air pollution impairs drivers’ fitness. However, other explanations are possible such as air pollution causing physical distractions, perhaps an itching nose, or limiting visibility.”
Air pollution can result from many different toxins, including carbon monoxide, nitrogen dioxide, sulphur dioxide, small particulate matter and ozone. But it was NO2 which was found to have the biggest impact.
Mr Sager, a postdoctoral candidate, divided the UK into a grid of 32 areas each covering about 4784 square miles (7700 sq km) and mapped accidents to the level of air pollution between 2009 and 2014 provided by the Department for the Environment (Defra)
He found a rise in the concentration of nitrogen dioxide of just one microgramme per cubic metre above the daily average is sufficient to increase the average number of accidents each day by two per cent, with the biggest effect occurring in cities.
Mr Sager calculated that in the area containing west London, which suffers from some of the highest levels of air pollution, a cut of about 30 per cent in the concentration of NO2 could reduce the number of road accidents every day by almost 5 per cent.
Levels of NO2 in polluted areas of London can reach beyond 97 microgrammes per cubic metre on average.
There are around 150,000 collisions in which someone is injured in Britain every year so preventing just two per cent of crashes could avert thousands of accidents.
Mr Sager added: “Whatever the exact mechanisms responsible, the robust finding of a significant effect of air quality on road safety is important given the high cost of road traffic accidents through damage to vehicles and deaths and injuries to people every day.
“Although this analysis has used data for the United Kingdom, I think my findings are relevant to other parts of the world. These additional costs from traffic accidents strengthen the case for reducing air pollution, particularly in congested cities.
“My analysis suggests that the causal effect of air pollution on road traffic accidents measured in this study more likely stems from nitrogen dioxide or other pollutant gases rather than particulate matter.”
However other experts were more sceptical about the link between air pollution and accidents.
AA president Edmund King said: “If you think about areas which are high in air pollution they are a lot busier, with taxis and buses and lorries and where you have a greater mix of traffic you tend to have more accidents.
“It would be hard to tease apart whether a crash is caused by a driver wiping his eyes because of pollution or the type of traffic which is to blame.
“If you look at Mumbai and New Delhi where you have some of the worst air pollution, yes you have far more accidents, but it is also far more chaotic.
“So I think this research may be far-fetched as I think it would be very difficult to prove that a driver’s fitness is impaired by pollution.”
The results of the study are published today as a working paper, and will be submitted for peer review in the coming weeks.