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Do nitrogen oxides – NO_x – directly damage health? Until recently their impact was considered mostly indirect, but that view is changing.

We have long known that NO_x, and in particular NO₂ – which form part of the discharge from car exhausts – are indirectly harmful. They contribute to the production of ozone and fine particulate matter less than 2.5 micrometres in size, called PM_2.5. These are the two most notorious pollutants that can have an impact on human health. But the production of those notorious pollutants has made it difficult to establish whether there are health problems connected with the NO₂ molecules themselves.

“NO₂ correlated with other damage from other emission gases, so its contribution couldn’t be disentangled,” says of King’s College London. “Now, there’s much stronger evidence for independent effects of NO₂.”

In the UK, a published earlier this month by the government as part of a plan to reduce NO_x concentrations estimated that the gases .

To put that in context, PM_2.5 prematurely killed an estimated 430,000 people in the 28 EU countries in 2012, . Ozone is estimated to lead to about 16,000 premature deaths across Europe each year.

Noxious problem

The Europe-wide impact of NO₂ hasn’t yet been established, says , head of the air pollution group at the European Environment Agency in Copenhagen, Denmark. “That’s something we will look into over the next year,” he says. But it’s clear that there is a health effect, he adds. “[NO₂] is responsible for aggravating cardiovascular and respiratory disease.”

The World Health Organization, too, recognises the problem. NO₂ is a direct hazard to health in its own right, the WHO concluded two years ago after an .

The switch from petrol to diesel vehicles is making things worse because it increases the toxicity of NO_x discharges from exhaust pipes. NO_x is a mixture of nitric oxide (NO), which is harmless, and nitrogen dioxide (NO₂). When diesel is burned in an engine instead of petrol, more NO_x is produced overall, and 70 per cent of the NO_x produced is NO₂, compared with only 10 to 15 per cent when petrol is burned.

The UK government has put forward a , especially in cities like London, where last year the levels were more than twice the EU limit of 40 micrograms per cubic metre of air.

Williams says a diesel ban in cities – as proposed last year by the Mayor of Paris – is workable, but may not be politically acceptable. But there are plenty of alternatives proposed in the consultation, including creation of clean air zones within cities, fuel duty to disincentivise diesel use and economic incentives for drivers to switch to cleaner transport, such as e-cars and hybrid vehicles.