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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 NO 2 – 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 NO 2 molecules themselves.


“NO 2 correlated with other damage from other emission gases, so its contribution couldn’t be disentangled,” says Martin Williams of King’s College London. “Now, there’s much stronger evidence for independent effects of NO 2 .”

In the UK, a document published earlier this month by the government as part of a plan to reduce NO x concentrations estimated that the gases kill up to 23,500 UK citizens prematurely each year.

To put that in context, PM 2.5 prematurely killed an estimated 430,000 people in the 28 EU countries in 2012, according to the European Environment Agency. Ozone is estimated to lead to about 16,000 premature deaths across Europe each year.

Noxious problem

The Europe-wide impact of NO 2 hasn’t yet been established, says Martin Adams, 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. “[NO 2 ] is responsible for aggravating cardiovascular and respiratory disease.”

The World Health Organization, too, recognises the problem. NO 2 is a direct hazard to health in its own right, the WHO concluded two years ago after an in-depth investigation of its impact.

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 (NO 2 ). 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 NO 2 , compared with only 10 to 15 per cent when petrol is burned.

The UK government has put forward a plan to tackle NO x pollution, 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.

When this article was first published, it confused nitrogen dioxide and nitrous oxide at one point.