Dieselgate and the unintended consequences of anti-idling drive

In the US, the Dieselgate scandal has resulted in prosecutions against VW personnel and multibillion dollar fines (Where’s there’s smoke…, 22 March). In Europe, no one has been charged and nobody has gone to jail, though the EU commission has threatened action against the UK government for failing to prosecute VW.

Defeat devices result in higher emissions of nitrogen dioxide, but the real danger from a health perspective are small particulates, notably the ultra-fine nanoparticles that can penetrate tissue, reach a placenta and cross the blood-brain barrier. These are largely present in exhaust emissions, so while all vehicles generate particulates from tyres and brakes, researchers have demonstrated that medical effects such as low birth weight are tied more closely to exhaust particulates than to friction particulates. This is important as the government likes to pretend that all particulates are equivalent, regardless of the source. Thus its clean air strategy emphasises the contribution of secondary particulates generated from agriculture etc, even though these contain little in the way of ultra-fine particles. It is disheartening that the UK government seems more anxious to protect the interests of car manufacturers than the health of its own citizens, but this situation is likely to worsen post-Brexit.
Dr Robin Russell-Jones Scientific adviser, Geraint Davies MP Chair, All-party parliamentary group on air pollution

• The proposals from Public Health England (PHE) that you reported (Ban cars from idling near schools, says UK public health agency, theguardian.com, 11 March) are welcome – positive action against all sources of air pollution, such as such as highly polluting vehicles and wood-burning stoves, is undoubtedly essential. However, I regret that PHE’s proposals on idling are not fully thought through.

Of course, an idling vehicle will emit more CO₂ and use more fuel than one that is switched off, but that is not the whole story. Other pollutants such as particulates, NOx, unburned fuel and carbon monoxide are carefully controlled on modern vehicles by filters and catalysts – and catalysts do not work when they are cold, and take some time to come up to temperature.

The studies mentioned by PHE in its report advocating anti-idling do not consider most of these other pollutants, and there is a real risk of unintended harm, as emissions on start-up (whether warm or cold) often form a substantial part of the total emissions from a vehicle’s journey. A study from the US Department of Energy showed that a vehicle would have to be idling for 10 minutes or three hours to emit as much NOx or carbon monoxide (respectively) as a single (warm) restart. There is a significant need for more research in this area – particularly on modern vehicles that use substantial levels of exhaust gas after treatment to control emissions.

There is a serious need to tackle air pollution, but to do so effectively will requires coherent interdisciplinary engagement and evidence-based policy making. The PHE document is a well-intentioned start, but we have a long way to go.
Dr Felix Leach
Associate professor of engineering science, Shell-Pocock fellow and tutor, Keble College, University of Oxford

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