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expert reaction to study looking at diesel emissions, nitrogen oxides and premature deaths

In a new study, publishing in Nature, scientists report that almost one-third of emissions from heavy-duty diesel vehicles, and over half of emissions from light-duty vehicles, in 11 markets around the world exceed certification limits for nitrogen oxides.


Prof. Richard Skeffington, Department of Geography & Environmental Science, University of Reading, said:

“The most surprising thing about this latest estimate of the deleterious effects of NOx emissions from diesel engines is that it does not address the same issue that led to the successful challenges to the Government’s Air Quality Plan by Client Earth – that is that the concentration standard for NOx is widely exceeded in British cities.  Instead, the authors calculate the secondary effects of NOx in producing two other pollutants – fine particles (PM2.5) and ozone.  NOx reacts with other atmospheric pollutants in the atmosphere to generate ozone and contribute to PM2.5 – note that the paper does not deal with PM2.5 directly emitted by diesels.  Because assumptions have to be made about the concentrations of these other pollutants in order to do the calculations (and the assumption is that these will not change in future) the numbers of excess deaths calculated has considerable uncertainty.

“In not calculating the direct effects of NOx on mortality, the paper authors seem to be following the view of the US Environmental Protection Agency (USEPA) that the effects of NOx apart from on respiratory complaints are only “indicative”.  The USEPA tends to be more cautious than the World Health Organisation in attributing effects to pollutants – they had lawyers from industrial lobby groups breathing down their necks well before the Trump era.  The direct effects of NOx can however be calculated with fewer assumptions and thus more certainty than the indirect effects mentioned in the paper, and DEFRA have included them in estimates of air-pollution induced mortality in the UK.  The deaths calculated in this paper are partly in addition to those (the present PM2.5 -related mortality is already accounted for by DEFRA).

“The paper uses estimates of real-world NOx emissions by diesels to calculate mortality – these can be three to five times higher than the certificated emissions by the same vehicles.  In most of the world, heavy vehicles dominate diesel NOx emissions – only in the EU is the use of diesel in light duty vehicles large enough to become the dominant source.  The paper authors calculate the effects of applying current EU standards in other markets and in enforcing standards using more stringent real driving emissions (RDE) testing.  What is very striking is that (according to the paper) application of current standards to emerging markets would prevent a predicted large increase in mortality as vehicle use rises, and also that enforcing RDE rigorously in the EU with current technology would lead to considerable reductions in mortality.  On the horizon are next generation standards which would more or less eliminate diesel NOx emissions.  It seems we have the technology to tackle the deleterious effects of diesel NOx emissions – we just need the will to apply it.”


Prof. Bill Collins, Professor of Meteorology, University of Reading, said:

“This study shows how the impact of NOx emissions from diesel vehicles is more widespread than just in the city streets.  Breathing in NO2 directly is damaging to health, but the NO2 also goes through reactions in the atmosphere that lead to fine particles (PM2.5) and ground-level ozone which are also harmful.  These last for longer in the atmosphere than NO2 and so can be spread much further.

“Anenberg et al have used global computer models of atmospheric pollution to show that these chemical products (called secondary pollutants) from diesels can spread far from the cities over wide regions, causing widespread health problems and damaging crops.  They estimate that the failure of vehicles to even meet their certified emission standards leads to 38,000 premature deaths per year, whereas imposing the most stringent current emissions standards globally could save over 100,000 premature deaths.  This would be likely to be even higher if the direct health impacts of NO2 were considered (not the subject of this study).  The authors find that these NOx pollution emissions slightly cool the climate, but by a negligible amount compared to the warming from the CO2 emissions.”


Prof. Roy Harrison FRS, Professor of Environmental Health, University of Birmingham, said:

“This is a rigorous study which highlights the serious consequences which have resulted directly from the irresponsible actions of the motor manufacturers in producing vehicles which meet regulatory requirements under test conditions, but emit far higher pollutant levels during on-road use.

“The study may well underestimate the full consequences for public health as it quantifies only the effects of particulate matter and ozone formed in the atmosphere as a result of excess nitrogen oxides emissions, but not the direct effects of the oxides of nitrogen themselves.  The latter effects remain controversial, but there is growing evidence for effects of nitrogen dioxide on premature deaths and disease incidence, which would likely well exceed the indirect effects via particulate matter and ozone.  Our own analysis of data from London using plausible risk coefficients from a WHO evaluation suggests that the premature mortality effects of nitrogen dioxide from road traffic far exceed those from exhaust emissions of particles1.  There are also very important consequences of nitrogen deposited from the atmosphere on terrestrial and aquatic ecosystems which need to be accounted for.”

  1. ‘Efficacy of Recent Emissions Controls on Road Vehicles in Europe and Implications for Public Health’ by Roy M. Harrison & David C. Beddows, Scientific Reports, April 2017


Dr Paul Young, Lecturer and Atmospheric Scientist, Lancaster Environment Centre, Lancaster University, said:

“High NOx emissions from diesel vehicles has been a large part of the reinvigorated public interest in air quality in recent years, including the realisation that the actual emissions while driving can be higher than those estimated from more controlled lab studies.

“This work is a thorough assessment of the size of the gap between real world and lab-estimated emissions, showing that the former can be up to five times higher than the emission standard.  This means more NOx, more particulate matter and more ground level ozone, all of which have bad effects on health, plants and even our buildings and infrastructure.  One can quibble with the authors’ estimates of the impacts – and they do acknowledge the uncertainty – but they all point in one direction: more deaths and lower crop yields.

This work points to the need to do two things: (1) get better real world estimates of vehicle emissions, and (2) have regulatory procedures that stipulate meeting real world driving emissions.  This study has made great strides towards number 1), and – as the authors say – regulators are moving towards number (2).”


* ‘Impacts and mitigation of excess diesel-related NOx emissions in 11 major vehicle markets’ by Susan C. Anenberg et al. published in Nature on Monday 15 May 2017.


Declared interests

Prof. Richard Skeffington: “I don’t think I have any significant vested interests. I teach a third year / masters course in air pollution at the University of Reading where I have been since 1999. From 1977-1999 I worked, mostly on air pollution, in the electricity supply industry, and I do consultancy work. Currently I don’t have any air pollution-related research grants.”

Prof. Bill Collins: “No conflict of interest. I have previously published with a few of the authors, but have not be involved in this study at all.”

Prof. Roy Harrison: “I am an Honorary Fellow of the Faculty of Public Health and an Honorary Member of the Chartered Institute of Environmental Health, both of which campaign for cleaner air.  This University has research contracts with Jaguar Land Rover and I am a partner in a bid to Innovate UK led by JLR to develop cleaner vehicles.”

Dr Paul Young: “Regarding air quality, I am organising a workshop in Africa on AQ and in receipt of (UK and international) funds for supporting research. More generally, I am in receipt of research funds from NERC, EPSRC and the ERDF to investigate various issues related to atmospheric composition, climate change and air quality.”

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