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expert reaction to reports that Brixton Road has measured 19 breaches of the hourly nitrogen dioxide threshold, exceeding the number of allowed breaches in a year

Brixton Road has become the first place in London to breach objectives for nitrogen dioxide for 2017. UK objectives and EU limits stipulate a maximum nitrogen dioxide concentration that must not be exceeded for more than 18 hours over the whole year.


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

“The exceedance of the annual short-term NO2 limit by a street in London in early January is becoming an annual event.  Last year it was Putney High Street after eight days, in 2015 Oxford Street, after only two days.  Which street gets there first is partly a matter of chance given that the majority of main roads in London will breach the limit at some stage.  Pollutant dispersion conditions have not been good, but nothing very unusual for this time of year.

“The limit allows 18 hourly exceedances a year – so a single episode can be enough to breach it.  This is particularly true in London because the capital is becoming ‘the city that never sleeps’ – traffic continues through most of the 24 hour cycle.  Looking at the trace for Brixton Rd in 2017, the NO2 concentration most days stays high until 1.30am after which it falls somewhat, but it starts to rise again about 4.30am.  On New Year’s Day there was no fall at all – not surprisingly perhaps.

“The monitor on Brixton Road is a kerbside monitor, drawing air in at breathing height – so it is measuring what a pedestrian might be exposed to when walking down the road.  It currently appears to be working well, and the technology is established and reliable.

“The evidence that NO2 at these concentrations is harmful to health is largely epidemiological (e.g. an increase in hospital admissions after a pollution episode).  Studies in the laboratory with controlled concentrations of NO2 tend to show that higher concentrations are required to cause measurable effects – but it is hard to relate these to real-world conditions when other pollutants are present.  Allowing 18 hourly exceedances is a largely arbitrary condition, probably designed so that a single episode does not trigger the limit – but as noted above this does not work very well in London.”


Dr Zongbo Shi, Programme Leader MSc in Air Pollution Management and Control, University of Birmingham, said:

What are the nitrogen dioxide limits?

“There are two limits:

– Annual limit of 40 µg/m3 (the concentration averaged over a year): no permitted exceedances.

– Hourly limit of 200 µg/m3: maximum of 18 exceedances.

If the annual average concentration is above 40 µg/m3, it breaks the limit; if there are more than 18 exceedances (over 200µg/m3), then it also breaks the limit.

Is this limit related to particulate pollution or is it different?

“No, they are different issues.  Road traffic contributes to particulate matter pollution but there are other sources; nitrogen dioxide pollution is primarily due to vehicle emissions.”

How does the UK compare to other countries?

“Our nitrogen dioxide pollution level is one of the highest in Europe and comparable to one of the most polluted megacities in the world: Beijing.”

Why does it matter – what is the strength of the evidence linking nitrogen dioxide to health problems?

“There is robust and growing evidence of the associations of ambient concentrations of nitrogen dioxide with a range of effects on health such as all-cause, respiratory and cardiovascular mortality, children’s respiratory symptoms and lung function.”


Prof. Ian Colbeck, Professor of Environmental Science, University of Essex, said:

“This is not surprising given yesterday’s report by the International Council on Clean Transportation (ICCT) that real world emissions of nitrogen oxide from new diesel cars (500 mg/km) were more than double those from lorries and buses and more than six times the legal limit.

“As part of the European Real-Driving Emissions (RDE) regulation, from September 2017 new vehicles can be tested under real-world driving conditions.  This is a significant step to help reduce emissions and so improve air quality.  While recognising the problem the European Commission is planning to bring forward discussion of additional revisions to the existing RDE regulations.”




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.”

Dr Zongbo Shi: “No conflict of interest.”

Prof. Ian Colbeck: “Member: Institute of Physics, Royal Meteorological Society, The Aerosol Society, International Society of Indoor Air Quality and Climate.  Funding: NERC, EU.  Voluntary appointments: Essex Air Quality Consortium.”

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