August 22, 2019

expert reaction to study looking at air pollution and mortality

Research, published in New England Journal of Medicine, reports a link between exposure to toxic air pollutants and increased cardiovascular and respiratory death rates. 

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“This is a good, careful study that looks very broadly at the short-term effects of air pollution across many parts of the world. The world coverage isn’t as comprehensive as you might think from the number of cities involved. Of the cities that provided data on the larger particulate matter (PM10), 78% are from just four countries (China, where nearly half the cities are, the USA, Japan and Spain). The position is even more extreme on the smaller particles (PM2.5), with 85% of the cities in just three countries (China, USA and Japan). But despite gaps the coverage is still impressive, and it emphasises yet again the very important effects of air pollutants on human health.

“Several previous studies have established that mortality rates go up in a city, in the short term, after periods of higher air pollution. This study gives a lot of detail on exactly what happens, and indeed on how it differs from place to place. A strong point of this type of research is that it compares each city with itself, by comparing death rates at times when the pollution is relatively low with times when it is relatively high, so overall differences between different cities cancel out to a large extent. It could still be the case that the rise in death rates after high pollution episodes is, in part at least, caused by some other difference between the times of high and low pollution that is only coincidentally linked with the air pollution. But the researchers took into account aspects of the weather and also the normal time trends in mortality that occur at the same times every year regardless of pollution peaks, which increase confidence that the issue really is the air pollution. Also, they considered the possibility that the changes in mortality might be caused by some other air pollutant apart from particulate matter. The levels of different air pollutants tend to go up and down together, so the higher mortality when particulates are high might really be due to some different pollutant being high. After allowing for other pollutant levels, though, there was still a correlation between particulate levels and mortality, which suggests that pollution by particulate matter may affect mortality in addition to any effects of the other pollutants they considered (ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide).

“The researchers found that the strength of the relationship between air particulate pollution levels and mortality at about the same time did vary quite a lot from place to place. For example, on average across all the cities they looked at, an increase of 10 µg per cubic metre in PM10 levels was associated with a 0.44% increase in mortality from all causes. But across the 15 UK cities that they considered, the increase was only 0.06%, which is far smaller, and their calculations indicate that an increase of this size can’t be safely distinguished from the noise (statistical variability) in the data. In other words, their UK figure is consistent with there being no short-term effect of PM10 levels on mortality at all. The UK figure is the second lowest across all the countries from which they had data, and the lowest is Colombia (0.03%) where they had data from only one city, that might not be very typical of the whole country. The research report does mention various possible reasons for the difference between countries and regions, to do with the overall levels of pollution, how far the populations might be acclimatized to pollution by having been exposed to a lot of it, and whether populations might be more or less vulnerable because of their overall levels of health and prosperity. But the researchers did not specifically analyse data on most of these things, and the exact reasons for the differences would need considerably more research. (Incidentally, the researchers had no data from UK cities on the smaller PM2.5 particles, which are generally more damaging to health. Such data might have clarified the UK’s position.)

“How big is the (overall average) 0.44% increase in all-cause mortality associated with an increase of 10 µg per cubic metre in PM10 levels? One way to get a handle on this is to see would happen in the UK, if the overall global average rate applied. In the UK in 2017 there were 607,172 deaths. That’s 1,633 a day. If the level of PM10 increased by 10 µg per cubic metre at all times across the whole country, then the number of deaths would go up by about 7 a day, that is, a bit less than 2,700 a year. Likewise, if it were possible to decrease the level of PM10 by 10 µg per cubic metre at all times across the whole country, that would mean 7 fewer deaths each day. However, such a reduction isn’t possible, because at many places and times, even in cities, the level of PM10 is already below 10 µg per cubic metre and couldn’t be reduced by that much. However, this figure is anyway only indicative because the research indicated that the effect of PM10 on mortality could well be smaller in the UK than the overall average, and the figure they actually produced for the UK was an increase of only 0.06% for each extra 10 µg per cubic metre of PM10 concentration. That would correspond to only about 1 extra death a day in addition to the existing 1,633 deaths. And we’ve got to bear in mind that a 10 µg per cubic metre change in PM10 is quite a substantial change by UK standards. The average PM10 level at UK roadsides, usually the most polluted spots, in 2018 was 18.6 µg per cubic metre, so reducing or increasing that by 10 is quite a big change. Overall, however, this quite large change of 10 µg per cubic metre in PM10 concentrations would not make a vast change to UK mortality levels – but the change is certainly measurable globally and, bearing in mind that everyone in a city is exposed to air pollution, this is yet more indication that air pollution really is a serious public health problem across the world.

“Similar calculations for PM2.5, where the global average increase in mortality corresponding to a 10 µg per cubic metre in the pollutant concentration is rather higher at 0.68%, would lead to an extra 11 deaths a day in the UK – still not a large number compared to the 1,633 daily deaths that occurred in 2017, but definitely worthy of consideration. But again, this is based on the global average increase, not on the actual UK increase, and the UK increase may well be quite a lot different. For PM2.5 I can’t do the calculation using the researchers’ figure for the UK increase because the research did not provide that estimate.”

Prof Jeremy Pearson, Assistant Medical Director, British Heart Foundation, said:

 “Air pollution is a public health emergency and is responsible for up to 11,000 heart and circulatory deaths every year in the UK alone.

“This latest research has confirmed previous findings from smaller studies that show a significant exposure to dangerous particulate matter over 1-2 days correlates to a small, but detectable, increase in deaths – the majority of which are due to cardiovascular events.

 “These findings were true in cities with low average ambient pollution and as well as those with poor air quality. This emphasises the desperate need for stricter standards to reduce excess deaths caused by urban air pollution.

 “Currently, the UK subscribes to EU guidelines on air pollution, which are far less strict than those set out by the World Health Organisation. We urgently need to see the Government protect the nation’s health by setting out a clear and robust plan to adopt these strict guidelines by 2030.”

Prof Sotiris Vardoulakis, Research Director, Institute of Occupational Medicine, said:

“This study has analysed daily air pollution (PM10 and PM2.5) and mortality data from 652 cities around the world to establish how changes in pollution levels affect the risk of death from respiratory and cardiovascular causes. It shows that increases in particulate air pollution (very small particles in the air) are linked with increased numbers of deaths recorded every day, even in cities where the air is relatively clean. This means that reducing particulate air pollution from road traffic and other sources in cities, even where levels are currently well below the regulatory limits, will reduce the number of deaths from lung and heart disease.”

Dr Stefan Reis, Science Area Head of Atmospheric Chemistry and Effects, Centre for Ecology and Hydrology (CEH), said:

“This comprehensive study of all-cause mortality associated with exposure to air pollution demonstrates the value of long-term studies which look at various regions as they provide a better measure of the relationship between different concentrations of air pollutants and the health impacts. This makes it easier to estimate public health gains from reducing air pollution.

“In addition, the results presented make a compelling case to consider reducing exposure to air pollutant concentrations that are below current national standards and even the WHO Air Quality Guideline Value of 10 µg m^-3. This is because the pooled concentration-response curves suggest marked public health benefits of reducing concentrations even at low current ambient levels. This raises an important question for policy interventions: should emission reduction efforts focus on interventions that help to reduce concentrations below current limit values at monitoring sites, or rather aim for reducing the exposure of the overall population exposure to air pollution as a priority.

“A caveat of the study is the location of sites from which long-term datasets are available, highlighting the need for monitoring in cooler and temperate, as well as other climatic regions in order to close vital data gaps and allow for more reliable, globally representative estimates.”

Prof Chris Griffiths, Professor of Primary Care, Queen Mary University of London, said:

“This study shows that in 652 cities across six continents the higher the pollution levels, the faster people are dying. The effects are consistent for deaths of any cause, and from cardiovascular disease and respiratory disease. These are avoidable deaths. Most concerning is that deaths relating to pollution occur at levels below international recommended pollution limits. The authors provide the strongest evidence yet that target air pollution levels are set too high.”

‘Ambient Particulate Air Pollution and Daily Mortality in 652 Cities’ by Chen et al. was published in New England Journal of Medicine at 22:00 UK time on Wednesday 21^st August. 

DOI: 10.1056/NEJMoa1817364

Declared interests

Prof Kevin McConway: “Prof McConway is a member of the SMC Advisory Committee, but his quote above is in his capacity as a professional statistician.”

Prof Sotiris Vardoulakis: “As a researcher, I have applied and received grants on a wide range of outdoor and indoor air pollution topics, health impact assessment, climate change risk assessment, urban health and sustainable development. I have received funding from UK Research Councils, NIHR, EU programmes (e.g. Horizon 2020), PHE, GLA, and the industrial sector (e.g. Dyson). I have been involved in the preparation of the health effects report for the Heathrow expansion consultation. I have acted as a temporary advisor to the WHO on air quality topics, and I head the WHO Collaborating Centre on Occupational Health at IOM. I have been involved as topic expert member in the NICE Public Health Advisory Committee on outdoor air pollution, and I am currently involved in the RCPCH Working Group on indoor air quality. I co-chair the Healthy-Polis International Consortium for Urban Environmental Health and Sustainability. I am currently employed as Principal Consultant at the Institute of Occupational Medicine (IOM), Edinburgh, UK. From 1st September, I will be employed by the Australian National University as Professor of Global Environmental Health. I am also an honorary Professor at the University of Exeter Medical School, European Centre for the Environment and Human Health.”

Dr Stefan Reis: I declare no conflict of interest and have not been part of the study, or have any current collaboration with the authors.

Prof Chris Griffiths: None

None others received.