February 15, 2018

expert reaction to study looking at rapid air pollution increases and heart attacks

A new paper, published in the European Journal of Preventive Cardiology, suggests that rapid increases in pollution may be as harmful to the heart as sustained high levels.

Prof. Metin Avkiran, Associate Medical Director at the British Heart Foundation, said:

“This study further highlights the daily danger that people face from breathing dirty air.  We already know that air pollution, including various gases and small particles, contributes to an estimated 40,000 premature deaths in the UK each year.  This research indicates that even temporary increases in nitrogen dioxide levels may elevate the risk of having a potentially fatal heart attack.

“If confirmed by larger studies, this is likely to reshape thinking around the impact of air pollution on heart health, to take into account the importance of sharp increases in pollution levels.  We also need research to determine how a temporary increase in nitrogen dioxide levels might trigger a heart attack.

“There is now a wealth of evidence which clearly demonstrates the harm air pollution causes.  Adopting the World Health Organisation’s air quality guidelines would help ensure the nation’s heart health is better protected from the potential risks they pose.”

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

“In a paper published in the European Journal of Preventative Cardiology, a group of researchers present evidence that a ‘rapid’ (daily) increase in nitrogen oxides in the air is associated with an increased risk of heart attacks two or three days later.  The study was conducted in Jena, a small city in central Germany, and is significant because Jena is a ‘clean air’ city, meeting the EU standards for nitrogen oxides on both an annual and hourly basis.  Nonetheless, the study showed that if the average nitrogen oxide (NOx) concentration rose by more than 20 micrograms per cubic metre in successive days, then the risk of a heart attack more than doubled, and even a rise of 8 micrograms per cubic metre increased the risk by more than 70%.  The study authors suggest that, if their findings are confirmed, there should be statutory limits in the rate of increase of nitrogen oxides as well as on the absolute levels.  This would be a difficult standard to implement.  I have to admit to being a little sceptical about these findings.  The authors claim to have controlled for meteorological changes (also known to affect the risk of cardiovascular events) but they do not present any evidence of the size of these effects in the paper.  The component of NOx generally considered to be most damaging is NO2, but increases in NO2 do not have a significant effect on the risk of a heart attack in the study.  The findings are intriguing, and as the authors say deserve further study elsewhere, but I would not be rushing to alter the EU standards just yet.”

Prof. Tim Chico, Professor of Cardiovascular Medicine and Honorary Consultant Cardiologist, University of Sheffield, said:

“The risk of someone having a heart attack at some point in their life is determined by lifestyle and genetics, but the reason it happens on a particular day rather than any other day remains unclear.  This study finds an association between rising levels of some air pollutants and the risk of having a heart attack in that period, and the authors suggest that it may be the rise in these pollutants that cause the increased risk of a heart attack (up to about a doubling of risk).

“These results are interesting, but certainly not conclusive.  It is known that people are more likely to have heart attacks at certain times of the year (winter), or even on certain days of the week (Mondays) and these may also be the times when air pollution is rising, which would also account for these results, but the authors have not accounted for other possible explanations.

“Nevertheless, it is clear that air pollution does pose a very serious risk to health, and does increase the risk of heart disease.  Leaving the car behind and walking reduces the risk of a heart attack in both the walker and the people around them.  We urgently need to redesign our society to encourage more physical activity and less traffic pollution.”

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

“Measuring health effects of air pollutants is not easy.  The levels of different pollutants tend to be related to one another, so it’s not easy to tease out their effects from each other.  Also, there are often several differences between people exposed to different levels of air pollution, that aren’t directly related to the pollution, and so it’s difficult to know whether it’s the pollution that is causing any health differences, or something else entirely.  (Such a ‘something else’ is called a confounder.)  Things can get even more difficult to disentangle if, like these researchers, you’re interested in the effects of changes in pollution levels, rather than just the levels themselves.

“This very interesting piece of research used a clever method to get round several of the difficulties.  Instead of comparing different people who had been exposed to different changes in air pollutants, the researchers used people who had had heart attacks as their own controls.  That is, they compared changes in the levels of common air pollutants, in the days before a participant had a heart attack, with the changes in pollutant levels a week earlier and a week later, when the same participant did not have a heart attack.  Comparing the same person at different times means that there are no systematic differences between the people being compared, so confounders (such as gender, age, smoking behaviour, and so on) that relate to differences between individuals are no longer a major problem.  Also, comparing pollution changes on days exactly a week apart avoids problems of comparability between different days of the week.

“Unfortunately, as the researchers acknowledge, this doesn’t get over all such difficulties of interpretation.  The study does provide evidence that heart attacks are more likely on days shortly after levels of nitrogen oxides in the air have increased rapidly, but it can’t tell us for certain whether the increased heart attack risk is caused by the rapid increases in these pollutants.  There may well be other differences between days when the nitrogen dioxide levels increase rapidly, and days when they don’t.  Perhaps some other air pollutant also increases at the same time as the nitrogen oxides.  Probably there are differences in weather conditions, or in traffic levels, that cause the changes in pollutant levels, and this sort of research can’t tell us directly whether it’s the rapid rise in nitrogen oxides that causes the increased heart disease risk, or these other changes that are related to the changes in nitrogen oxides.  These researchers did take account of certain changes in weather in their statistical analysis, but it’s impossible to be certain that all possible other variables have been taken into account completely.  The researchers themselves point out that one rather counter-intuitive finding, that heart attacks were rather more likely after a rapid decrease in ozone levels, could well be explained by other variables.  It seems quite likely that, when ozone decreases, some other air pollutants that were not included in this study (such as very fine particulates, PM2.5) tend to increase, so that the increased heart attack risk when ozone decreases might well really be caused by an increase in these other pollutants rather than by the decrease in ozone levels.  I did say that it’s difficult to sort one thing out from another in relation to air pollution.

“Another issue is that this study mostly didn’t find very strong statistical evidence of the possible effects of changes in pollutant levels that it considered.  That may well be just because there is a lot of variability, in pollutants and in health, from a whole range of other causes, and there just wasn’t enough information from the 693 heart attack patients that they considered to produce a clear statistical picture.  Also, Jena is just one city – it remains possible that there’s something special about Jena in relation to pollution and health and that things would be different elsewhere.  That’s why I welcome the researchers’ recommendation that larger scale studies should be carried out in different geographical regions.  If these larger studies do support the researchers’ suspicions, from this study, that rapid changes in pollutant levels may be an important risk factor for heart attacks, then that could have a considerable impact on the way we think about air pollution and health, and on policies for dealing with it.  But this study on its own can’t provide enough information to sort this out clearly.”

* ‘Rapid increases in nitrogen oxides are associated with acute myocardial infarction: A case-crossover study’ by Marius Rasche et al. published in the European Journal of Preventive Cardiology on Thursday 15 February 2018.

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. Tim Chico: “No conflicts.”

Prof. Kevin McConway: “Kevin McConway is the lead author of a chapter on Measurement and Communication of Health Risks from Pollution for a forthcoming Annual Report of the Chief Medical Officer.  He is a member of the Science Media Centre’s Advisory Committee.”