Two studies published in the BMJ found an association between exposure to high levels of aircraft noise and an increased risk of cardiovascular disease. One study looked at residents living close to Heathrow airport, whose results were presented at an SMC briefing and scrutinised in a before the headlines analysis, while the other looked at 89 North American airports.
Commenting on both the Heathrow and the USA studies:
Professor Kevin McConway, Professor of Applied Statistics, The Open University, said:
“Both of these studies are thorough and well-conducted. But, even taken together, they don’t prove that aircraft noise actually causes heart disease and strokes. Because of the kind of data the researchers used, the studies can’t do more than suggest very strongly that we should find out much more about aircraft noise and circulatory disease, using different kinds of study that could come much closer to sorting out what causes what. The researchers make these limitations clear in their reports.
“A major difficulty in interpreting what these studies tell us is that they are based on data for geographical areas, not for individual people. The Heathrow study used census output areas, with about 300 inhabitants on average, and also the larger (super output areas) areas with about 1500 inhabitants. The American study used even larger areas – zip (postal) code areas, each with (on average) about 2700 of the people aged 65 and over that they studied. Both studies found that hospital admission rates for heart disease and strokes were higher than average in the areas most affected by aircraft noise, and the Heathrow study found a similar relationship for death rates from these diseases. But geographical areas don’t get heart attacks and strokes – individual people do. Within any one of the areas they studied, individuals vary in terms of how much aircraft noise they personally are exposed to, depending on how much time they actually spend at home in the area, how good the sound insulation of their home is, and a whole host of other reasons. Within an area, these studies can’t tell us whether it’s the people most affected by aircraft noise who are most likely to get heart disease or have a stroke. So the studies can’t directly tell us whether it is the aircraft noise that is affecting individual people’s chances of these diseases.
“Another issue that makes these results hard to interpret is that the areas in question will differ in ways other than the level of aircraft noise, and it might be these other differences that are the real cause of the difference in cardiovascular disease rates. (This is the issue of ‘confounding’.) Both studies make very worthwhile attempts to allow for confounding in their statistical analyses. But, as they both acknowledge, these adjustments cannot be perfect. In the Heathrow study, the adjustments were all made using area-level data, so the problems I’ve already mentioned arise again. The American study did adjust for age, sex and ethnicity at an individual level, but their other adjustments were at area level, and they did not have data to adjust for smoking, even though smoking is a well-known and major risk factor for these diseases. The Heathrow study did adjust for smoking at least in part – but neither study had the data to adjust for differences in diet or exercise.
“The results of both these studies might have arisen because aircraft noise causes heart disease and strokes. But they might also have arisen in other ways. What they mainly tell me is that we need further studies to understand better what’s going on, and I hope such studies are done.
“One specific point worth making about the Heathrow study, to avoid confusion, is as follows. Although they did indeed study a large region around Heathrow, with 3.6 million inhabitants, and although all the areas they studied fell at least in part within the 50dB daytime noise contour for Heathrow aircraft, their results do not imply that all these 3.6m people might be at increased risk of cardiovascular disease. The research found significant evidence of increased risk only for areas where the noise levels were particularly high (over 60dB in the daytime), and only about 120,000 of the 3.6m people studied live in these areas. (The US research also found a significant relationship between aircraft noise and cardiovascular disease only at the higher noise levels.) I’m not saying that there is no problem if 120,000 people are exposed to a potential cause of disease – only that it’s not a problem affecting millions of people around Heathrow.”
Commenting on just the Heathrow study:
Professor David Coggon, Professor of Occupational and Environmental Medicine, University of Southampton, said:
“This study adds to the suspicion that noise from low-flying aircraft may cause heart disease. However, the method of study had limitations which are acknowledged by the authors, and the increased rates of disease that were observed may have been caused by factors other than noise, such as smoking and South Asian ethnicity. We know, for example, that South Asian ethnicity was more common in the areas with higher exposure to aircraft noise, and it could be that this was responsible for the increased risk of cardiovascular disease rather than the noise itself. The authors did attempt to account for smoking and ethnicity in their statistical analysis, but the scope for such adjustment was limited. Although the findings should be an encouragement to further more detailed research, they do not constitute strong evidence of a hazard.”
Dr Nic Timpson, Reader in Genetic Epidemiology, University of Bristol, said:
“This is an interesting paper which has considerable data collection behind it. The overarching hypothesis that aircraft noise exposure (which will vary for everyone and thus is different to directly measured aircraft noise) relates to a suite of biological factors, including the activity of the sympathetic nervous system, which can potentially influence the risk of cardiovascular events. Data on aircraft noise itself are taken from the Civil Aviation Authority where noise data was modeled using the UK Civil Aircraft Noise Contour Model ANCON using information on flight paths of arriving and departing aircraft along with factors such as height, speed, and engine power to derive noise at ground level. The combination of this information with health registry data is an exciting resource of good scale. Important to the interpretation of this work is that directly measured aircraft noise is not necessarily the same as aircraft noise exposure.
“My key concern with this work is that it is based on noise exposure estimates for given areas. This raises the question is as to what this variable exposure could be related to which could confound associations seen between directly measured aircraft noise and health outcome. It seems highly likely that estimated noise exposure will be correlated to the mediating effects of protection from it. This is likely correlated to living arrangements, schooling, workspace and others, many of which will be correlated with the complex suite of confounding factors, which also contain risk factors for stroke/cardiovascular disease. This therefore begs the question as to whether this potentially complex confounding structure (which I recognise as extremely difficult to measure) has been accounted for appropriately. The authors to take into account age, sex, ethnicity, deprivation, and a smoking proxy in their analyses, but this list is certainly not comprehensive.
“These concerns are extended by the observation that the greatest impact on observed associations was after adjusting for South Asian ethnicity. This group is known to have substantively different risk of stroke in particular and the impact of this adjustment (and all of the additional complications with respect to the confounding structure surrounding that demographic in this part of London) is one which potentially highlights the insufficiencies in confounder measurement in this case. Further to this, the residual effects (i.e. those after adjustment for the confounders available) that were found have lower bounds (i.e. the lowest values they are likely to be consistent with) consistent with small impacts on risk.
“The authors do not go on to provide an independent replication of these effects in other populations where the confounding structures may be different and which would provide greater evidence of a noise specific effect. They do however present the evidence context for this study which contains some similar (and some smaller) studies which have less than convincing suggestions of a causal impact of aircraft noise.
“Overall, this is an interesting study, however the authors’ own statement that: “As well as the possibility of causal associations, alternative explanations such as residual confounding and potential for ecological bias should be considered” should not be taken lightly in this instance.”
‘Aircraft noise and cardiovascular disease near Heathrow airport in London: small area study’ by Anna Hansell et al., published in the BMJ on Tuesday 8 October 2013.
‘Residential exposure to aircraft noise and hospital admissions for cardiovascular diseases: multi-airport retrospective study’ by Andrew W Correia et al., published in the BMJ on Tuesday 8 October 2013.