A study published in Current Problems in Cardiology looks at air pollution and blood pressure in adolescence.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“This is a well-designed and well-conducted piece of research that uses appropriate statistical methods, and reports its findings appropriately and clearly. But a major part of its conclusions is that there are some serious limitations in what it could find, because of limitations in the previous studies it reviewed, and that therefore more and better research should be carried out before we can really see what’s going on. So the interpretation isn’t straightforward, despite the (in my view) rather clear-cut tone of the press release. But, overall, that’s no reason to be complacent about harmful effects of air pollution.
“This is a systematic review and meta-analysis of previous studies that aimed to measure the association between air pollution levels in the places people lived and blood pressure in adolescence. Some of the limitations stem from the fact that all the studies that were reviewed were observational. People weren’t allocated by the researchers on those studies to live in places with different pollution levels – they lived where they lived, the air pollution was recorded, and their blood pressures were measured. As the researchers on the new study point out, people who are exposed to different levels of air pollution also differ in many other ways, such as gender, ethnicity, socio-economic conditions, or the traffic noise where they live, and all of those things can also be associated with blood pressure. So if it turns out that adolescents living in areas with higher air pollution also have higher blood pressure, that could be caused by the pollution, or by some of the other differences that are associated with pollution levels, or some combination of all of that. (In the jargon, the effects of air pollution could possibly be confounded with the effects of the other differences.)
“There are statistical ways to make adjustments to allow for these other factors to some extent, though that can be difficult when there are several factors involved that might not simply add up in their effect on blood pressure. But if the individual studies that were reviewed did not make these adjustments adequately, then there’s not a lot that can be done in a meta-analysis to make up for that. The upshot is that we can’t be sure whether the higher blood pressures in adolescents that were exposed to higher levels of air pollution, as this new study found, were actually caused by the air pollution or by something else. If the air pollution did play a part in the cause and effect, it’s difficult to say what the size of its effect really was.
“Another difficulty of interpretation arises from the design of some of the studies that were reviewed in the new research. Of the eight studies involved, five were so-called ‘cohort’ studies where people were followed up over time, and so their blood pressures at one date could be compared with the air pollution where they lived at an earlier date. Given that long-term effects of pollution on health obviously take time to develop, that makes sense. But the other three studies were cross-sectional, that is, they looked at the air pollution and the blood pressures at the same time. This makes them particularly unhelpful in looking at questions of cause and effect, because the possible cause (air pollution) has to happen before the possible effect (raised blood pressure), and a cross-sectional study can’t get at that time difference.
“But the researchers on the new study used standard methods to report on the quality of the studies that they reviewed. Four of the cohort studies were classified as being of only ‘fair’ quality, and the fifth was classified as ‘poor’. On the other hand, all three cross-sectional studies were classified as ‘good’. This quality classification doesn’t come into the statistical meta-analysis calculations, but it’s notable that the statistically significant associations that were found were to a considerable extent driven by the findings from the cross-sectional studies, particularly those in China.
“The upshot is that this new study does provide evidence of an association between the air pollution levels and where people live, but it’s based on studies that can’t be very clear on what causes what, and that to some extent were assessed as being subject to risks of bias (hence the ‘fair’ or ‘poor’ quality ratings). And that’s why the researchers, rightly, point out the need for more and better research. If the link between air pollution and adolescent blood pressure is one of cause and effect, then it’s very important in public health terms, and that’s why we need to know more.
“Despite the uncertainties in these new findings, personally I have to say that I’d be very surprised if it turned out that there was no involvement at all of air pollution in adolescents’ blood pressure levels. And even if that did turn out to be the case, that wouldn’t stop air pollution being an important cause of poor health. The great majority of studies of associations between air pollution and different aspects of health are observational, so each one on its own has some difficulty in establishing cause and effect. But associations have been found, generally in a consistent way, with many different aspects of health, and there has been research on just how different air pollutants can work in the body to worsen health. So my feeling is that we’re well beyond being able to say that it is all coincidence and confounding.”
‘Association of ambient air pollution with blood pressure in adolescence: A systematic-review and metaanalysis’ by Saniya Tandon et al. was published in Current Problems in Cardiology.
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. My quote above is in my capacity as an independent professional statistician.”