A study published in Environment International looks at exposure to an urban environment during early life and blood pressure in young children.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“This is an interesting study. It’s quite complicated, conceptually and statistically, and it’s very important to take into account that it isn’t just about air pollution. The press release concentrates on air pollution, but the research paper is about a much wider set of aspects of urban environments. It’s an observational study, so, as always with such studies, there will be complications in unravelling what actually causes what. The researchers acknowledge that they could not adequately measure or account for the patterns of cause and effect in the factors that they studies, because of such complications. Indeed their aim was not to do that, but instead to look at a whole range of different measurements on aspects of the urban environment – 125 of them – and look very broadly at which of them might be associated with blood pressure in young children. They used appropriate statistical methods, but those methods can’t answer many relevant questions about cause and effect.
“As I said, this study is not just about effects of air pollution on blood pressure, but I’ll use air pollution as an example to explain why it’s pretty well impossible to separate out what causes what. Children who live in areas with high air pollution will differ from children who live in areas with lower pollution in a lot of ways, in terms of characteristics of the children themselves, characteristics of their parents, and other characteristics of the places where they live. (For instance, often places with high air pollution will have high levels of traffic nearby, and so are likely to have high levels of noise too.) So if children in areas of high air pollution have higher blood pressure, on average, than children in less polluted areas, that doesn’t mean that the high blood pressure is caused by air pollution. That could be part of the pattern of cause and effect, but other factors might also be involved in causing high blood pressure, and it remains possible that air pollution has no direct causal effect at all. It’s possible to make statistical adjustments to allow for some other factors, but (as the researchers themselves acknowledge) this process can never be perfect. In a study like this, there are other complications too – for example, other studies have indicated that exposure to high air pollution during pregnancy can be associated with low birth weight, and that in turn might be associated with the child’s blood pressure, but there are several other factors that are associated with low birth weight too. Sorting out what’s going on in this complicated pattern of possible causes and effects, in such a broad study as this, is not going to be possible. So we can’t rule out that exposure to air pollution during pregnancy may increase the blood pressure of the resulting children in early life, but this study certainly can’t tell us that it definitely does have this effect. Another issue is that the associations between blood pressure and air pollution become less marked in some of the extra (sensitivity) analyses that the researchers carried out to verify their initial findings, so it’s not even completely clear how strong the associations between air pollution and blood pressure are.
“The study is called “Urban environment during early-life and blood pressure in young children”, a title that does not mention air pollution, because it is not a study that concentrates on air pollution. The researchers found associations between higher blood pressure in children and several other aspects of their environment as well as some measures of air pollution. These aspects include higher noise and higher ambient (outside) temperature during pregnancy, and high building density in the area where the child lived. They found associations between lower blood pressure on children and higher temperature during childhood and better levels of street connectivity. All these associations may or may not involve cause and effect, and they may or may not be reflecting some other association with another factor that wasn’t measured in this study. The researchers’ overall conclusion again isn’t directly about air pollution at all. They suggest that “living in a harmful urban environment may impact [blood pressure] regulation in children” and relate this finding to the importance for health of designing cities in ways that promote healthy environments. And what determines a healthy environment, for these researchers, clearly goes well beyond just the level of air pollution. I’d certainly agree with that.”
‘Urban environment during early-life and blood pressure in young children’ by Warembourg et al is published in Environment International.
Prof Kevin McConway: “I am a Trustee of the SMC and a member of the Advisory Committee, but my quote above is in my capacity as a professional statistician.”