The Herald reports on a new study that suggests a link between premature births and proximity to motorways. At first glance this seems like it is intuitively right – all that pollution and carbon monoxide could be expected to produce low birth-weight, premature babies. Unfortunately, this study, which can be accessed in full here (PDF), is not quite a slam-dunk.
Firstly, its numbers are small for an epidemiological study, only 1000 mothers. Admittedly, it seems that they had access to high quality information on these mothers, making the result a little more reliable but this type of prospective cohort study is best done with ten times this number, if possible.
The other, more serious, problem is that the most obvious confounding factor is the socio-economic status (SES) of the mothers. The authors admit that they could not properly correct for this (not surprising given the small numbers) which appears to be a fatal flaw in the analysis. There is a well-established strong correlation between low SES and low birth-weight, premature infants. There is also an obvious association between SES and distance from motorways and housing density (and therefore road and traffic density. It may well be that this study is merely measuring a proxy for SES rather than a direct causal link. Of course, the association between SES and birth weight may be a proxy for traffic density! Unfortunately, you can’t tell from this study.
The authors also postulate that noise from freeways may disturb mother’s sleep and cause stress. Although this may sound plausible, it is not very likely. There are a multitude of studies that have demonstrated that humans quickly adapt to constant noise. Motorways have not been demonstrated to cause an increase in insomnia.
I must say I find this sort of study a little pointless anyway. We already have a good deal of evidence that suggests that increasing pollution is related to low birth-weights. Adding that there is more pollution near motorways seems somewhat redundant and not particularly useful to mothers who live in such areas, as they usually lack the resources to move away. If they had managed to persuade half of their mothers to wear N95 particulate masks for the whole of their pregnancy then that would have been useful information. Possibly.