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A new study, in The BMJ, examines the association between exposure to air pollution from road traffic in London during pregnancy and an increased risk of low birth weight babies born at full term.
Prof. Jonathan Grigg, Professor of Paediatric Respiratory and Environmental Medicine at Queen Mary University of London (QMUL), said:
“Previous studies have found an association between exposure of pregnant women to air pollution at the home address and low birth weight at term. This study strengthens the argument that a relatively short-term exposure of 9 months or less to vehicle exhaust emissions adversely affects fetal growth. The authors linked birth weight to modelled exposure to a range of pollutants, including inhalable particulate matter (PM2.5) from traffic, PM2.5 from other sources, and nitrogen dioxide NO2, and found a robust association between traffic-derived PM2.5 and low birth weight at term. Since exposure was modelled for women living with the M25, the researchers were able to calculate that 3% of term low birth weight cases in London are directly attributable to exposure of pregnant women to PM2.5 concentrations above 13.8 μg/m3.
“The importance of this study is that it makes a very strong case for the need to drive down traffic-derived emissions from roads as soon as possible. Children in London and other large urban areas are currently inhaling traffic-derived pollutants that increase the risk of pneumonia and asthma, and stunt lung growth. This study suggests that damaging effects of air pollution start well before birth. Since the vast majority of the most toxic vehicles on our roads are diesel cars and vans, the UK Government must introduce a national ‘remove and replace’ diesel vehicle policy to protect the health of children – both born and to be born.”
Prof. Kevin McConway, Emeritus Professor of Applied Statistics at The Open University, said:
“This study isn’t the first to find an association between exposure to air pollution and low birth weight. But such studies aren’t easy either to carry out or to interpret. A major issue is that they are observational – that is, they are based on observing and collecting data on the mothers and babies, because it’s obviously unacceptable to experiment on mothers by deliberately exposing them to different levels of polluted air. Such a study might find that mothers who were exposed to higher levels of air pollution during their pregnancy might be more likely to have a baby whose birth weight is low, but the reason for the low birth weight could have nothing to do with the pollution. Perhaps mothers who live in places with poor air quality tend to be different from those in other places for some reason that has nothing to do with any effect of the pollution on their babies, and it is these unrelated differences that actually cause the low birth weight.
“This complication is unavoidable, and there are statistical ways to try to clarify the results. Statistically, one can allow for differences between the mothers on which the researchers have data. This study did carry out such adjustments, taking into account the age and ethnicity of the mother, the baby’s sex, the season of the birth, and so on. But the researchers cannot adjust for things on which they had no data. They had no information on exposure to pollutants outside the home, for instance. Importantly, they had no data on whether the mothers smoked, so could not adjust for that. They did take account of the level of tobacco sales in the area where the mother lived, but that’s not the same as looking at whether individual mothers smoked, and it is known from many studies that whether the mother smoked during pregnancy has a very substantial effect on the chances of low birth weight. The researchers also had no individual data on exposure to passive smoking, or on socio-economic deprivation (though again they did have data on average socio-economic deprivation in the area where they lived).
“Thus we really can’t be sure, from this study, whether any increase in low birth weight in areas with higher pollution was actually caused by the pollution. Maybe, if the researchers had been able to allow for individual data on whether the mothers smoked, the relationship between air quality and birth weight would have been much weaker or even non-existent. We just can’t be sure.
“It’s certainly plausible that air quality affects birth weight, and previous studies have also found such an association. This new, thorough, careful study does add to what was known, in that it works with very localised measures of exposure to poor air, and it specifically covers London, while many previous studies have looked at places where the conditions are very different from those in London. Also, its finding that there was no evidence of an association between traffic noise and birth weight, once measures of air pollution have been taken into account, is interesting and potentially important. But this study, on its own, can’t prove that poor air quality causes babies’ health to be worse.
“It’s also important to interpret properly the effect sizes given in the press release, as not all the required information is given there. For instance, the release says ‘increases in traffic related air pollutants … were associated with 2% to 6% increased odds of low birth weight…’. But how big an increase in pollutants? The research report gives details on the size of the increases involved, and they are pretty substantial. And you have to bear in mind that low birth weight, in a baby who was not born prematurely, is not very common. Of the births in this study, only 2.6% had low birth weight, and an increase in odds of between 2% and 6% on that would change it to somewhere between 2.65% and 2.75%. Worth taking into account, given that air pollution is everywhere (in London), but not huge.
“The fact that the observed associations are not very large increases my concern about the unobserved variables, such as individual maternal smoking, that the researchers could not take into account. It would not take a particularly strong effect of one of these variables to entirely account for the observed association between an individual pollutant and birth weight.”
Prof. Barbara Maher, expert in magnetic tracing of airborne nanoparticulate pollution at the Lancaster Environment Centre, said:
“This careful study finds robust associations between maternal exposure to airborne particulate pollution and low birth weight in > 500,000 babies, born in the London/Greater London area between 2006 and 2010. Although it’s always difficult to separate out the effects of co-associated pollutants, the authors report strongest association with the fine-grained particulate matter (PM2.5) and less with either the gaseous nitrogen oxides or road noise. The study uses established models to map air and noise pollution at very high spatial resolution, and attempts to account for possible confounders (such as census area-level tobacco expenditure). The analysis also indicates some increased association between low birth weight and PM2.5 exposure in the second and third trimesters of pregnancy. The associations between low birth weight and PM2.5 exposure are evident nothwithstanding the exposure variations experienced inside the home (from indoor pollution sources, e.g. from cooking) and away from the maternal home (e.g. at work, and/or during travel to and from work).
“Previous work has shown that nanoscale, metal-bearing and potentially toxic pollution particles can gain entry to the amniotic fluid surrounding the developing child. This new study re-emphasises the damage which air pollution is causing, not only from cradle to grave, but even before birth. In the international context, such damage is likely affecting the life-course of millions of newborns every year.”
* ‘Impact of London’s road traffic air and noise pollution on birth weight: retrospective population based cohort study’ by Rachel B Smith et al. published in The BMJ at on Tuesday 5 December 2017.
Declared interests
Prof. Kevin McConway: “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.”
Prof. Jonathan Grigg: “I am a founding member of Doctors Against Diesel.”
None others received.