May 24, 2016

expert reaction to review paper looking at air pollution and risk of stillbirth

A systematic review and meta-analysis published in the journal Occupational & Environmental Medicine on prenatal ambient air pollution exposure has reported an association between exposure to air pollution and the risk of stillbirth.

Dr Alexander Heazell, Senior Clinical Lecturer in Obstetrics, University of Manchester, said:

“Stillbirth is an important topic to investigate. We don’t understand why babies die and environmental factors are a sensible area to research. This study combined 13 individual studies to investigate whether air pollution was associated with increased risk of stillbirth. The study authors did not find a clear link between air pollution and stillbirth, but demonstrated there is a need for further well controlled studies. Critically, we need to understand how environmental exposure relates to levels of chemicals in individual women and how this relates to stillbirth risk.”

Prof. Jean Golding, Emeritus Professor of Paediatric and Perinatal Epidemiology, University of Bristol, said:

“This is an important gathering together of the studies that have been done on this topic.  It should be noted that the air pollution measurements are related to the area in which the pregnant mother lives, but no actual measurements were made on the mothers’ exposures. This would be an important next step, but not easy to do. I agree with the authors, that this is a topic that warrants detailed further research.”

Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“Stillbirth is an important health problem worldwide, and there is no simple answer to the question of what causes it. Many factors contributing to stillbirth risk have been identified. Also, air pollution is known to have various adverse effects on health. So it’s natural to investigate whether there is any link between a mother’s exposure to pollutants in the air she breathes and the risk of stillbirth. Several such studies have been done in various parts of the world, and no clear pattern has emerged from their findings yet. So it made good sense for this new review to be carried out, to try to make more sense of the complexity in the individual studies.

“In statistical terms, in my opinion, this review was generally conducted appropriately, and the research report provides a good level of detail on what they found. But, through no fault of these researchers, they really didn’t find anything very clear. A reasonable headline for a press release on this work could have been “Air pollution and stillbirth – we still don’t know whether they are linked.” None of the main statistical conclusions in the review shows a statistically significant link between air pollutants and stillbirth. This is made perfectly clear in the research report, though it is not explicitly mentioned in the press release.

“One point that the research report also makes very clear is the difficulty of trying to pick apart what causes what here. It’s difficult to measure how much pollution there is in the air that an individual mother actually breathes, so most of these studies are based on data from air quality monitoring stations, which may have been some distance away. There is a huge range of other potential causes of stillbirth that need to be taken into account in some way (generally statistically). So the studies that were reviewed used a broad range of study designs and statistical methods, making it harder to synthesise their findings in a meta-analysis. An important part of a systematic review like this is to assess the quality of the studies being reviewed, and to include only those that were addressing broadly the same question and show no important signs of bias in the statistical results. Here, this process meant that only three studies (two from the USA and one from Taiwan) contributed to most of the overall results, with a fourth study from South Korea also playing a part in one set of findings. (This certainly does not mean that all the other studies were bad – really it again just indicates the difficulty of research on this question.)

“Now, of course, lack of statistical significance does not tell us that there is definitely no association between air pollution and stillbirth. It just means that the results are consistent with there being no association. The results are also consistent with there being a small increase in stillbirth risk when the air is polluted. This review just can’t tell us whether there’s no risk, or a small risk, because the evidence it reviewed is still not clear-cut enough.

“Since this review doesn’t provide a smoking gun (to make a dreadful pun), it’s reasonable that it calls for more and better evidence. That evidence won’t be quick or easy to find. Whether or not there’s a link to stillbirth, it remains a good idea for governments and authorities to act on air pollution, since it has so many other health consequences. And I don’t think these new findings should be a serious cause for concern for individual pregnant women – if there is an increased risk of stillbirth, this review indicates that the increase is pretty small.”

‘Prenatal ambient air pollution exposure and the risk of stillbirth: systematic review and meta-analysis of the empirical evidence’ by Nazeeba Siddika et al. published in Occupational & Environmental Medicine on Tuesday 24 May 2016.

Declared interests

Prof. Jean Golding: “I have no conflicts of interest.”

Prof. Kevin McConway: “I have no relevant interests to declare.”

No others received.