A study published in JAMA Network Open looks at exposure to particulate matter air pollution and semen quality among men in China.
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“This study does provide some evidence of an association between exposure to fine particles in the air, which are one form of pollution, and some measures of sperm quality in men of reproductive age, based on data from China. However, it’s important to note that the authors are quite restrained in what they conclude – they are careful to say the exposure to particulates in the air MAY adversely affect sperm motility (measures of how mobile the sperm are). They, rightly, can’t conclude more than that. The main reason is that this is, unavoidably, an observational study, and one can basically never conclude from a single observational study that one thing causes another. But, also, there are some specific limitations that arise because of the way the study was carried out, and the report draws attention to these.
“I should say that none of these issues mean that air pollution cannot cause problems with sperm quality – only that this study has not shown that that is the case. Its findings are consistent with there being a cause-and-effect association, but the problem is that they are also consistent with other explanations of the association instead. Also, there are many other studies that show associations between high levels of air pollution and other aspects of ill health, some of them with more persuasive evidence of cause and effect than this one. Even if it eventually turns out that fine particulates in the air don’t cause problems with sperm, there are plenty of other reasons for reducing air pollution.
“The issue about this being an observational study is that the researchers obviously couldn’t make the participants live in places where the particulate air pollution was low or high – the participants just lived where they lived, and the researchers recorded the air pollution level, measures of sperm quality, and several other quantities. There would be many differences between the men who lived in areas with low and high particulates in the local air, apart from the particulate levels. Any of these other differences (known as ‘potential confounders’) might, in whole or in part, have been the cause of the differences in sperm motility, rather than the differences in particulate levels. Statistical adjustments can be made to try to allow for the potential confounders, and the researchers made adjustments for several, including for ethnicity, age, body mass index, smoking and alcohol consumption, some aspects of the weather, and more. But, as they themselves say, they couldn’t adjust for everything that might have been relevant. They mention that they didn’t adjust for dietary habits and physical condition, and I’d add that they didn’t adjust for the men’s occupations (though they did adjust for educational level, which would be related to occupation to some extent). Therefore they really can’t (and don’t) conclude that this study has shown that particulate air pollution causes issues with sperm motility.
“Since it’s never going to be possible to carry out large-scale studies of this sort of association that are not observational, the question arises as to how we might ever know whether or not air pollution can have a causal effect on sperm quality. Really we’d need more data from a wider range of studies, but we’d also need to know about plausible biological ways in which the pollutants could actually work inside the body to affect sperm quality. And, as these researchers point out, more (and different) studies are needed to investigate possible biological mechanisms.
“A limitation also arises from the way that air pollution was measured. The levels of pollution exposure for each man in the study were taken from the nearest fixed air monitoring station to where he lived. I have no information on how densely these stations are sited in the parts of China studied here – but generally particulate pollution levels can often vary quite a lot between places that are fairly close to one another, and so it’s possible that the particulate levels to which a man was exposed are not well approximated by the levels at the nearest fixed station to his home. That could be true also if the man spends a lot of his time nearer a different monitoring station, for example if he does not work very close to his home.
“I’ll finish with one more detailed point. It’s interesting that the researchers made statistical adjustments for the levels of several gaseous air pollutants (nitrogen dioxide, sulphur dioxide, carbon monoxide, and ozone). They do not report on any associations between those pollutants and sperm quality measures – but because they made statistical adjustments to allow for them in looking at the association between particulates and sperm quality, that gives an indication that the association between particulates and sperm quality did not arise simply because areas with high particulate levels tended also to have high levels of the gaseous pollutants. It doesn’t look likely that it was the gases that were really behind the association between fine particle levels and sperm quality.”
Prof Allan Pacey, Professor of Andrology, University of Sheffield, said:
“The possibility of a link between air pollution and semen quality has been suggested in a number of studies over the years, although not all of them have agreed with this conclusion. This paper adds to the evidence base suggesting the link is real, and is impressive because it uses semen quality data from over 30,000 men.
“The conclusions (that increased air pollution in this particular part of China may be associated with reduced sperm motility) is interesting but of course is correlation and not causation. Having said that, the sperm quality measures they have used seem robust and are strengthened by the fact that they made them with a computer system, rather than relying on the manual assessment by a human observer which we know is an error prone method.
“I am not able to comment on the quality of their statistical analysis (it’s a bit beyond my skill set) but if it is robust, then it does seem to suggest a link between air quality and sperm motility is real. However, to what extent this is clinically meaningful (i.e. whether it will reduce the ability of men in high pollution areas to become fathers) is not clear and we should remember that the link between sperm quality and fertility is a weak one.”
‘Association of Exposure to Particulate Matter Air Pollution With Semen Quality Among Men in China’ by Yan Zhao et al. was published in JAMA Network Open at 16:00 UK time on Thursday 17 February 2022.
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.”
Prof Allan Pacey: “Chairman of the advisory committee of the UK National External Quality Assurance Schemes in Andrology, Editor in Chief of Human Fertility, Trustee of the Progress Educational Trust (Charity Number: 1139856) and Trustee of the British Fertility Society (Charity Number:1075661) (all unpaid).”