November 25, 2019

expert reaction to study looking at particulate matter air pollution and glaucoma risk

Research, published in Investigative Ophthalmology & Visual Science, reports that living in a more polluted area is associated with a greater likelihood of having glaucoma.

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

“This is a careful study that uses data from a large number of people.  But, as the researchers themselves make clear, it certainly doesn’t answer all the questions.  That’s for several different reasons.

“The researchers point out that, although they found correlations between glaucoma (and thinning of the retina that is related to glaucoma) and the levels of air pollution outside people’s houses, their study cannot establish whether higher levels of pollution cause the risk of glaucoma to increase.  That’s because the study is observational.  They didn’t experimentally change the levels of pollution that people were exposed to.  That would have been impossible (and clearly unethical).  Instead they simply recorded a measure of air pollution outside people’s addresses, and also made eye measurements on the participants and asked them whether they had been diagnosed with glaucoma.  The trouble is that there are likely to be a whole range of differences between people that live in areas of high and low pollution, apart from the pollution level.  Any of these differences might, at least in part, be the real reason for the differences in glaucoma risk, and not the different levels of air pollution at all.  It’s possible to adjust the results statistically, to take account of other differences on which data is available.  These researchers did that using several factors, including age, sex, a measure of material deprivation in the area where they lived, smoking, BMI, and some others.  But they could not take into account factors on which they have no data, and these adjustments can never be perfect.  So there has to be some doubt about whether high air pollution, as measured in this study, is the cause of any increased glaucoma risk.  One possibility is that the increased risk is caused by a different air pollutant, and not the fine particulates (PM2.5) that were used in this study – but it remains possible that the cause has nothing direct to do with air pollution at all.

“The research could not take into account the actual levels of air pollution inside the participants’ homes, or indeed inside their workplace or other places where they spent a lot of time, because they had no data on those pollution levels.  The researchers argue that, if they had had that data, the correlation between glaucoma risk and pollution might have been higher.  Well, it might have been, or it might not have been.  That all depends on the correlation between the pollution levels outside people’s homes and the actual air pollution level they are exposed to, inside their homes and workplaces and elsewhere, and the researchers can’t know that because the measurements couldn’t be made.

“So we can’t be sure that high levels of particulate air pollution are actually causing increased glaucoma risk – they might or might not be.  It’s certainly plausible that air pollutants might cause a risk increase, but being plausible doesn’t necessarily make it true.  And we don’t know how air pollution levels inside buildings come into it.  Also, the research did not find a clear link between air pollution levels and the pressure inside the participants’ eyes, and increased pressure inside the eyes is well known to increase the chance of glaucoma.  As they point out, that might be because air pollution affects glaucoma risk in some other way, and they did find an association between air pollution levels and thinning of the retina (though again they cannot know whether that association is one of cause and effect).  So there are several reasons why more research would be needed to investigate how air pollution might affect glaucoma chances, if indeed it does do that.

“Just to see how big the effect might be, if the effect of air pollution on glaucoma is indeed causal, we can look at some more data from this study.  In all, 1.8% of the participants said they had had a glaucoma diagnosis.  Imagine a group of 1000 people, comparable to those in this study.  Then 18 of them would have had a glaucoma diagnosis.  Now imagine that these 1000 people had all lived at a place with worse air pollution than the place they really lived – enough worse, in terms of the pollutant being measured (fine particulates, PM2.5), to move someone on average from being just in the lowest quarter of participants’ homes  to just in the highest quarter.  That’s a reasonably large change, compared to the experience of participants in this research at any rate.  If the effect of this pollution increase on glaucoma risk is really one of cause and effect, that would increase the number of the 1000 people who have glaucoma from 18 to 19.  Not a large increase, though certainly bad news for the one person involved, of course.  There’s some statistical uncertainty about this number – out of the 1000, the statistics are consistent with there being essentially no change in the number of glaucoma cases out of 1000 people, or two extra, rather than one extra.”

‘The relationship between ambient atmospheric fine particulate matter (PM2.5) and glaucoma in a large community cohort’ by Sharon Y. L. Chua et al. was published in Investigative Ophthalmology & Visual Science at 15:00 UK time on Monday 25 November 2019.

DOI: 10.1167/iovs.19-28346

Declared interests

Prof Kevin McConway: “Prof McConway is a member of the SMC Advisory Committee, but his quote above is in his capacity as a professional statistician.”