A study published in the British Journal of Ophthalmology suggests air pollution is linked to higher risk of sight loss from age related macular degeneration (AMD).
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“There is considerable evidence of associations between air pollution and a range of health problems, and the researchers describe reasons why associations between air pollution and AMD may be plausible. Therefore this large and generally well-conducted study, using a respectable and well-understood data source, is very timely. Having found some evidence of association, the researchers recommend more research to investigate further, including follow-up research in the same cohort of people (UK BioBank). I think that’s a good recommendation. There are aspects of this study that mean its conclusions can’t answer all the relevant questions. That’s not because it is a poor study, but because any such study has limitations.
“Perhaps the most important issue is that the study is observational. Inevitably, there will be many differences between people who live in areas with different levels of air pollution, besides the pollution difference. Any difference in rates of AMD or of changes in the retina between people in high-pollution and low-pollution areas might be caused, in part or entirely, by these other differences rather than by the differences in pollution levels. It’s possible to make statistical adjustments to the results to allow for other relevant differences on which the researchers have data, and these researchers did that for several potentially relevant factors. But one can never be certain that everything relevant was dealt with, and that’s why the results can’t establish that air pollution causes an increased risk of AMD. However, just because we can’t be sure that the pollution causes AMD, that doesn’t rule out the possibility that it does have a causal effect.
“A study that followed people up over time, rather than (as this one does) just looking at the pollution level outside homes and the condition of their eyes at one point in time, would help to clarify the position on cause and effect. Apart from anything else, this study wouldn’t pick up how long people had lived in an area of high or low pollution, and when any changes to their retinas might have occurred in relation to that. That’s why the researchers propose that that should be done. In a situation like this where experimenting on people would not be possible, the way that an understanding of cause and effect is built up is to put together evidence from different sources. This study is part of that process, and there have been studies elsewhere in the world with broadly similar results, but more information is needed. The position is also complicated because this study could only take into account air pollution outside people’s homes, and did not look at indoor air pollution levels (at home or in workplaces).
“It’s worth pointing out that, on the basis of these results, the evidence for an association between having had an AMD diagnosis and air pollution is not particularly strong statistically. The evidence for associations between air pollution and structural changes in the retina is generally stronger. The researchers found evidence of an association between only one of the six pollutant levels that they considered and AMD diagnosis, though that pollutant is the very fine particles (PM2.5) which have also been associated with several other health issues. The level of statistical evidence even for that association is not particularly strong. With the changes in retinal structure, the researchers applied a standard statistical adjustment to deal with the fact that they had looked simultaneously at six different pollutant levels, and after that adjustment there was still strong evidence for association. With the association between pollution and AMD, this adjustment was not made, and if it had been made, the evidence for an association between PM2.5 and AMD would have had very weak statistical support. Again, that does not mean that there is no association, just that the study, large as it was, didn’t find clear evidence of an association – another reason for further studies to clarify the position.
“AMD is a horrible condition that is a major cause of blindness. In this study, about 11 people in every thousand had had a diagnosis of AMD. Suppose all those people had lived in a place with a much higher level of PM2.5, enough to move from the lowest quarter of PM2.5 levels just into the highest quarter. If the association between PM2.5 and AMD is in fact one of cause and effect, then that number would increase to 12 in every thousand, though there’s still some statistical uncertainty about that and the number might still be about 11 in every thousand or could be a bit larger, about 13 in every thousand. That’s not a huge increase. However, AMD becomes more common as people get older. For people in the UK aged 65 and over, about 48 in every thousand have AMD (according to a study quoted by these researchers). An increase in air pollution by the amount I’ve just considered, again assuming that the association is causal, would put that number up to somewhere between just under 49 and 56 in every thousand. Reductions in air pollution could move the AMD rates down by similar amounts (though it’s hard to be precise because some people live in areas where the pollution is so low that it can’t be reduced by as much).”
Prof Anna Hansell, Professor in Environmental Epidemiology, University of Leicester, said:
“Air pollution has been linked to increased risk of many diseases affecting many parts of the body, but potential for damage to the eye has only recently been realised. This large study looked at over 100,000 middle-aged and elderly individuals living in the UK. Authors found associations between air pollutants and both self-reported age-related macular degeneration and measurements of retinal layer thickness, an indicator of risk of age-related macular degeneration. In particular, associations with eye damage were linked the with air pollution components from combustion sources including transport. It is of concern that air pollution, even at the lower levels seen in the UK (compared with many other parts of the world), is associated with premature ageing of the eyes.
“The study used a very large dataset and took account of other factors that may have affected the findings, such as smoking and deprivation. The estimates of air pollution exposure were modelled at home address and use standard methods. However, the authors averaged air pollution estimates from the two different air pollution models available in the dataset rather than picking one model, which may have introduced some misclassification into exposure estimates. This may mean the size of the association between air pollution exposure and outcome is not precise. However, it should not invalidate the findings of a link. Also, findings are consistent with other recent studies that have also detected a link between air pollution and eye damage including AMD and glaucoma.
“The authors suggest that one of the ways in which air pollution may possibly damage the eye is through oxidative stress. We know from other research that a diet high in fruit and vegetables provides anti-oxidant nutrients that is protective against age-related macular degeneration and further research is needed to see the impact of increasing fruit and vegetables in the diet on those in areas of high air pollution.”
Prof Robert MacLaren FMedSci, Professor of Ophthalmology, University of Oxford, said:
“The UK Biobank is like a population census, only it focusses on medical diagnoses and biological measurements across a group of half a million people in the UK. In this study, the researchers found that small particles in ambient air pollution linked to where people lived, was associated with an increased risk of age-related macular degeneration by about 8%. Age-related macular degeneration is the most common cause of sight loss in the developed world and so this finding is significant. Furthermore, participants in the study had an average age of around 60 and this small increase risk of 8% is likely to be compounded further over ensuing decades. The association of macular degeneration with smoking is well recognised, but this new finding of an environmental link related to atmospheric pollution will add further to the climate change debate.”
Prof Chris Inglehearn, Professor of Molecular Ophthalmology, University of Leeds, said:
“This UK-based study is similar to a recent study from Tiawan1. Both show a link between air pollution and age-related macular degeneration, a common cause of blindness in older people. The profile of pollutants the two groups looked at is slightly different but the source is the same, combustion. Of course, correlation does not prove causation, but the fact that these two independent studies reach similar conclusions gives greater confidence that the link they make is real.
“This study is timely, given the recent ruling by a UK coroner that road traffic fumes contributed to the death of a young girl with asthma. These studies provide further evidence that links air pollution with detrimental impacts on human health.”
‘Association of ambient air pollution with age-related macular degeneration and retinal thickness in UK Biobank’ by Chua et al was published in the British Journal of Ophthalmology at 23:30 UK time on Monday 25th January.
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee, but my quote above is in my capacity as a professional statistician.”
Prof Anna Hansell: “I have no conflicts of interest to declare.”
Prof Chris Inglehearn: “No COI.”
None others received.