© Freepik

Numerous studies are now reporting the harmful effects of air pollution on the central nervous system (neurodegenerative diseases in adults, neurodevelopmental disorders in children). Glaucoma, the second leading cause of blindness worldwide, is a neurodegenerative disease of the optic nerve whose principal characteristic is thinning of the retinal nerve fiber layer. In a study of a cohort of 683 elderly Bordeaux residents followed up over 10 years, researchers from Inserm and Université de Bordeaux at the Bordeaux Population Health research center showed accelerated thinning of the retinal nerve fiber layer in the participants more exposed to air pollution, especially those who had a greater exposure to PM_2.5 (fine particulate matter of a diameter of 2.5 microns or less). This study therefore suggests a possible increased glaucoma risk for the inhabitants of areas with fine particulate pollution, even at levels below the current European Union regulatory thresholds (25 micrograms/cubic meter). The results have been published in Environmental Research.

Air pollution is a global public health issue. The harmful effects of air pollutants on respiratory and cardiovascular functions have been widely documented in the scientific literature. It is also increasingly evident that chronic exposure to air pollution has adverse effects on the central nervous system, with an increased risk of neurodegenerative diseases in adults and neurodevelopmental disorders in children.

The retinal nerve fiber layer (RNFL) is part of the central nervous system, and its thinning represents the main characteristic of glaucoma[1], an eye disease associated with the progressive destruction of the optic nerve, most often caused by excessive pressure inside the eye. This disease is the second leading cause of blindness in the developed countries.

Researchers from Inserm and Université de Bordeaux studied the effect of exposure to higher concentrations of air pollutants (fine particulate matter[2] and nitrogen dioxide) on the ocular neurodegenerative processes. To do that, they followed up for a 10-year period a population of 683 people aged 75 years or over who were resident in Bordeaux at the time of their inclusion in the Aliénor[3] cohort. This is the first prospective study on this subject.

As part of this study, the volunteers underwent eye exams every two years between 2009 and 2020 in order to measure changes in RNFL thickness.

In addition, their exposure to air pollution over the previous 10 years was determined using their home address, with the help of annual exposure maps for each pollutant. These detailed maps, with a resolution of 100 meters, were produced using air quality control station measurements and meteorological and geographical characteristics (proximity of a road, population density, distance from the sea, altitude, etc.)[4].

According to the results of this study, people having been exposed to higher concentrations of fine particulate matter had more rapid thinning of the retinal nerve layer over time.

These results are represented in the figure above which shows that the participants exposed to a 25 µg/m³ concentration of PM_2.5 had a faster reduction in the thickness of this layer compared to those exposed to a 20 µg/m³ concentration.

These findings suggest that exposure to a high concentration of pollutants over time could increase the risk of glaucoma.

As regards PM_2.5, the 10-year average exposure estimates were below the annual regulatory threshold of the European Union (established at a maximum of 25 μg/m³) for all participants, but above the limit values recommended by the WHO in 2005 (10 μg/m³) and further reduced in 2021 (5 μg/m³).

“The results of this study confirm the previous observations on the effects of air pollution on neurodegenerative processes – at ocular level in our case. They constitute an additional argument in favor of lowering the European regulatory thresholds[1], as recommended by the WHO, as well as lowering the effective exposure of the French population, which continues to exceed current regulatory thresholds in some places,” explains Laure Gayraud, PhD student in epidemiology and first author of the study.

“More generally, our study documents the effects of air pollutants on neurological aging. Taking the example of ocular aging, it suggests that exposure to high concentrations of pollutants over time could lead to an acceleration of neurological ageing, as was observed in studies on brain aging,” explains Cécile Delcourt, Inserm research director and last author of this research.

The aim is now for the scientists to expand the field of study to national level, using data from other French cohorts in order to learn more about the effects of pollutants on eye aging.

Diagram of the eye and retina

© Adobe stock

[1]To learn more, consult our in-depth article on glaucoma (only available in French).

[2]PM_2.5 corresponds to fine particulate matter of a diameter of 2.5 microns or less.

[3]Alienor is an epidemiological study in the general elderly population that explores the relationships between age-related eye diseases and other major determinants of these diseases (genetic, nutritional, environmental, or vascular factors).

[4]These maps were previously produced by the team of Kees de Hoogh and Danielle Vienneau from the Swiss Tropical and Public Health Institute (co-authors of the publication).

[5]The Members of the European Parliament’s Environment Committee recently voted in favor of lowering the thresholds for several pollutants by 2030, including fine particulate matter.

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