From simple throat irritation to cancer, the deleterious effects of air pollution on respiratory and cardiovascular functions were already numerous. It seemed less obvious at first glance that it could have an impact on our eyes. A study conducted by French researchers confirms that prolonged exposure to fine particles, in particular those produced by road traffic, would promote ocular aging and could lead to glaucoma, the second cause of blindness in the world. Ocular aging is characterized, among other things, by a progressive degeneration of the nerve layer of the retina, which can ultimately alter the field of vision. It is also the main manifestation of glaucoma, a neurodegenerative eye disease affecting the optic nerve.
While the link between air pollution and eye health is new, researchers had already established a cause and effect relationship with neurodegenerative diseases. But “we often forget that the eye, and more particularly the retina, is an “extension” of the central nervous system, and therefore of our brain. From this, we therefore hypothesized that pollution can also have a harmful effect on our eyes,” explains Laure Gayraud, doctoral student in epidemiology and main author of the study.
To demonstrate this, the team of researchers from Inserm and the University of Bordeaux followed 683 people, aged over 75, residing in Bordeaux between 2009 and 2020. The participants had eye exams every two years in order to follow the evolution of the thickness of the nerve fibers of the retina and thus indirectly determine the speed with which their eyes were aging. At the same time, the scientists estimated the exposure of these people to air pollution by measuring the concentration of different air pollutants with an accuracy of 100 meters around their place of residence. These data were obtained using annual exposure maps based on measurements from air quality monitoring stations. Among the pollutants taken into account: nitrogen dioxide and two types of fine particles, those with a diameter of less than 2.5 microns, “PM2.5”, and carbon soot.
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The results are final. Bordeaux residents exposed to higher concentrations of fine particles were victims of an accelerated thinning of the nerve cell layer of their retinas. In other words, these pollutants would promote the early deterioration of retinal cells, signifying more rapid aging of the eyes. “We remain cautious with regard to glaucoma, because its diagnosis is based on a battery of examinations and we have not studied the direct link with the disease but one of its characteristics”, underlines Laure Gayraud. From a physiological point of view, the very small size of fine particles allows them to penetrate deep into the body and cause inflammatory reactions and oxidative stress in the body. “We think that this is the mechanism at work in this study: the fine particles could penetrate the barrier of the retina and cause degeneration of the ocular cells”, explains the doctoral student.
Even more alarmingly, the levels of fine particles were on average below the regulatory thresholds for annual exposure in the European Union. “For PM2.5, the threshold is set at a maximum of 25 micrograms per cubic meter. In this study, the measurements ranged from 16 to 25 micrograms per cubic meter, with an average around 20,” says Dr. Cécile Delcourt, research director at Inserm, who led the study. This suggests that the risk is already significant for the inhabitants of polluted areas even below current standards. Therefore, the effect of fine particles on ocular aging adds an additional dimension to the health issues related to air pollution.
The authors underline the importance of reassessing the European thresholds, reinforcing the WHO recommendations in favor of a reduction to 5 micrograms per cubic meter for PM2.5 in particular. “The EU has just voted for 2030 a lowering to 10 micrograms per cubic meter, so there is already an evolution”, notes Dr Delcourt. “Nevertheless, it is a process that will take time and unfortunately, at the individual level, it remains difficult to act,” she continues. In the meantime, the researchers intend to deepen the understanding of the mechanisms involved and document the effects of fine particles on age-related macular degeneration (AMD), which remains the main cause of blindness.
