Ambient Air Pollution May Cause Irreversible Eye Damage, UCL Researchers Claim

A global review formerly concluded that air pollution could be damaging every organ in the human body,[1] with the World Health Organisation (WHO) documenting that 92% of the world population lives in areas where air quality levels exceed limits[2] and that this has led to approximately 7 million premature deaths, annually[3] - see editions 316 (here) and 317 (here) of BC Disease News for more detailed epidemiological analyses.

In 2019, Chua et al published an article in the Investigative Ophthalmology & Visual Science Journal, linking exposure to ambient atmospheric fine particulate matter [PM2.5 – particles with an aerodynamic diameter less than 2.5 micrometres (µm)] and glaucoma (optic nerve damage).[4] A suspected link has also been drawn between ambient air pollutants, such as PM2.5, and cataracts (clouding in the lens of the eye) – see Shin et al (2020).[5]

Earlier this year, The Guardian reported that small increases in air pollution are associated with yet another form of eye-related medical condition – age-related macular degeneration (AMD).[6]

AMD is a common disorder, affecting middle vision (as opposed to edge, or peripheral, vision) in one/both eyes of people typically in their 50s and 60s (5% of British people over the age of 65 have been diagnosed).

It occurs when the small, central portion of the retina (the light-sensing nerve tissue at the back of the eye), called the macula, wears down and produces distorted vision. 90% of cases are ‘dry form’ AMD, where vision is impaired by yellow deposits (drusen) in the macula. The remaining 10% of AMD cases are ‘wet form’, where vision is impaired by blood vessels that grow from underneath the macula leaking blood and fluid into the retina, eventually forming a scar.[7]

Whilst not ordinarily causing total blindness, AMD is irreversible and can worsen without treatment. The exact cause of degeneration is unknown, but cigarette smoking, high blood pressure, obesity and genetic makeup are suggested risk factors.[8]

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Source: Wikimedia Commons – myUpchar (1 October 2019) ‘Depiction of a woman suffering from Age-related Macular Degeneration (AMD)’]

Hypothesising that AMD risk may also be affected by exposure to combustion-related particles, a team of University College London (UCL) academics conducted a large-scale cross-sectional study – the first of its kind – to assess the connection between air pollution and AMD diagnoses and harmful retinal changes.

They evaluated UK Biobank database data on 115,954 people, aged 40-69, who had no eye problems at baseline.

As test subjects were assessed over time, overt disease was identified by self-reported AMD. Moreover, 52,602 people underwent spectral-domain optical coherence tomography (SD-OCT) scans to examine retinal health, as a biomarker of AMD [looking specifically at photoreceptor sub-layer (PSL) thickness and retinal pigment epithelium (RPE) layer thickness]. Indeed, dry AMD is characterised by progressive dysfunction of the RPE, photoreceptor loss and retinal degeneration.

Air pollution estimates were provided by the Small Area Health Statistics Unit, as part of the BioSHaRE-EU Environmental Determinants of Health Project, with annual average concentrations being calculated by a model developed by the European Study of Cohorts for Air Pollution Effects (ESCAPE) Project.

Documenting findings in the British Journal of Ophthalmology,[9] it was revealed that participants exposed to higher concentrations [an additional 1 microgram per cubic metre (µg/m3)] of PM2.5 were 8% more likely to have self-reported AMD, whilst also displaying retinal thicknesses typical of AMD. PM2.5 may account for stronger associations because they are able to penetrate deeper and reach the smaller airways and alveoli prior to bloodstream transmission.

For all other pollutants [PM2.5 absorbance (a proxy for elemental/black carbon), PM10, nitrogen dioxide (NO2) and nitrogen oxides (NOx)], associations with differences in retinal layer structure (both PSL and/or RPE) were observed.

No significant associations were recorded for PMcoarse (aerodynamic diameter between 2.5µm and 10µm), which ‘may be explained by differences in the sites of deposition in the respiratory tract and the sources and chemical composition for these different-sized particles’.

Some similarities were drawn with a longitudinal population-based Taiwanese study [Chang et al (2019)[10]] on traffic-related pollution, which discovered that, compared to participants in the lowest exposure quartile of ambient NO2 and CO exposures, those in the highest quartile had an increased risk of self-reported AMD.

Interested (but uninvolved) University of Leeds Professor, Chris Inglehearn, commented:

‘The fact that these two independent studies reach similar conclusions gives greater confidence that the link they make is real’.

In fact, more broadly, the study authors were able to conclude, that:

‘Taken together [with Chua et al (2019), Wang et al (2019)[11] and Chang et al (2019)], our results support published findings of increased risk of eye diseases or association with retinal structures in participants with higher exposure to ambient air pollution’.

Endeavouring to justify their results, Lead Investigator and UCL Professor, Paul Foster, explained that the eye, being one of the highest oxygen-consuming tissues in the body, relies on an ‘enormously high flow of blood’ and it is presumed that when microscopic pollutants are inhaled, ‘the distribution of pollutants is greater to the eye than to other places’. Vulnerable cells could potentially be damaged through oxidative stress/inflammation and this may be why multiple studies have now underscored statistical relationships (with AMD).

To establish a causal connection, i.e. that air pollution is an important modifiable risk factor for AMD, it is accepted that findings will need to be replicated. Future studies (possibly using UK Biobank data again, due to the large sample size) will not only re-examine the effects of outdoor ambient air pollution estimates on incident disease, but also probe the impact of indoor pollution (currently postponed by the coronavirus pandemic), as most individuals spend a large amount of time indoors:

‘We suspect there’s a lot more that is relevant going on inside the house’.

An especially worrying outcome of this latest research is that ambient air pollution can be damaging, even in settings of relatively low exposure levels. Acknowledging the ‘significance’ of this, Chief Executive of the British Safety Council (BSC), Mike Robinson, declared that:

‘The Government needs to act now to ensure it adopts the World Health Organisation’s (WHO) air pollution targets. The Environment Bill currently making its passage through Parliament is the perfect opportunity to do so’.[12]

Of course, BSC is already heavily invested in work that aims to reduce human contact with pollution. As we have previously reported in BCDN, the Council recently developed a mobile phone App, called Canairy, in partnership with King’s College London, which calculates workers’ occupational hourly exposure to nitrogen dioxide, ozone and PM2.5/PM10.

 

[1] Schraufnagel DE et al., Air Pollution and Noncommunicable Diseases: A Review by the Forum of International Respiratory Societies' Environmental Committee, Part 1: The Damaging Effects of Air Pollution. Chest. 2019 Feb;155(2):409-416. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6904855/pdf/main.pdf> accessed 30 March 2021.

Damian Carrington, ‘Revealed: air pollution may be damaging “every organ in the body”’ (17 May 2019 The Guardian) <https://www.theguardian.com/environment/ng-interactive/2019/may/17/air-pollution-may-be-damaging-every-organ-and-cell-in-the-body-finds-global-review> accessed 6 May 2021.

[2] ‘Ambient air pollution: A global assessment of exposure and burden of disease’ (2016 WHO) <https://apps.who.int/iris/bitstream/handle/10665/250141/9789241511353-eng.pdf?sequence=1> accessed 30 March 2021.

[3] 7 million premature deaths annually linked to air pollution’ (25 March 2014 WHO) <https://www.who.int/mediacentre/news/releases/2014/air-pollution/en/> accessed 30 March 2021.

[4] Chua SYL et al., The Relationship Between Ambient Atmospheric Fine Particulate Matter (PM2.5) and Glaucoma in a Large Community Cohort. Investigative Ophthalmology & Visual Science. November 2019, Vol.60, 4915-4923. <https://iovs.arvojournals.org/article.aspx?articleid=2756049> accessed 30 March 2021.

[5] Shin J et al., Association between Exposure to Ambient Air Pollution and Age-Related Cataract: A Nationwide Population-Based Retrospective Cohort Study. Int J Environ Res Public Health. 2020 Dec 10;17(24):9231. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7763970/pdf/ijerph-17-09231.pdf> accessed 30 March 2021.

[6] Damian Carrington, ‘Air pollution linked to higher risk of irreversible sight loss’ (26 January 2021 The Guardian) <https://www.theguardian.com/environment/2021/jan/26/air-pollution-linked-to-higher-risk-of-irreversible-sight-loss#img-1> accessed 2 February 2021.

[7] Alan Kozarsky MD, ‘Age-Related Macular Degeneration’ (26 October 2019 WebMD) <https://www.webmd.com/eye-health/macular-degeneration/age-related-macular-degeneration-overview> accessed 8 April 2021.

[8] ‘What is AMD?’ (27 March 2018 NHS) <https://www.nhs.uk/conditions/age-related-macular-degeneration-amd/> accessed 30 March 2021.

[9] Chua SYL et al., Association of ambient air pollution with age-related macular degeneration and retinal thickness in UK Biobank. British Journal of Ophthalmology. 2021 Jan 25;bjophthalmol-2020-316218. <https://discovery.ucl.ac.uk/id/eprint/10122001/1/Chua_Ambient_air_pollution_with_AMD_clean_copy_BJO_v6.pdf> accessed 31 March 2021.

[10] Chang KH et al., Traffic-related air pollutants increase the risk for age-related macular degeneration. Journal of Investigative Medicine. 2019;67:1076-1081.

[11] Wang W et al., Epidemiological variations and trends in health burden of glaucoma worldwide. Acta Ophthalmologica. 2019; 97(3): e349-e55. <https://onlinelibrary.wiley.com/doi/epdf/10.1111/aos.14044> accessed 7 May 2021.

[12] ‘Air pollution is being linked to an increasingly wide range of diseases’ (26 January 2021 British Safety Council) <https://www.britsafe.org/about-us/press-releases/2021/air-pollution-is-being-linked-to-an-increasingly-wide-range-of-diseases/> accessed 6 May 2021.