Impact of air pollution on eye – The New Tobacco

Ishwarya Suresh, Master’s in Optometry

Research Optometrist, Vision Research Foundation, Sankara Nethralaya, Chennai, India.

 

Introduction

Air pollution is a serious health issue that impacts the quality of life and leads to about 7 million premature deaths per year. Long term exposure to air pollutants is associated with respiratory disorders, cardiovascular problems, and impact on eye disease.⁽¹⁾ Increased industrialization and an increase in the number of vehicles give out smog that causes excessive air pollution. This could be a major threat to general and ocular health which increases hospitalization and healthcare spending.⁽²⁾ According to the World Health Organization (WHO), air pollution is a “silent public health emergency” and called as “the new tobacco” of the environment.⁽³⁾  Figure 1 shows the main sources of air pollution, which are as follows: Mobile sources – such­ as cars, buses, planes, trucks; Stationary sources – such as power plants, oil refineries and industrial facilities; Anthropogenic sources – such as agricultural areas, cities, and wood burning fireplaces; and Natural sources – such as wind-blown dust and wildfires. These air pollutants are chemically transformed, and it is absorbed by human cells leading to general and ocular health effects.

 

Figure 1: Sources of air pollution (Image Courtesy: Guarnieri, M., & Balmes, J. R. (2014). Outdoor air pollution and asthma. The Lancet, 383(9928), 1581-1592)

 

Air pollution consists of different pollutants like primary and secondary pollutants. The primary pollutants include particulate matter PM2.5 derived from combustion sources such as wood and biomass fuels and PM10 generated from construction and road dust, ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2), and sulphur dioxide (SO2) from industrial and vehicle waste as depicted in the figure 1 and secondary pollutants include sulphur trioxide (SO3), ozone (O3), ammonium (NH4+) and particulate matter PM matter. Regardless of the type of pollutant, pollution can affect occupations both outdoors by ambient air pollution and indoors by household air pollution. ^(4,5)

 

Figure 2:  Effects of air pollution caused by air pollutants on various ocular tissues.

 

A review of how and what are the effects of air pollution on eyes ⁽⁶⁾ ; Air pollution affects the eye, primarily leading to ocular disorder as illustrated in the figure 2. Eyes are protected by a thin layer of tear film which is very sensitive to external factors and high concentration of toxin in the air that can cause clinical manifestation leading to serious visual impairment. The conjunctiva is a mucous, translucent membrane that extends from the lid margins to the limbal region, which is well vascularized, with two portions (i.e.) tarsal (palpebral) and bulbar conjunctiva. Chronic exposure to air pollutants causes hyperplasia of the goblet cells of the epithelium of the eyes surface. These pollutants increase expression of cytokines, chemokine’s and growth factors in the conjunctiva that activates inflammation in the conjunctiva leading to degenerative changes like Pterygium and inflammatory changes like conjunctivitis. The tear film is composed of 3 distinct layers called mucin, aqueous and lipid layer, any disorder in lacrimal production, vaporization, and composition of tear film results in the unstable tear film. The air pollutants mainly PM2.5 and PM10 affects the stability of the tear film by altering the goblet cell and mucin concentration in the tears leading to dry eyes. Meibomian glands are sebaceous glands that prevent the tears from vaporizing from the surface of the eye. Exposure of air pollutants on eye lid margin results in oxidative stress on the site of particulate matter deposition and cause chronic inflammatory changes leading to Meibomian gland dysfunction.^(7,8)

 

Mechanism of pollutants on tissue damage

Pollutants in the environment can predispose the immune triggers, which leads to loss of immune tolerance that increases the pro-inflammatory cytokines and is suspected to cause tissue damage as demonstrated in figure 3. People with chronic allergic or atopic tendencies may have heightened mucosal immune response and have less tolerance for further environmental antigenic stimulation.⁽⁹⁾

 

Figure 3: The schematic diagram showing mechanism of various environmental air pollutants inducing pathology in the ocular tissues. (Image Courtesy: Jung, S. J., Mehta, J. S., & Tong, L. (2018). Effects of environment pollution on the ocular surface. The ocular surface, 16(2), 198-205)

 

To conclude, the eye is constantly exposed to the atmospheric components that are vulnerable to damage by several atmospheric pollutants. Though eyes are naturally structured and protected with eyebrows, eyelashes, eyelids, and tear film, constant contact with ambient air, can directly and indirectly, affect ocular functions. ⁽⁹⁾

 

References

1. Sanchez-Carrillo CI, Ceron-Mireles P, Rojas-Martinez MR, et al. Surveillance of acute health effects of air pollution in Mexico City. Epidemiology 2003;14: 536e44.
2. Brunekreef, B., & Holgate, S. T. (2002). Air pollution and health. The lancet, (London, England), 360(9341), 1233-1242.
3. WHO global air-quality guidelines. Lancet 2006;368:1302.