Air quality is an integral component of our overall well-being, impacting various aspects of health.
While it is widely recognized that air pollution causes illnesses such as asthma, lung cancer, chronic obstructive pulmonary disease (COPD), cardiovascular diseases, and neurological disorders, its impact on ocular health is often overlooked. Eyes, which are only protected by a thin layer of tear film, can be very sensitive to external factors.
In this article, we will explore the critical relationship between air quality, encompassing both indoor and outdoor air pollution, and the development of eye-related illnesses. We will also delve into precautions and treatments and offer insights for safeguarding our eyes in an increasingly polluted world.
Overview of air quality and air pollution
Before we dive into their effects on ocular health, it is essential to understand what air quality and air pollution entail. There are two categories of air pollutants: outdoor and indoor.
Outdoor air pollutants arise from vehicle emissions, fuel consumption, and smoke from forest fires. These pollutants include carbon dioxides, particulate matter, and nitric oxides.1 Wildfire smoke is particularly toxic as it contains carbon monoxide, polycyclic aromatic hydrocarbons, and particulate matter, which can change based on the conditions of the incineration.2
It often goes unnoticed that indoor air pollutants in our own homes can also pose a significant risk to our health as well. Everyday tasks like cooking with oil on high heat or using aerosol cleaning products can add pollutants to our homes.1
Indoor air pollution can also arise from indoor smoking, pesticide use, and poor ventilation.1 These pollutants include particulate matter, aldehydes, volatile organic compounds, and cigarette smoke extract.1
Understanding air quality measurements
When we watch the news or look at our weather app, there is often an air quality reading. But what does this mean? Air quality is measured with the air quality index (AQI). This value ranges from 0 to 500. The greater the density of pollutants in the air, the higher the AQI level is. The table below shows the ranges and recommendations for each level.
An AQI below 50 is what we strive for and is what is considered safe for everyone. When the AQI increases above 50, sensitive groups like the elderly, pregnant individuals, children, and people with cardiovascular and/or respiratory illnesses are impacted first. As the AQI increases over 100, the general public becomes at risk.3
Table 1 outlines AQI levels and guidelines.3
| Grading | AQI Level | Recommendations |
|---|---|---|
| Good | 0 to 50 | Little to no risk. |
| Moderate | 51 to 100 | Acceptable air quality but some sensitive individuals should avoid exposure due to the risk of respiratory symptoms. |
| Unhealthy for Sensitive Groups | 101 to 150 | The general public and sensitive individuals should avoid exposure due to the risk of irritation and respiratory symptoms. |
| Unhealthy | 151 to 200 | Health risk increased for the general public but greater risk for sensitive individuals. Avoid exposure. |
| Very Unhealthy | 201 to 300 | The risk of health effects is increased for everyone. Avoid exposure. |
| Hazardous | 301 to 500 | High risk of serious health effects for everyone. Avoid exposure. |
Table 1: Courtesy of IQAir.
Recent years have seen a growing concern over air quality worldwide. According to the World Health Organization (WHO), in 2019, 99% of the world's population was living in air quality that was below the recommended guideline.4 Major cities have faced alarming levels of air pollution, primarily due to increased urbanization, industrialization, and fossil fuel consumption. In cities such as New Delhi, Dubai, and Beijing, the AQI is consistently over 100.
Air pollution caused by wildfires
Another source of air pollution that is becoming more and more prevalent every year is wildfires. Wildfire smoke can cause serious health complications. The smoke causes inflammation and oxidative stress in the body, which can lead to conditions such as pneumonia, bronchitis, and respiratory failure.2
Cellular damage can result from exposure and can lead to organ failure, which is the leading cause of death in emergency burn care centers.2 Many of the pollutants contained in wildfire smoke are similar to those found in cigarette smoke, which sheds light on the seriousness that exposure to wildfire smoke can bring.2
With such profound effects on the body, it's no surprise that poor air quality can also have destructive effects on our eyes.
The effects of air quality and pollution on the eye
Poor air quality can negatively affect the eyes, culminating in a range of conditions. Below are some of the most common conditions associated with pollution.
Dry eye disease
The ocular surface is most susceptible to poor air quality. The abrasive nature of pollutants can irritate the eyes, leading to discomfort, redness, and an increased risk of developing or worsening chronic ocular surface diseases.5 This occurs as air pollutants disrupt and create an unstable tear film.
Tear film instability is an entry point into the vicious circle of dry eye disease and can lead to the death of the cells on the ocular surface, including the corneal epithelium and conjunctival goblet cells.6 Disruption of the tear film can be seen as a rapid tear film breakup time (Figure 1), and this can result in punctate epithelial erosions (Figure 2). Examples, as seen in clinical practice, are below.
Figure 1 highlights a slit lamp exam of a patient with a rapid tear film breakup time.
Figure 1: Courtesy of Mahnia Madan, OD, FAAO.
Figure 2 demonstrates a slit lamp exam of a patient with punctate epithelial erosions.
Figure 2: Courtesy of Mahnia Madan, OD, FAAO.
Studies on the link between poor air quality and ocular surface disease
There are several studies indicating that exposure to poor air quality can lead to worsening of ocular surface diseases. A study by Saxena et al. found that inhabitants of strongly polluted areas of Delhi complained of reddening and irritation of the eyes two times more often than the control group. These participants also had increased signs of ocular surface disease as measured by the Schirmer test and tear breakup time.7
A Canadian study, which included 77,439 people who presented to the emergency department with conjunctivitis, looked at the associations between conjunctivitis and ambient air pollution levels. A strong association between conjunctivitis and air pollution, especially NO2 level, was shown.8
Exposure to air pollutants such as vehicle emissions or forest fires also results in an increase in matrix metalloproteinases (MMPs); in particular, MMP-9.9 MMP-9 is an inflammatory marker that has been associated with increased inflammation seen in patients with dry eye.10
A study concluded that patients with exposure to air of a higher AQI due to forest fires had chronic dry eye and increased MMP-9.11 This study also found that treatment of the dry eye with intense pulsed light (IPL) therapy resulted in an improvement in MMP-9 levels; however, there was reduced improvement in patients given IPL treatment while being exposed to poor air quality.11
Glaucoma
Glaucoma can arise from exposure to outdoor and indoor air pollutants. Particulate matter less than 2.5μm in diameter has been shown to be positively associated with the development of glaucoma.12
Particulate matter was not found to be a cause of increased intraocular pressure, thus alluding that this mechanism is likely neurotoxic or vascular and not pressure-dependent.13
However, exposure to higher concentrations of particulate matter is associated with structural characteristics seen in glaucoma, such as thinning of the macular ganglion cell-inner plexiform layer.1
Retinopathy
Exposure to air pollution has also been associated with increased levels of retinopathy, in particular, diabetic retinopathy. A study by Shih-Chun et al. concluded that diabetic patients who are exposed to pollutants, such as particulate matter, are more likely to develop diabetic retinopathy.14
The exact mechanism was unknown; however, it was thought to be due to the particulate matter inducing oxidative stress, increasing glucose levels, and causing inflammation.14
Maculopathy
Maculopathy, such as age-related macular degeneration (AMD), can also arise from exposure to air pollutants. A longitudinal study in Taiwan investigated the ocular effects of chronic exposure to carbon monoxide and nitric oxide.
It was concluded that exposure to these traffic-related air pollutants significantly increased the risk of developing AMD.15
Cataracts
Currently, there is no strong evidence connecting cataract formation with air pollution. Some studies from developing countries have found that household burning of biomass (charcoal, wood, and animal feces) used in stoves for cooking by poorer social groups is responsible for high levels of air pollutants.
This study in India showed a positive connection between using biomass in household stoves and cataract formation in women; however, this is not something seen in developed countries.16
Precautions to recommend to patients to avoid ocular damage
The first line of treatment is often to avoid the offending agent. It is important to limit exposure to outdoor air pollutants by staying indoors. Therefore, the indoor air must be free of pollution as well.
Exposure to indoor air pollution can be reduced by installing air purifiers, changing filters on heating and cooling systems, and ensuring that stoves and fireplaces are vented.17,18 Avoiding open flames at home can prevent pollutants from entering the air.
Another way to reduce indoor air pollutants is to avoid the use of aerosol products indoors. These products can include pesticides, hairsprays, and cooking sprays.18 In the general population, smoking has been found to have a statistically significant relationship to dry eye.19 Therefore, advocating for smoking cessation is crucial in limiting exposure to pollutants in the air and preventing dry eye symptoms.
Shielding the ocular surface with barriers
Similar to wearing a mask to prevent pollutants from entering our respiratory system, we can wear glasses or moisture chamber frames such as Ziena Dry Eye Frames to prevent pollutants from entering our eyes. This will not stop all pollutants but will reduce the exposure.
Contact lenses are not the best option to wear when there is significant pollution in the air, as the particles can get trapped in the contact lens and can further irritate the eyes.20 Goggles and/or face shields are recommended for people who are required to work when air quality is poor, such as firefighters or construction workers.
Artificial tears can provide a barrier between offending environmental agents and the ocular surface. They can help dilute and wash away allergens from the ocular surface. We recommend preservative-free artificial tears as the presence of preservatives can cause irritation and dryness in some patients. When dealing with substantial ocular inflammation, the use of anti-inflammatory eye drops and therapies is warranted.
Monitoring higher-risk patients
Patients who are at risk for glaucoma, AMD, diabetic retinopathy, and dry eye or have these ocular conditions should undergo regular monitoring and receive counseling on the impact of air pollution on their ocular health.
It is also important to note that adopting a healthier lifestyle, including a balanced diet, regular exercise, and management of systemic conditions like diabetes, can also help reduce the impact of poor air pollution on ocular health.
Conclusion
Air quality significantly affects ocular health, with implications for both short-term discomfort and long-term vision preservation.
As air pollution continues to be a global concern, eye doctors should remain informed about its effects and provide appropriate guidance to patients.
