Wildfire Particulate Matter Poses Immediate and Long-Term Threats to Eyes

Smoke and other particulate matter stemming from a cluster of large Canadian wildfires has invaded much of the Northeastern US this week, engulfing New York City, Philadelphia, Washington D.C., and thousands of points across the Eastern Seaboard in pollutants and, in New York, an eerie sepia-toned haze. For many areas, the air quality index (AQI) intermittently swung between the highest levels of health concern, from “dangerous for sensitive groups” to “hazardous emergency for all.” Most health safety measures being issued advised residents of affected areas to remain indoors with windows closed and, if they must venture out, wear an N95 or KN95 mask to filter harmful particulate matter from the respiratory system. But, while these safety measures can help prevent flare ups of cardiovascular or lung conditions, fewer news resources are addressing the potential ocular effects of this situation. 

However, the eye is certainly vulnerable to environmental pollutants.

While the AQI is now dropping in many regions, climate scientists have been been predicting an increase in Canadian wildfires — and a resulting decrease in air quality — for years.¹ It’s likely that cities across both Canada and the United States can begin to expect more events like this one. Some of those events are likely to result in short-term ocular irritation. Others can instigate long-term damage, potentially making chronic eye diseases more prevalent.

These air quality threats have been known to ophthalmology researchers in Asia, Canada, and other areas for some time now. US ophthalmologists may want to spend some time reviewing their findings in the event that wildfire-associated ocular presentations appear in their offices this week, or the future. 

Pollution and The Ocular Surface

The primary concern with the air quality on the East Coast following this event is the particulate matter it transports. Particulate matter can elevate conjunctival disease and dry eye signs and symptoms, trigger oxidative stress in the cornea, and even increase the odds of age-related eye diseases, such as glaucoma and macular degeneration.^2-6 

The aftermath of the wildfires put New York City briefly on top of the world’s worst air quality list last week, as levels of particulate matter with a width of 2.5 microns (PM_2.5) soared. “PM_2.5 carries toxic components, such as metal, nitrate, sulfate and polycyclic aromatic hydrocarbons (PAHs), and a biological fraction (such as endotoxin), and is known to induce cell death in various ways, including autophagy and pyroptosis,” a research team explains.⁷

While this is rare on the East Coast of the US, high levels of PM_2.5 are common in cities such as New Delhi, India. 

“Conjunctival disease caused by air pollution can manifest as subclinical ocular surface changes that frequently cause major discomfort, such as burning and grittiness,” according to a 2022 review that took its findings from research into the high levels of traffic- and industry-related airborne byproducts in New Delhi.² “Moreover, persistent exposure to air pollution can result in cellular transformation, including goblet-cell hyperplasia in the human conjunctival epithelium.” 

A study from Hangzhou, China shows that daily concentrations of PM_2.5 and the larger PM₁₀ can predict the likelihood that ophthalmologists will see patients with dry eye complaints that day, suggesting an immediate impact of high particulate matter levels.³ 

The same study shows both concentrations have a lasting effect on dry eye diagnosis for up to 2 and 3 days, respectively. Additional research out of China shows that exposure to PM_2.5 levels greater than 81.21 μg/m³ for longer than 143 days in 1 year can increase an individual’s risk of a dry eye disease diagnosis by 2.01 (95% CI=1.79-2.25; P <.0001).

In the cornea, that cellular transformation can manifest in the form of increased oxidative stress and inflammatory markers. This kind of ocular environment can be especially risky for patients with Pseudomonas aeruginosa infections, exacerbating keratitis and contributing to rapid corneal perforation.⁴

Fine Particulate Matter and The Posterior Segment

It’s intuitive to suspect that smokey, smoggy air can irritate the ocular surface. But long-term exposure to pollutants can instigate inflammation, elevate intraocular pressure, and lead to chronic retinal disease, as well. “The relationship between PM_2.5 and glaucoma is biologically plausible,” according to a Canadian research team. “Emerging evidence from mouse models suggests that PM_2.5 exposure may contribute to glaucoma and ocular hypertension.” Additionally,  individuals living in areas with higher PM_2.5 are more likely to report glaucoma. Even childhood glaucoma appears to have a link to particulate matter exposure (HR=1.22; 95% CI, 1.15-1.28).⁵

The same investigation shows that age-related macular degeneration (AMD) can be significantly worsened in patients who are exposed to high levels of PM_2.5.^5,6 “We also found that higher levels of PM_2.5 were associated with visually impairing AMD in a single pollutant model,” the study authors explain. “However, this relationship was attenuated and had only borderline statistical significance in the multi-pollutant model, particularly after entering SO₂. PM_2.5 was not associated with all AMD in either single or multi-pollutant models. We assume the reason for this is that PM_2.5 does not affect the early AMD process, which does not affect visual acuity, but rather only the late AMD neovascularization process, which does.”⁵

A review shows that 4 studies conducted in 2021 all show rates of glaucoma elevated in areas with high exposure to PM_2.5. Additionally, research shows other pollutants (such as NO₂, SO₂, and O₃) all have a null impact on glaucoma rates.⁶

Additional research shows more than just an epidemiological connection. PM_2.5 can penetrate the cornea and affect tissue within the eye, according to a study conducted by researchers at the Shanghai Medical College in China.⁷

PM_2.5 pollutants, like those associated with the recent wildfire smoke, can instigate ocular hypertension (and glaucoma) because once it penetrates the ocular surface, it induces cell pyroptosis and inflammation in the trabecular meshwork and other tissue, consequently elevating intraocular pressure (IOP). This reduces production of reactive oxygen species.⁷

“PM_2.5 has a direct toxic effect on intraocular tissues and may contribute to the initiation and development of ocular hypertension and glaucoma,” the researchers report. “This occurs as a result of increased oxidative stress and the subsequent induction of NLRP3 inflammasome mediated pyroptosis in trabecular meshwork cells.”⁷

Treatment and Prevention 

With widespread events such as the Canadian wildfire, it’s vital to obtain the best patient history possible. In particular, knowing whether a patient works indoors or outdoors can help establish their risk of particulate matter exposure. 

Those who work outdoors are certainly at greater risk of exposure and may consider protective eye wear when the AQI is elevated.⁸ But windows and doors can’t keep all particulate matter at bay. 

AirNow — a partnership of the US Environmental Protection Agency, National Oceanic and Atmospheric Administration (NOAA), National Park Service, NASA, Centers for Disease Control, and tribal, state, and local air quality agencies — recommends using indoor air cleaners with high-efficiency mechanical filters, or electronic air cleaners that remove particles. The group also advises using a central air conditioning system, preferably one with a “fresh air” option, that does not draw air from outdoors and has an up-to-date filter.⁹ 

For ocular surface inflammation, artificial tears can help relieve some symptoms. A study out of Korea shows that it may be possible to wash some fine dust particles out of the eye before they cause extensive damage.¹⁰

“In addition, eye drops, such as artificial tears, diquafosol, and cyclosporin A, can be used to prevent and treat ocular surface disease and deterioration of the damage,” according to that report. “In patients who already have ocular surface disease, the harmful effects of PM exposure may be greater and more attention should be paid to eye health management.”¹⁰

The effects of climate change are likely to result in more events like the East Coast experienced last week. Ophthalmologists practicing in geographic regions where the population is not accustomed to these kinds of events can consider this a wake-up call. The time to begin organizing a response toolkit is now. With the available research, clinicians can begin to identify which of their patients are likely to be at risk, how that particulate matter can further increase an individual patients’ likelihood of disease development or progression, and develop a protocol patients can enact to keep their eyes and vision healthy.  

This article originally appeared on Ophthalmology Advisor