While the coronavirus pandemic may have affected people in various ways throughout the world, its impact has not been felt equally among different regions. Some people are more likely than others to be infected by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and to die from the coronavirus disease 2019 (COVID-19), the disease caused by SARS-CoV-2. Among the many factors causing this unequal impact, one significant one is air pollution, especially particulate matter concentration. As evidenced by research, high particulate matter concentrations increase COVID-19 mortalities because they cause damage to the human body and serve as a carrier for SARS-CoV-2.
Particulate matter is one of the many types of air pollution. Its concentration is usually measured in micrograms per cubic meter (μg/m3), and it is made up of chemicals like nitrates, carbon, sulfates, and mineral dusts. The most harmful types of particulate matter are PM2.5, which is 2.5 micrometers in length, and PM10, which is 10 micrometers in length. In fact, PM2.5 is the most harmful form of air pollution in the United States, as it can travel deeply into the lungs (National Institute of Environmental Health Sciences, 2020; Comunian et al., 2020). Because car ownership has rapidly increased in the past several decades, one of the largest sources of particulate matter globally is transportation, especially in urban areas (Magazzino et al., 2020). Other sources of particulate matter include other forms of fossil fuel combustion, wildfires, and cigarette smoke (National Institute of Environmental Health Sciences, 2020).
Numerous studies conducted throughout 2020 show positive correlation between particulate matter concentration and COVID-19 mortalities. One such study used environmental monitoring stations to collect data on the particulate matter concentrations of three French cities: Paris, Lyon, and Marseilles. The researchers also collected COVID-19 mortality data from the French National Public Health Agency and then used a machine-learning algorithm to find trends between the two data sets. The data shows a positive correlation between particulate matter concentration and COVID-19, and the researchers were able to determine particulate matter concentration thresholds for each city that must not be exceeded. These thresholds were 17.4 μg/m3, 15.6 μg/m3, and 14.3 μg/m3of PM2.5 and 29.6 μg/m3, 20.6 μg/m3, and 22.04 μg/m3 of PM10 for Paris, Lyon, and Marseille, respectively (Magazzino et al., 2020). Another study examined county-level COVID-19 mortality data for 98% of all counties in the United States The researchers found that an increase of just 1 μg/m3of PM2.5 yields an 8% increase in the COVID-19 mortality rate (Wu et al., 2020).
Clearly, there is a strong correlation between particulate matter concentration and COVID-19 mortalities. But what fraction of all COVID-19 deaths can be traced back to particulate matter? A study published in the journal Cardiovascular Research attempted to find out. Using satellite PM2.5 data, the researchers attributed approximately 15% of all COVID-19 deaths to PM2.5. That figure was 18% in the United States and 17% in North America (Pozzer et al., 2020). The clear relationship between particulate matter concentration and COVID-19 mortalities is likely one of the reasons why certain races have had higher COVID-19 mortality rates than others. For example, as of April 9th, 2020, African-Americans accounted for 42% of all COVID-19 deaths in the United States even though they constitute only 13% of the population. One reason for COVID-19’s disproportionate effect on African-Americans: they are more likely than other races to be exposed to air pollution (Gupta, 2020).
How do high particulate matter concentrations increase COVID-19 mortalities? There are many ways and theories, but not enough research has been done to know the exact answer. One way is that particulate matter can serve as a carrier for SARS-CoV-2, increasing its spread. SARS-CoV-2 RNA has been found in PM10 samples. By coagulating, or changing from a fluid state to a solid-like state, viruses can accumulate onto particulate matter and stay in the air for days, possibly infecting people who are nearby (Tung et al., 2021; Comunian et al., 2020).
Another way high particulate matter concentrations increase COVID-19 mortalities is by causing oxidative stress. Normally, electrons orbit atoms in pairs. An atom or molecule containing an electron that is not paired with another electron is called a free radical. Free radicals try to react with random molecules to oxidize them, or steal electrons from them. This can be dangerous, as it leads to important molecules being denatured. To maintain homeostasis, the body uses antioxidants, which selectively react with free radicals to reduce them, or give them an electron, without causing much damage. When reduced, free radicals become much more stable and less dangerous. Oxidative stress happens when there are too many free radicals and not enough antioxidants (Kelly, 2003). Studies, including one conducted in 1996 by inserting PM10 into the tracheas (windpipes) of rats’ lungs, show that particulate matter has metallic and organic components that contain free radicals (Li et al., 1996; Kelly, 2003; Comunian et al., 2020). Therefore, exposure to particulate matter causes oxidative stress in the lungs. When oxidative stress occurs in the lungs, inflammatory cells from the immune system release more free radicals in an effort to attack. Unfortunately, these free radicals injure the lung’s epithelial tissue (the membranous tissue that covers and protects the lungs), making the lungs weaker and more susceptible to severe effects of viruses like SARS-CoV-2 (Kelly, 2003; Li et al., 1996; Comunian et al., 2020). In addition to causing epithelial injury, the free radicals from particulate matter prevent the immune system from focusing its attention on attacking an invading virus, as the immune system has to simultaneously respond to the free radicals and to the virus (Comunian et al., 2020).
A third way that particulate matter increases COVID-19 mortalities is by causing angiotensin converting enzyme 2 (ACE-2) to be overexpressed in the lungs. ACE-2 has multiple jobs, and one of its jobs is to inhibit oxidative stress and excessive inflammation, both of which particulate matter causes in the lungs. So, when exposed to particulate matter, the body increases its expression of ACE-2 in order to maintain homeostasis. However, ACE-2 also serves as a receptor for SARS-CoV-2 on the cell membrane. This is because SARS-CoV-2 has a spike protein that can bind with ACE-2, allowing the virus to invade cells through receptor-mediated endocytosis. Thus, increased exposure to particulate matter results in an increase in the number of cells SARS-CoV-2 can invade, making the body more vulnerable to death from COVID-19 (Paital & Agrawal, 2020; Comunian et al., 2020; Magazzino et al., 2020).
The coronavirus pandemic has affected the lives of billions, and long-term exposure to particulate matter pollution has only made its impact worse. Approximately 15% of all worldwide COVID-19 mortalities can be attributed to PM2.5 exposure (Pozzer et al., 2020). By serving as a carrier for the virus and damaging the body’s lungs, particulate matter is a clear biological danger to humans. In order to lessen COVID-19 mortalities, particulate matter concentrations must be immediately reduced around the world.
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