The novel coronavirus can be carried through the air by aerosol particles to unsuspecting people who breathe them in. This is why we wear masks and try to stay socially distant. For months the rule of thumb has been for individuals to stay 1.5 meters apart to lessen transmission, although experts have more recently shown that very fine particulate matter, or PM, could carry SARS-CoV-2 virus particles farther. What has received little media coverage is that diesel exhaust particles could be a big part of this spread.
Diesel exhaust, emitted by diesel engines, is a major source of fine particles suspended in city air. They can cause a range of health problems, from heart disease to cancer. They have been shown to carry viruses. Most of the fine particles come from transportation, caused by incomplete combustion. The emissions are worst from low-quality fuel-oil diesel commonly used in shipping, and in some developing cities in old trucks and auto-rickshaws. But high-quality diesel still results in significant emissions, even under the latest vehicle technology standards. The particles are not visible: 90 percent of them are less than one micrometer in diameter. That is far smaller than the two size categories implicated in health effects—less than 10 micrometers (PM10) and less than 2.5 micrometers (PM2.5), meaning they can spread in air even easier.
A number of studies show a positive correlation between the incidence and mortality of COVID-19 and the level of air pollution. In early 2020, studies conducted in Wuhan, China, found that viruses exhaled by infected people could reach new people who were not very close. The researchers suggested that air pollution may have been a factor in the spread. The Italian Society of Environmental Medicine found a significant correlation between the geographical distribution of PM10 orPM2.5 particles in 110 Italian provinces and the incidence of COVID-19 infection in those areas.
Other scientists in Italy later found that COVID-19 mortality in the high-particulate air pollution areas of Lombardy and Emilia Romagna was about 12 percent, compared with about 4.5 percent in the rest of Italy. They also found that the virus’s genetic material, its RNA, was present in particulate matter. They suggested that in places with high particulate concentrations and stable air, particulates holding the virus grouped into airborne clusters, enhancing its persistence in the atmosphere.
Global data support these ideas. One paper found a correlation between cities with high particulates noted in annual indexes of air quality with high incidence of COVID-19 infection—in China, Iran, Italy, Spain, France, Germany, the U.K. and U.S. Following that, a survey conducted by the Harvard School of Public Health found there was a strong association between an increase in particulate matter and mortality: An increase of one microgram per cubic meter of air in long-term exposure to PM2.5 was associated with an 8 percent increase in the COVID-19 mortality rate.
A recent study in 355 Dutch communities examined the associations between atmospheric concentrations of PM2.5, nitrogen dioxide (NO2), sulfur dioxide (SO2) and COVID-19 cases, both hospital admissions for them and deaths from them. It found compelling evidence of a statistically significant positive relationship for PM2.5, and to a lesser extent NO2. The SO2 levels were not a big factor. The researchers concluded that a one microgram per cubic meter increase in PM2.5 concentration was associated with 9.4 more COVID-19 cases, 3.0 more hospital admissions and 2.3 more deaths.
These various studies lead us to conclude that particulate air pollution either makes people more susceptible to COVID-19 infection or brings the virus into their lungs. The studies are primarily based on variations in human exposure to air pollution and occurrences of COVID-19 among large populations, however. Evidence of a linkage could be strengthened by studies of small, local populations in many large cities that have excellent data on standard measures of air pollution and the COVID-19 caseloads. That is what originally clinched the case for associating small-particle air pollution with heart disease and cancer.
Should we wait for this evidence before acting to lessen the disease burden COVID-19 brings? We argue no. Many high-income cities and countries are already working to reduce diesel exhaust pollution and the health complications it creates. Their efforts should be redoubled, especially in COVID-19 areas.
Pollution reductions have begun serendipitously, too, in many cities where “lockdowns” have been imposed, naturally curbing fossil-fueled transportation. Some national and local governments are already seeking to maintain these reductions as lockdowns are eased, in some cases by closing streets or building more infrastructure that encourages cycling.
Prior to the pandemic, some cities were already starting to replace diesel trucks and buses with electric versions, and planning new electric transit such as trackless trams. But the transition away from diesel is slow. There are only around 400,000 electric buses in the world, mostly in cities trying to reach goals for 2040 established in the Paris climate accord. Cities such as Canberra, Sydney and Brisbane in Australia have begun to electrify their bus fleets. The European leader, the Netherlands, has now electrified 10 percent of its bus fleet, and all new buses after 2025 will be electric. China has electrified 17 percent of its bus fleet. The U.S. plans for a third of all buses to be electric by 2045.
Electrification of diesel transport has several environmental advantages, but now that there is a powerful health dimension, such agendas should accelerate. Plans to phase out new diesel trucks and other new diesel vehicles by 2030 should be adopted. An international study published in October suggests that around 15 percent of COVID-19 deaths can be linked to PM2.5 air pollution, and it has called for regulations to quicken the switchover to electric transportation. It concludes: “A lesson from our environmental perspective of the COVID-19 pandemic is that the quest for effective policies to reduce anthropogenic emissions, which cause both air pollution and climate change, needs to be accelerated.”
We agree, and note that reducing diesel exhaust could lessen risk from future airborne pandemics as well. Business as usual should no longer be an option.
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