Air Pollution Particles: More Deadly the Smaller They Get
Air pollution is one of our most serious global health threats. Annually, seven million people die from the effects of air pollution and, according to the World Health Organization, 9 out of 10 people in the world are breathing air that contains unsafe levels of pollutants.
Manmade sources of air pollution flood the atmosphere with an array of particles and gases. Heavy industry, transportation, and the burning of fossil fuels among other human activities release often toxic pollutants into our ecosystems, soil, water, and food chain.
Exposure to particulate matter (PM)—tiny solid particles or liquid droplets in the atmosphere—is considered the main source of air pollution-related health risks. A study estimated deaths from PM exposure in the U.S.—both indoor and outdoor—at 230,000 to 300,000 in a single year.
Pollution can travel great distances over extended periods of time. This makes it both challenging to identify specific sources of pollution and to isolate the human health effects of individual pollutants.
PM can be made of a variety of substances. From exhaust fumes and asphalt particles to microbes and allergens, PM comes from both manmade and natural sources.
Traffic is a major source of PM, stemming from the wear of vehicle components such as brakes and tires as well as the stirring up or suspension of road dust. The inorganic particles that are produced by pavement abrasion contain silicon, aluminum, potassium, sodium, and calcium, while particles from brakes and tires may contain metals such as copper, antimony, lead, cadmium, and zinc.
The intensity of PM exposure is influenced by a combination of factors that include emissions concentrations, weather (dry means more dust), seasons, topography (valleys can be pollution traps), and microenvironments, such as school classrooms.
The most significant health effects come from pollution particles smaller than the width of a human hair—a diameter of 10 micrometers or less.
Coarser particles settle rapidly and tend to lodge in the trachea (upper throat) or the bronchi. If inhaled, coarse PM is initially collected in the nose or throat, sometimes causing the body to react and expel the intruder(s) through sneezing and coughing.
It is generally acknowledged that the particles with the most significant health effects have a diameter of less than 10 micrometers (smaller than the width of a human hair). PM in the 5-10 micrometer range is most likely to be deposited in the tracheobronchial tree, while those smaller (1-5 micrometer) in size tend to settle in the respiratory bronchioles and the alveoli where gas exchange occurs.
Particles smaller than 1 micrometer behave in ways similar to gas molecules and will therefore penetrate—powered by diffusion—down to the alveoli from where they can then move further into the cell tissues and circulation system.
What Diseases do PM Exposure and Penetration Lead to?
Exposure to PM can produce health conditions that require treatment for a variety of respiratory symptoms, including decreased lung function and the exacerbation of chronic respiratory and cardiovascular diseases, all of which can lead to premature death. Milder respiratory symptoms and problems associated with PM exposure include shortness of breath, chest discomfort and pain, and coughing and wheezing. A national U.S. epidemiologic study found a strong, consistent correlation between adult diabetes and PM exposure after adjusting for other risk factors like obesity and ethnicity.
How does PM Exposure Affect Children?
Results of scientific studies suggest that exposure to high particle levels may lead to poor childbirth outcomes that include low birth weight, preterm deliveries, and even fetal and infant mortality. Exposure to PM was also reported to affect lung development in children, including reversible reductions in lung function as well as reduced lung growth rates and weakened long-term lung function.
When children are exposed to excessive levels of PM for twenty-four hours, they incur a significantly higher risk of experiencing respiratory symptoms, asthma medication use, and reduced lung function. A study in 2014 found that children with asthma in Guadalupe experienced a greater risk of visiting an emergency room, likely due to PM pollutants in Saharan dust. A study of two-to-six-year-old predominantly African American children in East Baltimore, Maryland found associations between exposure to PM and worsening asthma symptoms.
And What About the Elderly?
It is well-established that PM exposure is linked to an increased risk of hospitalization for heart attacks among the elderly, as well as exacerbation of congestive heart failure. One study suggested that there is a possible association between rises in PM levels and increases in plasma viscosity, inflammation markers, endothelial (thin membrane that lines the heart) dysfunction, and altered autonomic control of the heart. Moreover, it was observed that PM had a significant role in initiating and promoting a condition—atherosclerotic progression—that is responsible for most cardiovascular diseases.
A panel study in Los Angeles using data from 798 participants in two clinical trials found that increases in PM were associated with increased thickness of the carotid artery lining, a condition that can lead to thrombosis. In another study, based on a survey of Boston residents, it was seen that exposure to PM led to decreased tone of the vagal nerve in the brain as well as reduced heart rate variability.
Effective Management of Air Quality is Crucial
There is growing public awareness that even at relatively low concentrations the health burden of air pollution is significant. Consequently, effective management of air quality is necessary to minimize health risks.
This article is based on the author's original presentation at the Twenty-Sixth International Conference on the Unity of the Sciences held in 2020.
*Ki-Hyun Kim, Ph.D., is a distinguished professor of the Department of Civil & Environmental Engineering at Hanyang University in Seoul, South Korea. He has published more than 850 articles in leading scientific journals, including Chemical Society Reviews and Progress in Materials Sciences.