Particulate matter 2.5, commonly known as PM2.5, is a significant air pollutant. Understanding its origins and formation is important for comprehending its impact on air quality and public health. It is found both indoors and outdoors, making its study relevant for environmental health.
Understanding PM2.5
PM2.5 is particulate matter with a diameter of 2.5 micrometers or less. To grasp its minuscule size, an average human hair is approximately 70 micrometers in diameter, making PM2.5 particles about 30 times smaller than a single strand of hair.
This fine particulate matter consists of solid and liquid particles, including dust, soot, metals, and various chemicals. Its small size allows PM2.5 to remain suspended in the air for extended periods, traveling long distances from its source. This characteristic also enables these particles to bypass the body’s natural defenses, penetrating deep into the respiratory tract and entering the bloodstream.
Primary Contributors
Primary contributors are sources that directly release PM2.5 into the atmosphere. These emissions can stem from both human activities and natural events. The combustion of various materials is a major global source, directly releasing fine particles.
Vehicle exhaust, particularly from diesel and gasoline engines, is a significant source of directly emitted PM2.5. Industrial processes also contribute, with emissions from power plants, factories, and smelters. Domestic burning of fuels like wood in stoves and fireplaces, along with outdoor burning of waste or agricultural residues, also directly emits PM2.5.
Beyond combustion, non-combustion sources also directly release PM2.5. Dust from construction sites, unpaved roads, agricultural activities, and wind-blown dust from natural sources like deserts, contributes to primary PM2.5 levels. Natural events such as volcanic eruptions and wildfires also release large amounts of PM2.5 directly.
Atmospheric Formation
Beyond direct emissions, PM2.5 can also form in the atmosphere through complex chemical reactions, a process known as secondary PM2.5 formation. This occurs when certain gaseous pollutants, termed precursor gases, react to create new solid or liquid particles.
Key precursor gases include sulfur dioxide (SO2), nitrogen oxides (NOx), volatile organic compounds (VOCs), and ammonia (NH3). These gases undergo chemical reactions, often with sunlight and specific atmospheric conditions, to form substances like ammonium sulfate, ammonium nitrate, and secondary organic aerosols. For instance, ammonium nitrate forms from reactions involving nitrogen oxides and ammonia, favoring high humidity environments.
Sources of these precursor gases include human activities such as emissions from power plants, industrial facilities, and vehicle exhaust. Agricultural emissions of ammonia also contribute to the formation of secondary PM2.5, particularly ammonium nitrate. This secondary formation is a substantial contributor to overall PM2.5 levels, especially in certain regions or during specific seasons.
Impact on Health
Exposure to PM2.5 can lead to adverse health effects, impacting multiple bodily systems. The small size of these particles allows them to penetrate deeply into the lungs, irritating the alveolar walls. This deep penetration can lead to conditions such as asthma, bronchitis, and reduced lung function. Long-term exposure has been linked to increased hospital admissions for respiratory causes and exacerbated respiratory diseases.
The cardiovascular system is also affected by PM2.5 exposure. Fine particles can enter the bloodstream, triggering systemic inflammation and stress, which can increase the risk of heart attacks, strokes, and other cardiovascular problems. Studies have shown a link between increased daily PM2.5 exposure and higher cardiovascular hospital admissions, emergency department visits, and deaths.
Beyond respiratory and cardiovascular issues, research suggests other health impacts, including increased risks of neurodegenerative diseases, diabetes, and adverse birth outcomes. Vulnerable populations, such as children, the elderly, pregnant individuals, and those with pre-existing heart or lung conditions, are more sensitive to PM2.5 pollution. Long-term exposure to PM2.5 contributed to over 4 million deaths globally in 2019.