Dust is a complex mixture of microscopic solid or liquid particles suspended in the air, varying in chemical composition. The size of these particles significantly impacts their behavior in the atmosphere and their potential effect on human health and the environment.
Defining Dust Particle Sizes
The size of dust particles is measured in micrometers (µm). Particulate matter (PM) is classified into categories based on their aerodynamic diameter. This classification helps in understanding how deeply particles can penetrate the respiratory system.
PM10 refers to inhalable particles 10 micrometers or less in diameter. These are sometimes called coarse particles. For context, an average human hair is approximately 70 micrometers in diameter, meaning a PM10 particle is about seven times smaller than the width of a human hair.
PM2.5 includes fine inhalable particles 2.5 micrometers or less in diameter. PM2.5 particles are smaller than PM10 particles, and about 30 of them could fit across the width of a single human hair. Ultrafine particles (UFPs) are even smaller, defined as having a diameter of less than 0.1 micrometers.
Sources of Dust Particles by Size
The origin of dust particles correlates with their size. Larger particles, such as PM10, come from sources that directly release them into the atmosphere. Examples include dust from construction sites, unpaved roads, and agricultural activities.
Natural occurrences like wind-blown dust from open lands, wildfires, and pollen also contribute to PM10 levels. These coarse particles are primarily direct emissions.
Smaller particles, like PM2.5, originate from combustion processes. Vehicle exhaust, industrial emissions, and the burning of wood or other biomass are sources of these fine particles. PM2.5 can also form in the atmosphere through chemical reactions involving gases like sulfur dioxide and nitrogen oxides.
Ultrafine particles (UFPs) are byproducts of similar combustion reactions and industrial processes. These can include emissions from power plants, gas and coal combustion, and even tire wear from vehicles. Natural sources like volcanic lava and ocean spray can also produce UFPs.
How Particle Size Affects Health and the Environment
The size of dust particles directly influences how deeply they can penetrate the human respiratory system, impacting health. Larger particles, such as PM10, are filtered by the nose and throat, leading to irritation of the eyes, nose, and throat. These particles can still reach the upper parts of the lungs, affecting lung function and respiratory health.
PM2.5 are small enough to bypass natural defenses and travel deep into the lungs, reaching the alveoli and terminal bronchioles. From there, they can even enter the bloodstream. Exposure to PM2.5 is associated with various health problems, including aggravated asthma, decreased lung function, and increased respiratory symptoms like coughing and difficulty breathing.
Long-term exposure to PM2.5 has been linked to serious conditions such as cardiovascular diseases, heart attacks, irregular heartbeat, and premature death, particularly in individuals with pre-existing heart or lung conditions. Ultrafine particles (UFPs), being the smallest, pose a greater health risk due to their ability to penetrate deeper into the lungs and potentially cause enhanced cardiovascular toxicity and oxidative stress. There is no safe level of particulates, and worldwide, PM2.5 exposure contributed to 7.8 million deaths in 2021.
Beyond human health, particulate matter has environmental impacts. Fine particles, especially PM2.5, are a primary cause of reduced visibility, creating haze in both urban and scenic areas. Particles can travel long distances and settle on land and water, leading to environmental damage.
Depending on their chemical composition, settled particles can make lakes and streams acidic, alter the nutrient balance in coastal waters, and deplete soil nutrients. This can harm sensitive forests and farm crops, affect ecosystem diversity, and contribute to acid rain effects that damage materials like stone monuments.