Dust is a complex mixture of minute particles found in indoor and outdoor environments. Often measured in microns, these units reveal the microscopic nature of airborne components.
The Microscopic World of Dust
A micron, also known as a micrometer (µm), is a unit of length equal to one-millionth of a meter. To put this into perspective, a single human hair typically has a diameter ranging from 50 to 70 microns. Objects generally need to be at least 40 to 50 microns in size to be visible to the naked eye. The world at the micron level includes biological components like bacteria (1 to 10 microns) and red blood cells (7 to 8 microns in diameter).
Dust is a heterogeneous mixture of solid particles and liquid droplets suspended in the air. Its composition varies, encompassing inorganic ions, metallic compounds, elemental carbon, organic compounds, and earth’s crust components.
Measuring Dust Particle Size
The size of dust particles is categorized to describe their behavior and potential effects. Household dust, for instance, can have a broad particle size distribution, typically ranging from 0.5 to 100 microns. For air quality regulatory purposes, particulate matter (PM) is often classified by its diameter.
PM10 refers to inhalable particles with diameters of 10 micrometers or less. This category includes particles such as pollen, mold spores, and some types of dust from construction sites, landfills, and agriculture. PM2.5, known as fine particulate matter, consists of particles 2.5 micrometers or less in diameter. These fine particles are a portion of PM10 and can originate from sources like combustion of gasoline, oil, diesel fuel, or wood. Examples of PM2.5 particles include smoke (around 2.5 microns) and some pet dander (smaller than 2.5 microns).
Sources and Composition of Dust
Common indoor sources of household dust include human skin flakes (20 to 40 microns) and human hair (18 to 180 microns). Textile fibers from clothing, carpets, and upholstered furniture also contribute. Dust mites (100 to 300 microns) produce fecal pellets and body fragments (10 to 20 microns) that become airborne.
Outdoor particles like soil, pollen, and industrial byproducts enter homes via shoes, pets, clothing, or open windows. Pollen grains range from 10 to 1000 microns, with many between 10 and 100 microns. Mold spores often measure 3 to 40 microns. Combustion activities, such as burning candles or cooking, also generate fine particulate matter.
Why Dust Particle Size Matters
The size of dust particles directly influences their behavior in the air and their potential impact. Larger particles, generally those above 100 microns, tend to settle quickly due to gravity and are often visible. Conversely, smaller particles can remain suspended in the air for extended periods, making them more likely to be inhaled. For example, a 20-micron particle may settle in minutes, while a 1-micron particle can remain airborne for 12 hours.
Particle size is also a determinant of health implications. Particles larger than 10 microns are typically trapped in the nose, mouth, or throat, but can still cause irritation. Fine particles, PM2.5, pose a more significant concern because their small size allows them to penetrate deeply into the lungs and even enter the bloodstream, potentially leading to cardiovascular and respiratory issues. Exposure to PM2.5 has been linked to aggravated asthma, decreased lung function, and increased respiratory symptoms. Air filtration systems, such as HEPA filters, are designed to capture particles of various sizes, including those as small as 0.3 microns, through mechanisms like sieving, impaction, and diffusion.