Household dust is a complex mixture of organic and inorganic particles, including skin flakes, textile fibers, pollen, mold spores, and combustion byproducts. Quantifying the size of these airborne materials is the first step in understanding their behavior and impact on indoor environments. This quantification is achieved using the micron, a unit of measurement that allows for the precise discussion of extremely small particulate matter.
Understanding the Micron
A micron, formally known as a micrometer (\(mutext{m}\)), is a unit of length equal to one-millionth of a meter. This tiny scale is necessary when discussing airborne particles, as they are largely invisible to the naked eye, which can typically only detect objects larger than about 40 microns.
For perspective, the diameter of an average human hair is approximately 50 to 100 microns, making it many times larger than the most hazardous dust particles. A human red blood cell is significantly smaller, measuring about 6 to 8 microns across.
The Typical Size Range of Dust Particles
Airborne dust is categorized into distinct groups based on its size, with the most common being the inhalable coarse fraction and the fine fraction. The coarse fraction, referred to as PM10, consists of particles with an aerodynamic diameter of 10 \(mutext{m}\) or less. These particles generally range from 2.5 to 10 microns and commonly originate from mechanical processes like construction, road dust, pollen (10 to 1000 \(mutext{m}\)), and mold spores (3 to 30 \(mutext{m}\)).
The fine particle fraction, or PM2.5, includes any particle with a diameter of 2.5 \(mutext{m}\) or less and is often composed of hundreds of different chemicals. This fraction includes combustion byproducts like smoke, vehicle exhaust, and industrial emissions, which form in the atmosphere through complex chemical reactions. Ultrafine particles, which are less than 0.1 \(mutext{m}\), are a subset of PM2.5 and are produced by sources such as welding and certain chemical reactions.
How Particle Size Determines Health Risk
The size of a dust particle is the primary factor determining where it will deposit within the respiratory system upon inhalation. Particles larger than 10 \(mutext{m}\) are considered non-respirable, meaning they are efficiently trapped and filtered out by the hairs and mucous membranes in the nose and throat, preventing them from traveling deep into the lungs.
Particles in the 2.5 to 10 \(mutext{m}\) range can bypass the upper respiratory defenses and penetrate the trachea and larger bronchi of the lungs. Their presence in the tracheobronchial region can cause irritation and contribute to respiratory symptoms.
The most significant health risk is posed by fine particles (PM2.5) and ultrafine particles, which are small enough to reach the alveoli, the deepest part of the lungs where gas exchange occurs. Once in the alveoli, these particles can cross the lung barrier and enter the bloodstream, allowing them to affect other organs, including the heart and brain.
Filtration and Air Quality Standards
Air quality standards and filtration technologies are designed to target the micron sizes of particulate matter that pose the greatest health risks. The Minimum Efficiency Reporting Value (MERV) scale rates air filters based on their ability to capture particles between 0.3 and 10 microns. Filters with higher MERV ratings, such as MERV 13 to 16, are more effective at capturing smaller, hazardous particles like bacteria and fine dust.
High-Efficiency Particulate Air (HEPA) filters represent a higher standard, certified to capture at least 99.97% of particles that are 0.3 microns in diameter. This standard is relevant because 0.3 \(mutext{m}\) is the size range that particles are most likely to pass through a filter due to a combination of physical mechanisms.