Masks offer a degree of protection against poor air quality by filtering ambient pollutants, such as those found in smoke and smog, before they are inhaled. The effectiveness of any mask against air pollution is determined by two primary factors: the material’s ability to filter tiny particles and the mask’s ability to form a tight seal against the face. Masks are not a solution for cleaning the surrounding air, but they act as a personal air purification system, providing a cleaner pocket of air for the person wearing them.
Defining the Particulate Threats to Air Quality
Poor air quality is primarily defined by the concentration of airborne particulate matter (PM), which represents a complex mixture of extremely small solid particles and liquid droplets suspended in the air. These particles originate from various sources, including vehicle exhaust, industrial processes, wildfires, and dust storms. The size of these particles is the critical factor determining both the health risk and the difficulty of filtration.
The most concerning pollutants are designated as PM10 and PM2.5, referring to particles with diameters of 10 micrometers and 2.5 micrometers or less, respectively. PM10 is small enough to bypass the body’s natural defenses in the nose and throat, allowing it to enter the lungs. The smaller PM2.5 is particularly harmful because its microscopic size allows it to travel deep into the respiratory tract and even pass into the bloodstream.
Exposure to these fine particles is linked to serious health problems, including aggravated asthma, reduced lung function, nonfatal heart attacks, and an increased risk of premature death in individuals with pre-existing heart or lung disease. Minimizing inhalation is the main goal during periods of high air pollution. The challenge for protective masks is capturing these tiny particles, especially those in the sub-micron range, before they can be drawn into the lungs.
How Different Mask Materials Filter Airborne Particles
Masks filter airborne particles not by acting as a simple sieve, but by employing several complex physical mechanisms that vary based on the filter material. One mechanism, called inertial impaction, works well for larger particles (around 1 micrometer and larger); these particles have enough momentum that they collide with and stick to the filter material. The smallest particles, generally 0.1 micrometers and smaller, are captured through diffusion, where their random movement (Brownian motion) causes them to collide with the mask fibers and become trapped.
Particles in the mid-range, particularly around 0.3 micrometers, are the most difficult to filter and represent the “Most Penetrating Particle Size” (MPPS) because they are too small for effective inertial impaction and too large for efficient diffusion. This is where electrostatic attraction becomes important, particularly in high-efficiency masks like N95 respirators. N95s use layers of fine, nonwoven polypropylene fibers that carry a static electric charge, which acts like an invisible magnet to attract and capture these moderately sized particles, significantly boosting filtration efficiency.
N95 respirators are designed to block at least 95% of airborne particles at the MPPS under laboratory test conditions. Surgical masks, primarily designed as a fluid barrier, typically have a lower filtration efficiency for fine aerosols, sometimes ranging from 42% to 88% at the 0.3 micrometer size even under a perfect seal condition. Cloth masks, which rely mostly on mechanical interception and straining, offer minimal and highly variable protection against fine particulate matter, often showing filtration efficiencies between 16% and 23% for fine particles, depending heavily on the fabric type and number of layers.
Why Proper Seal Determines Real-World Protection
The theoretical filtration efficiency of a mask material is meaningless if the mask does not form a complete seal against the wearer’s face, a problem known as “leakage.” Air follows the path of least resistance, meaning that if there is a gap around the nose bridge, cheeks, or chin, unfiltered ambient air will rush through that opening instead of being drawn through the filter material. This effect drastically reduces the amount of clean air the wearer actually receives.
High-efficiency respirators, such as the N95, require a tight-fitting design to ensure that all inhaled air passes through the filter media. To provide their stated protection level, users in occupational settings must undergo a process called fit testing. This test, which can be qualitative or quantitative, confirms that a specific size and model of respirator seals correctly to an individual’s unique facial structure, including performing movements like talking and moving the head.
Surgical and cloth masks, which are typically loose-fitting by design, cannot achieve the necessary seal to effectively block fine particulate matter from air pollution. While they may filter a significant amount of air, the gaps around the edges allow fine, hazardous particles to bypass the filter entirely. Without a reliable seal, a mask’s real-world effectiveness against fine air pollution can drop significantly, sometimes yielding an actual performance efficiency of 60–80% for an N95 and even lower for other types.