Air purifiers are an effective supplementary tool for reducing the concentration of SARS-CoV-2 aerosols indoors. These devices utilizing High-Efficiency Particulate Air (HEPA) technology actively remove airborne viral particles from shared spaces. By continuously filtering the air, they lower the overall risk of exposure in a given room. This technology offers a significant layer of protection when combined with other public health measures.
How Respiratory Viruses Spread Indoors
The primary way SARS-CoV-2 spreads is through the air via respiratory particles released when an infected person breathes, speaks, coughs, or sneezes. These particles are categorized into two main groups: larger droplets and smaller aerosols. Droplets are relatively heavy, typically falling quickly to surfaces within a few feet of the source.
Aerosols are microscopic particles that are small enough to remain suspended in the air for minutes or even hours. These tiny, virus-laden particles can travel well beyond six feet and accumulate throughout an entire room. This airborne transmission presents the greatest risk in poorly ventilated and crowded indoor environments. Air purification directly addresses this risk by systematically removing these lingering aerosol particles from the environment.
The Mechanism of Air Filtration and Particle Capture
Air purifiers rely on mechanical filtration, most often using a HEPA filter, to remove virus-sized particles. A HEPA filter is composed of a dense, randomized mat of fine glass fibers arranged in pleats. The official standard requires that a HEPA filter must capture a minimum of 99.97% of particles that are 0.3 microns in diameter. This 0.3-micron size is known as the Most Penetrating Particle Size because it is the most difficult to trap.
HEPA filtration uses three main physical mechanisms to capture particles of all sizes. Larger particles are stopped by impaction, where they collide directly with the filter fibers, and interception, where they follow the air stream but come close enough to a fiber to stick to it. For the smallest particles, including the SARS-CoV-2 virus itself (around 0.1 microns), the mechanism of diffusion is highly effective.
Diffusion causes ultrafine particles to move erratically due to collisions with gas molecules, increasing the probability that they will contact and stick to a fiber. This means HEPA filters are highly efficient at capturing particles significantly smaller than 0.3 microns, including the virus. Unlike some other technologies, such as ionization or ultraviolet (UV-C) light, mechanical HEPA filtration physically removes the particles from the air stream.
Selecting a Purifier Based on Room Size and Clean Air Delivery Rate
The effectiveness of an air purifier is determined by its ability to process a sufficient volume of air for the size of the room. The most important metric for sizing a unit is the Clean Air Delivery Rate (CADR), which measures the volume of filtered air a unit produces per minute (CFM). For virus mitigation, the CADR rating for smoke particles is the most relevant, as smoke particles are closest in size to viral aerosols.
The goal is to achieve an adequate number of Air Changes Per Hour (ACH) in the space. ACH is the number of times the entire volume of air in the room is theoretically filtered in one hour. For enhanced protection in a shared indoor space, experts recommend aiming for a minimum of 4 to 5 ACH. This rate ensures a continuous and rapid reduction in the concentration of airborne particles.
To calculate the required CADR, first determine the room’s volume by multiplying its length, width, and ceiling height in feet. Multiply the volume by the desired ACH (e.g., 5) and divide the result by 60 minutes. The resulting number is the minimum CADR (in CFM) needed for that room. Choosing a unit that meets or exceeds this calculated CFM requirement ensures the air is cleaned at a rate sufficient to mitigate airborne risk effectively.
Placement and Usage Guidelines
To maximize the air purifier’s performance, the unit should be placed in a centralized location within the room. Keep the unit away from walls, furniture, or curtains that could obstruct the airflow intake and exhaust. For maximum benefit, air purifiers should be operated continuously, running 24 hours a day, at the highest speed that is comfortable for the occupants.
Regular maintenance is necessary for the unit to function correctly, which involves replacing the HEPA filter and pre-filter according to the manufacturer’s instructions. Air purifiers are a supplemental mitigation strategy and cannot remove viral particles that have already settled onto surfaces. They do not replace the need for vaccination, masking, or maintaining physical distance from others.