Air purifiers are widely used to improve indoor air quality and mitigate airborne pathogens. Since people spend significant time indoors, the potential for viruses to circulate in shared spaces is a major public health concern. The effectiveness of these devices depends on their ability to capture or neutralize microscopic biological particles. Evaluating air purification technology requires understanding how viruses travel through the air and the operational mechanics of the devices. This analysis explores the core technologies and practical considerations for using air purifiers to mitigate airborne viruses.
Understanding Airborne Virus Transmission
Respiratory viruses, including influenza and coronaviruses, spread primarily through particles expelled during breathing, speaking, coughing, or sneezing. These particles are categorized by size: larger droplets and smaller aerosols. Large droplets (greater than 5 to 10 micrometers, or µm) settle quickly onto surfaces near the source.
Smaller particles, known as aerosols (less than 5 µm), behave differently. These minute particles remain suspended in the air for extended periods and travel further on air currents, enabling longer-distance transmission. Although the virus itself is tiny (0.07 to 0.2 µm), it is always encased within a respiratory fluid particle. Therefore, effective air purification must target these virus-laden aerosols, which typically range from 0.1 to 1 µm, to reduce the concentration of infectious agents.
The Core Technology: HEPA Filtration
The most dependable technology for removing airborne viruses is the High-Efficiency Particulate Air (HEPA) filter. A true HEPA filter must capture a minimum of 99.97% of particles at 0.3 microns (µm) in diameter. This size is known as the Most Penetrating Particle Size (MPPS) because it is the most difficult size for the filter to capture. HEPA filters are mechanical filters composed of a dense mat of randomly arranged fibers that employ three distinct capture mechanisms.
Impaction
Larger particles are trapped through impaction, where their inertia causes them to crash directly into the fibers.
Interception
Medium-sized particles are caught by interception, where they follow the airflow but brush against and stick to a fiber.
Diffusion
Diffusion is important for capturing the smallest ultrafine particles, including virus-laden aerosols, which are often smaller than the 0.3 µm MPPS. These tiny particles move erratically due to Brownian motion, increasing the probability they will collide with and adhere to a fiber. Because of diffusion, a HEPA filter is often more efficient at trapping particles smaller than the 0.3 µm MPPS than it is at capturing particles at that size.
The Clean Air Delivery Rate (CADR) is the most reliable metric for determining a purifier’s real-world effectiveness. CADR measures the volume of clean air delivered in cubic feet per minute (CFM). For virus mitigation, consumers should look specifically at the CADR rating for “smoke.” Smoke particles are the smallest of the standard test pollutants (smoke, dust, and pollen), making this rating the most relevant proxy for removing virus-sized aerosols. A higher CADR rating means the unit can clean the air in a space more quickly, which is essential for rapidly reducing airborne virus concentration.
Assessing Alternative Air Cleaning Methods
While HEPA filtration is the gold standard for particle removal, other technologies are sometimes incorporated to address biological contaminants.
Ultraviolet Germicidal Irradiation (UV-C)
UV-C uses short-wave UV light to damage the DNA or RNA of viruses and bacteria, rendering them inactive. For UV-C to be effective, airborne pathogens must be exposed to the light at sufficient intensity and duration. In many consumer purifiers, the air moves too quickly past a low-intensity UV-C lamp, reducing the necessary “dwell time” for inactivation. This often makes the technology ineffective as a standalone feature. Adding UV-C light to a HEPA filter unit offers no additional infection control benefit beyond the physical particle removal already provided by the HEPA filter.
Ionization and Plasma Technologies
Other methods, such as ionization and plasma technologies, generate charged ions that attach to airborne particles. This causes particles to clump together or settle onto surfaces. While these technologies remove particles, a major concern is their potential to generate ozone as a byproduct. Ozone is a lung irritant that can cause chest pain, coughing, and throat irritation. Health organizations generally advise against using air cleaning devices that intentionally produce ozone.
Operational Factors for Maximum Protection
The maximum benefit of an air purifier is realized only when the unit is properly selected and used. A primary factor is ensuring the unit’s CADR is correctly matched to the room size. A general guideline suggests selecting a purifier with a CADR that is at least two-thirds of the room’s square footage. This ensures the air is cleaned frequently enough to reduce virus concentration effectively.
Optimal placement is also important for maximizing the air cleaning effect. Purifiers should be situated in a location with good airflow, away from corners, walls, and furniture that could obstruct the air intake and exhaust. Placing the unit in the area where people spend the most time, such as a living room or bedroom, offers the greatest exposure reduction.
Finally, the unit’s performance depends heavily on timely maintenance, particularly replacing the HEPA filter. An old, clogged filter significantly reduces airflow and the unit’s effective CADR, negating its ability to clean the air. Air purification should be considered a single layer of protection that complements other measures, such as increasing outdoor air ventilation or using high-quality masks when exposure risk is high.