High-Efficiency Particulate Air (HEPA) filters are mechanical air filters that capture a high percentage of airborne contaminants, including dust, pollen, pet dander, and other fine particles. This technology has attracted significant attention for its potential use in mitigating the transmission of airborne pathogens. The following sections explore the scientific mechanism and practical application of HEPA filtration specifically against viral aerosols, such as those that carry the SARS-CoV-2 virus.
The Science of HEPA Filtration
HEPA filters are composed of a dense mat of randomly arranged fibers, which collect particles through several simultaneous physical mechanisms rather than acting as a simple sieve. The smallest airborne particles are captured primarily through diffusion, where their random movement causes them to eventually strike and adhere to a fiber. Conversely, larger, heavier particles are captured by impaction, where their inertia prevents them from following the air stream’s curved path around the filter fibers, leading to a direct collision.
Mid-sized particles are collected through interception. These three mechanisms work together, requiring true HEPA filters to capture 99.97% of particles that are 0.3 micrometers (microns) in diameter. This specific size represents the Most Penetrating Particle Size (MPPS), meaning it is the most challenging size for the filter to capture efficiently. Particles both larger and smaller than 0.3 microns are collected with even higher efficiency due to the dominance of impaction and diffusion, respectively.
Although the SARS-CoV-2 virus itself is extremely small, it rarely travels alone in the air. The virus is typically suspended within respiratory aerosol droplets expelled during breathing, talking, coughing, or sneezing. These virus-carrying aerosols are generally larger than the filter’s MPPS, ranging from a fraction of a micron up to several microns, which makes them highly susceptible to HEPA filtration. Real-world studies conducted in hospital settings have confirmed that portable HEPA filtration units can substantially remove SARS-CoV-2 from the air.
Measuring Air Cleaning Power
The Clean Air Delivery Rate (CADR) measures the volume of clean air a purifier produces per minute, typically expressed in cubic feet per minute (CFM). This rating combines the fan speed and the filter efficiency, providing a practical benchmark of how quickly the device can process and clean the air. When evaluating units for viral aerosol removal, the “Smoke” CADR rating is the most relevant measurement, as it tests the unit’s effectiveness against the smallest particles.
The CADR of a unit determines the resulting Air Changes Per Hour (ACH) for a specific room size. ACH is the theoretical number of times the total volume of air in a room is replaced by filtered air each hour. Achieving a high ACH is directly correlated with a lower concentration of airborne virus particles over time, which reduces the potential for transmission. For virus mitigation in occupied indoor environments, public health guidelines suggest aiming for a minimum of 4 to 6 air changes per hour.
To determine the required CADR, one must calculate the room’s total volume and then multiply that volume by the desired ACH. This calculation ensures the purifier is powerful enough to meet the recommended air exchange rate for the space. Using a unit with an insufficient CADR for a large room will result in a lower ACH, compromising the effectiveness of the air cleaning effort.
Strategic Use for Virus Mitigation
Air purifiers should be positioned in a central location to promote the best circulation and most even distribution of filtered air throughout the space. Obstructions like walls, furniture, and corners can block the intake and exhaust vents. Maintaining a clearance of at least a few feet around the device ensures the fan can move air freely, optimizing the unit’s Clean Air Delivery Rate.
For targeted particle removal, the unit can be positioned closer to the area where occupants are spending the most time, such as near a primary breathing zone or seating area. When an infected person is isolating in a room, the air purifier should be placed within a short distance of the patient to capture aerosols near the source of emission. Running air purifiers continuously, even when the room is empty, is generally recommended to maintain a consistent baseline level of air cleanliness. In situations where natural ventilation with outdoor air is also employed, the air purifier should operate simultaneously to manage particles that may not be fully exhausted.
For buildings with central Heating, Ventilation, and Air Conditioning (HVAC) systems, the system can be augmented by installing higher-rated filters, such as those with a MERV 13 rating or higher. This adjustment requires checking the HVAC system’s capacity, as not all residential systems can handle the increased airflow resistance of a denser filter. While HEPA filters can become slightly more efficient as they load with particles, a clogged filter will eventually reduce the airflow, decreasing the unit’s overall CADR and ACH.
Filters should be replaced according to the manufacturer’s recommended schedule. When replacing a filter, particularly after use during a period of illness, it is advisable to wear protective gear such as a face mask and gloves. The old filter should be carefully sealed in a plastic bag before disposal to contain any trapped pathogens.