The transmission of COVID-19 primarily occurs through the air, where infectious respiratory particles are expelled by an infected person when they breathe, talk, cough, or sneeze. These particles can remain suspended as aerosols for extended periods, especially in indoor environments with limited airflow. Air purifiers function as a complementary intervention, removing these airborne contaminants to reduce the overall concentration of the virus in a shared space. These devices should be viewed as an added layer of protection, working in conjunction with established measures such as vaccination, masking, and physical distancing. A portable air purifier draws room air through specialized filters to capture particulate matter, delivering cleaner air back into the environment.
The Core Mechanism of Viral Particle Capture
The foundational technology for removing airborne viral aerosols is the High-Efficiency Particulate Air (HEPA) filter. A true HEPA filter must capture 99.97% of particles that are 0.3 micrometers (\(\mu\)m) in size. This size is referred to as the Most Penetrating Particle Size (MPPS) because it is the most difficult size for the filter to capture.
Although the SARS-CoV-2 virus is about 0.1 \(\mu\)m, it rarely travels alone; it is contained within larger respiratory droplets and aerosols ranging from 0.5 \(\mu\)m to over 10 \(\mu\)m, which are easily trapped. Furthermore, HEPA filters are highly efficient at capturing particles smaller than 0.3 \(\mu\)m due to a phenomenon called diffusion.
Diffusion causes the smallest particles to move randomly in a zig-zag pattern, increasing the likelihood of collision with filter fibers. Larger particles are captured through interception (sticking to a fiber as air flows past) and impaction (inertia causing them to slam into a fiber). These three mechanisms ensure HEPA filters successfully capture the vast majority of virus-carrying aerosols across all relevant size ranges.
Essential Features for Effective Mitigation
When selecting an air purifier for viral mitigation, the most important performance metric is the Clean Air Delivery Rate (CADR). CADR is measured in cubic feet per minute (CFM) and represents the volume of filtered air a unit produces, factoring in both fan speed and filter efficiency. A high CADR signifies that the unit can move a large volume of air through its filter effectively, which is necessary for quick particle removal.
CADR is typically tested for three types of particles: pollen, dust, and smoke, with the smoke CADR being the best proxy for viral aerosols due to the similar small size of smoke particles and virus-carrying droplets. The effectiveness of a purifier is also assessed by its ability to achieve a sufficient number of Air Changes Per Hour (ACH). This metric indicates how many times the entire volume of air in a room is theoretically filtered within one hour.
Experts often recommend aiming for a minimum of four to six ACH to significantly reduce the concentration of airborne pathogens. To determine the necessary CADR for a specific room, calculate the room’s volume and select a unit or combination of units that can provide the targeted air exchange rate. Beyond the CADR rating, the unit’s housing must also be properly sealed, preventing air from bypassing the HEPA filter element.
Understanding and Avoiding Ineffective Technologies
While the market offers various air purification technologies, consumers should be wary of devices that rely on methods other than proven mechanical filtration. Some air cleaners utilize ionization, plasma, or bipolar ionization, which work by charging airborne particles so they stick to surfaces or each other. However, these technologies can be less effective than claimed and may sometimes generate harmful byproducts.
Ozone generators should be strictly avoided, as ozone gas is a known lung irritant that damages the respiratory system and is ineffective at removing viruses at safe concentrations. Other technologies, such as Ultraviolet-C (UV-C) light, can be used for disinfection, but the light must be completely contained within the air purifier’s housing. If UV-C light is improperly integrated or escapes the unit, it poses a risk to human health, including damage to eyes and skin.
The most reliable approach is to choose devices that prioritize a certified HEPA filter and a high CADR, which are backed by decades of established scientific data on mechanical filtration. Devices that generate reactive oxygen species or charged particles should be treated with caution due to the potential for unintended chemical reactions in the indoor air.
Effective Placement and Usage Strategies
To maximize the benefit of a portable air purifier, continuous operation is highly recommended, ideally running the unit 24 hours a day, seven days a week. Running the fan on a higher speed setting increases the CADR and ACH, leading to faster particle removal, though this may increase noise levels.
Placing the air purifier in a central location within the room or near the occupants who are most at risk or are generating aerosols is generally the best strategy. The unit should be positioned away from walls, large furniture, or curtains to ensure that the air intake and exhaust vents are not obstructed, which would restrict airflow and reduce efficiency. In shared spaces, placing a unit in an area of high occupancy, such as a common area, will provide the greatest benefit.
To maintain the unit’s high performance over time, regular maintenance is necessary, which primarily involves replacing the HEPA filter according to the manufacturer’s schedule. A dirty or clogged filter reduces the airflow, decreasing the effective CADR and undermining the unit’s ability to clean the air.