Most respiratory infections happen indoors. The CDC estimates that Americans spend roughly 90% of their time inside buildings, and indoor air typically contains 2-5 times more pollutants than outdoor air according to EPA data. During a respiratory outbreak, the air inside your home, office, or school becomes the primary transmission medium. Cleaning that air is one of the most effective things you can do, and it works whether the pathogen is influenza, tuberculosis, COVID-19, or something we haven't seen yet.
How HEPA filtration works
HEPA stands for High Efficiency Particulate Air. To qualify as true HEPA under US DOE standards, a filter must capture at least 99.97% of particles at 0.3 microns in diameter. That 0.3-micron threshold isn't arbitrary - it's the most penetrating particle size (MPPS), the size hardest for the filter to catch. Particles both smaller and larger than 0.3 microns are actually captured at even higher rates.
This matters because respiratory pathogens travel primarily on aerosol particles ranging from 0.5 to 10 microns. Individual virus particles are much smaller (SARS-CoV-2 is roughly 0.1 microns), but they don't float freely. They ride on respiratory droplets and aerosols generated by breathing, talking, coughing, and sneezing. A HEPA filter captures these aerosol particles with extremely high efficiency.
HEPA filters work through three mechanisms: interception (particles following airflow get caught on fibers), impaction (larger particles can't follow air curves and slam into fibers), and diffusion (the smallest particles bounce randomly and collide with fibers). Together, these mechanisms capture particles across the full size range relevant to airborne disease transmission.
Sizing a purifier: CADR and the two-thirds rule
The most important specification on any portable air purifier is its Clean Air Delivery Rate (CADR), measured in cubic feet per minute (CFM). CADR tells you how much clean air the device actually delivers after accounting for filter efficiency and airflow rate.
The Association of Home Appliance Manufacturers (AHAM) recommends that a purifier's CADR should be at least two-thirds of the room's area in square feet. For a 150-square-foot bedroom, you need a CADR of at least 100 CFM. For a 300-square-foot living room, 200 CFM. For a 500-square-foot open office area, 333 CFM - likely requiring two units.
During a respiratory outbreak, aim higher than the standard recommendation. Instead of two-thirds, target a CADR equal to the full room area or higher. More air changes per hour means faster removal of infectious aerosols.
Air changes per hour: the target metric
ACH (air changes per hour) measures how many times per hour the entire volume of air in a room gets replaced with filtered air. It's the metric that infection control professionals actually use.
The CDC recommends at least 6 ACH for airborne infection isolation rooms in hospitals. Standard office buildings typically achieve 2-4 ACH through HVAC systems alone. Homes with windows closed and no dedicated ventilation may be as low as 0.5 ACH.
During a respiratory outbreak, aim for 5-6 ACH in occupied rooms. To calculate ACH from your purifier's CADR: multiply CADR (in CFM) by 60, then divide by the room volume in cubic feet (length x width x ceiling height). A purifier with a CADR of 200 CFM in a room measuring 12 x 15 feet with 8-foot ceilings (1,440 cubic feet) delivers 200 x 60 / 1,440 = 8.3 ACH. That's excellent.
If you're setting up a home isolation room for someone with a confirmed infection - maintaining proper quarantine vs isolation protocol - target the higher end of this range. Position the purifier between the sick person and the door so that air flows from the clean zone toward the infected person, gets pulled through the filter, and exits clean.
The Corsi-Rosenthal box: $60 and peer-reviewed
In August 2020, Richard Corsi (then dean of engineering at UC Davis) and Jim Rosenthal (a Texas-based filter manufacturer) designed a DIY air purifier using a standard 20-inch box fan and four MERV-13 furnace filters taped together in a cube shape, with the fan mounted on top.
The total cost: approximately $60-80 in materials. The performance: surprisingly strong. A peer-reviewed study published in the journal Aerosol Science and Technology in 2022 measured Corsi-Rosenthal boxes delivering CADR values of 400-600 CFM for smoke particles, depending on fan speed and filter type. That matches or exceeds many commercial units costing $300-800.
Studies conducted at Brown University and the Harvard T.H. Chan School of Public Health confirmed that Corsi-Rosenthal boxes significantly reduce airborne aerosol concentrations in classroom-sized spaces. A 2022 study in the journal Indoor Air found that a single Corsi-Rosenthal box in a classroom of 1,000 square feet reduced aerosol concentrations by roughly 70-80%.
The main trade-off is noise. Box fans at high speed are loud - typically 55-65 decibels, roughly equivalent to a normal conversation. Commercial HEPA purifiers on lower settings run at 25-40 decibels. If noise is a concern, run the DIY unit on medium speed and accept the lower CADR, or use it in rooms where noise tolerance is higher.
Placement and operation tips
Where you put an air purifier matters as much as which one you buy.
Place the unit in the breathing zone. Tabletop height or slightly above, not on the floor in a corner. Infectious aerosols concentrate at the height where people breathe, roughly 3-6 feet above the floor.
Don't block the intake or exhaust. Keep at least 12-18 inches of clearance on all sides. Pushing a purifier against a wall restricts airflow and drops the effective CADR.
Run it continuously. Air purification is not like disinfecting a surface; you don't run it once and call it clean. Infected people continuously generate new aerosols by breathing. The purifier needs to run as long as the room is occupied. Turn it off when the room is empty and the air has had time to clear (roughly 30-60 minutes at 5+ ACH).
Close the doors and windows. This sounds counterintuitive - doesn't fresh air help? It does, but an open window creates unpredictable airflow that may pull contaminated air from other rooms or reduce the purifier's effectiveness. In general, if you're relying on a portable HEPA unit, keep the room sealed and let the purifier do the work. If you're using natural ventilation (open windows) as your primary strategy, you don't need a purifier running simultaneously.
Replace filters on schedule. HEPA filters lose effectiveness as they load with particles. Most manufacturers recommend replacement every 6-12 months under normal use. During heavy use in an outbreak, check monthly and replace when visibly discolored or when the purifier's airflow drops noticeably. MERV-13 filters in a Corsi-Rosenthal box should be replaced every 3-4 months during continuous use.
Costs and running expenses
A quality portable HEPA purifier suitable for a bedroom costs $100-250. Units sized for large rooms or open-plan spaces run $300-600. Replacement HEPA filters cost $30-80 each.
Electricity costs are modest. A typical portable HEPA purifier draws 30-70 watts on medium settings. Running continuously, that's about $3-6 per month at average US electricity rates of $0.16/kWh. A box fan in a Corsi-Rosenthal box draws about 70-100 watts, costing roughly $8-12 per month in electricity.
These costs are minor compared to lost workdays from illness, medical bills, or the economic impact of a household outbreak. Think of it as part of your outbreak preparedness supplies, alongside masks and hand hygiene products.
What air purification cannot do
HEPA filtration reduces airborne pathogen concentrations. It does not eliminate them. A purifier does not make an indoor space safe for someone to remove their mask while sharing a room with an actively infected person at close range. It adds a layer of protection, not total protection.
Air purification also doesn't address surface contamination (fomites), direct droplet exposure from close-range coughs or sneezes, or pathogens that transmit through routes other than airborne aerosols.
Think of air purification as one layer in a multi-layer infection prevention strategy. Combined with masking, ventilation, distancing, hand hygiene, and vaccination when available, it measurably reduces risk. On its own, it reduces risk but doesn't eliminate it.