COVID-19 is airborne and good indoor ventilation can help reduce the risk of spread. These calculators are designed to help you ensure proper ventilation.

No single prevention measure is foolproof: high-quality masks, vaccines, and distancing are still important.

These calculators are intentionally simplified and only provide estimates.

CFM - Cubic Feet per Minute. Measures how much air is being moved into or out of a space, usually by fans or HVAC systems.

CADR - Clean Air Delivery Rate. Measures how much air is being filtered or purified.

ACH - Air Changes per Hour. Measures how many times per hour the volume of indoor air is replaced by outdoor (or purified) air.

99.9% clearance time - The time it takes for 99.9% of particles to be cleared from the room.

CO2 - Carbon dioxide can be used to estimate indoor air quality. The main source of indoor CO2 is human breath. Fresh, outdoor air removes CO2. Thus, indoor CO2 levels estimate how much air is being "rebreathed"--and how likely COVID is to spread among people in the room.

This is a complex question and answers can vary substantially depending on who you ask. The CDC recommends between 6 and 15 air changes per hour in medical settings. Of those, 0-3 ACH must be fresh outdoor air.

ASHRAE standards specify between 5 and 20 CFM per person of outside air, depending on what the room is being used for, plus additional CFM for every square foot of floor space.

It's worthing noting that the ASHRAE standards are not for infection control, and should probably be considered a bare minimum for COVID safety.

Joey Fox provides a wealth of information on preventing COVID spread through better indoor air quality on his Twitter account.

The calculator gives you approximations of air quality for your room. If minimum ventilation standards aren't met, it will warn you.

• Most consumer fans have CFM values between 400 and 2000 depending on their size and power. Most manufacturers list the CFM values.
• Air purifiers have three different CADR ratings. To be safe, use the smallest one in the calculator.
• Do not rely on HVAC systems or window AC units for fresh air. They typically recirculate indoor air.

Good ventilation reduces, but does not eliminate, the chance of COVID spread. Wearing high quality masks and isolating when sick are still crucial prevention measures