Introduction: planning clean air delivery in occupied rooms
Indoor air exchange planning makes invisible airflow easier to evaluate for classrooms, homes, meeting rooms, and other shared spaces. A window opened slightly or a fan set to high may change conditions, but neither action by itself states how much clean air reaches occupants. By converting dimensions and airflow into CFM and ACH, this planner gives a common basis for comparing mechanical outdoor air, portable filtration, and estimated infiltration.
People may notice stale air, odors, fatigue, or a stuffy feeling without knowing the room’s actual clean-air delivery. CO₂ monitoring can provide useful context for occupancy and outdoor-air ventilation, but it does not directly count the particle-cleaning contribution of portable filters. Separating those sources in an air-exchange plan helps users identify whether a target calls for more outdoor air, more filtration, or a different room setup.
This tool combines room geometry with the airflow inputs entered on the form. It reports current ACH, the CFM shortfall for the selected ACH target, and CFM per person. When event inputs are supplied, it also calculates a Wells–Riley style infection probability. The generated comparison rows apply fixed example additions, allowing users to see how the same room and event assumptions respond to more CADR, more modeled outdoor airflow, or both.
From room volume to indoor air exchange and infection probability
Indoor air exchange begins with room geometry. A rectangular room with length L, width W, and height H has volume V equal to cubic feet. Mechanical ventilation, infiltration, and filtration are added as CFM to form the clean-air total: . The planner converts that total to ACH with . A target ACH requires of total clean air.
For event risk, the indoor-air planner applies a Wells–Riley style expression. If infectious occupants emit q quanta per hour, each susceptible person breathes p cubic meters per hour, and exposure lasts t hours, it calculates . The 1.699 factor converts CFM to m³/hour. This is a comparative estimate based on the entered assumptions, not an individualized prediction of transmission.
The planner also divides total clean airflow by the number of people in the room. CFM per person is useful for comparing occupancy choices with available clean air, although it is not a substitute for examining distribution, source control, or actual contaminant measurements. Reducing occupancy raises this metric when airflow stays fixed; adding ventilation or CADR raises it when occupancy stays fixed.
Worked example: air exchange options for a multipurpose room
Consider the default 20-by-15-foot room with a 9-foot ceiling, 180 CFM of mechanical ventilation, 40 CFM of infiltration, and a 250-CADR purifier. The room volume is 2,700 ft³ and combined clean air is 470 CFM. That equals about 10.4 ACH and 47 CFM per person for ten occupants. The 8 ACH target is met because the target flow is 360 CFM.
With one infectious person, a two-hour event, 25 quanta per hour, and a breathing rate of 0.6 m³/hour, the calculator’s displayed formula produces an estimated per-susceptible-person probability of about 3.7%. Changing quanta, breathing rate, duration, or infectious count changes that estimate directly. Setting infectious individuals to zero intentionally removes the infection-probability calculation while preserving the airflow metrics.
The built-in comparison adds either 250 CADR, 300 CFM of modeled fan airflow, or both. For the default geometry, those alternatives produce 16.0 ACH, 17.1 ACH, and 22.7 ACH respectively. They show how additional clean-air capacity changes the model, but a real fan’s delivered airflow, the quality of incoming outdoor air, noise, drafts, and equipment placement still need separate evaluation.
Comparison of indoor air upgrade strategies
The indoor air upgrade table is generated from the values currently entered in the form. It includes the baseline and three fixed example strategies, listing clean-air flow, ACH, CFM per person, estimated infection probability, and whether each reaches the selected ACH target. Because the table is recalculated after submission, it is more useful than a static illustration for checking a particular room and event.
The table’s download button exports the displayed scenario rows as CSV after a successful calculation. Treat the rows as planning comparisons rather than equipment specifications: the added purifier is always modeled as 250 CADR and the fan as 300 CFM. For a purchase or operational decision, enter the baseline carefully and verify the actual clean-air contribution of any proposed equipment.
Indoor air exchange limitations and assumptions
This indoor air exchange planner assumes that the room is well mixed, meaning clean air dilutes contaminants evenly and immediately. Real spaces can have short-circuiting, stagnant corners, or occupants located close to an emission source. Purifier placement, supply and return locations, doors, and furniture can therefore matter even when the calculated ACH is high. CADR also assumes maintained filters and unobstructed airflow.
The infection calculation is a simplified Wells–Riley style model. It does not explicitly include masks, vaccination or immunity, aerosol settling, time-varying concentrations, or close-range exposure. Infiltration is likewise uncertain because weather, stack effect, and door use can change it. For smoke events, use the airflow results with care: filtration can help remove particles, while increased outdoor air may not be beneficial if outdoor air quality is poor. The planner does not estimate humidity, temperature, or HVAC energy effects, but it provides a transparent starting point for comparing indoor clean-air options.