"Open a window" is the lay version. The engineering version is in two ASHRAE standards. ASHRAE 62.1 sets ventilation rates for commercial and institutional buildings, offices, schools, hospitals, restaurants. ASHRAE 62.2 sets ventilation rates for residential buildings.
The 62.2 formula (current 2022 edition) is: 7.5 CFM per occupant + 0.03 CFM per square foot of floor area, delivered continuously. A 2,000 ft² home with four occupants needs about 90 CFM of continuous outdoor-air intake. That is roughly equivalent to running a single bathroom exhaust fan around the clock, and that is by design.
In practice, U.S. and Canadian homes meet 62.2 in one of three ways: continuous-running bathroom or kitchen exhaust fan, a balanced HRV/ERV unit (energy-recovery ventilator: the right answer in cold or hot-humid climates), or intentional natural ventilation (cracked windows, trickle vents). Terrestream estimates a room's air-change rate from how quickly CO2 falls after people leave, and flags a room where CO2 repeatedly clears slowly. That is one room's air exchange, not a whole-house ventilation measurement, so it cannot tell you whether your home meets 62.2; an HVAC contractor or energy rater can measure that.
Fisk's reviews are unambiguous that increases above 62.2 minimums produce measurable productivity and health benefits, schools and offices benefit from 2× to 3× the minimum rate. The 62.2 minimum is a floor; it is not an optimum.
