Health, Safety, and Indoor Environmental Quality
Combustion safety first
Any naturally drafting appliance - atmospheric water heater, furnace, oven, fireplace - is at risk when the combustion appliance zone is depressurized. Test worst-case depressurization with interior doors closed and all exhaust fans running; a common limit is about -5 Pa in the zone for Category I appliances, with tighter limits used by some programs, so know the standard you are tested to. Look for draft hood spillage, rust, soot, a yellow lazy flame, and elevated carbon monoxide. CO alarms belong near sleeping areas; CO is odorless.
Ventilation and pollutants
Indoor air quality rests on source control, dilution, and filtration, in that order. ASHRAE 62.2 provides the mechanical ventilation rate. Key pollutants: moisture (mold risk climbs above roughly 60% RH, dust mites above about 50%), CO and nitrogen dioxide from combustion, radon (test every home; mitigate above 4 pCi/L), VOCs and formaldehyde from furnishings and products, asbestos and lead in older homes, and garage fumes drawn in by pressure imbalances.
Moisture and comfort
Keep indoor relative humidity roughly in the 30-50% band. Solve the source rather than masking it: exhaust at the source (bath fans typically 50-80 CFM intermittent, kitchen about 100 CFM), maintain a continuous air barrier, and dry any bulk water intrusion.
Traps
Treating air cleaning as a substitute for ventilation. Recommending ozone generators. Assuming a tight house is automatically unhealthy - an unventilated house is the problem, not a tight one. Forgetting that bath fans ducted into the attic dump moisture there.
Worked example
ASHRAE 62.2 default ventilation is about 7.5 CFM per occupant plus 3 CFM per 100 ft2, with occupants taken as bedrooms + 1. For a 3-bedroom, 2,000 ft2 home: 7.5 x 4 = 30, plus 3 x 20 = 60, for a total of about 90 CFM continuous.