What is the safest first step before choosing window ventilation or controlled outdoor air? Compare outdoor humidity, smoke, pollen, temperature and pressure with indoor conditions before using windows as the default air-quality response.
What to compare
Record the first location, timing, trigger, recent change and an unaffected comparison area. Keep weather, occupancy and system state with every photograph or reading.
What the evidence cannot prove alone
A single reading, image, smell or temporary improvement cannot certify a concealed source. Keep at least two plausible pathways visible until repeatable evidence separates them.
Safe next step
Stop ordinary investigation when there is active electrical danger, combustion warning, sewage, structural instability, rapid water spread, unsafe access or acute occupant symptoms. Continue with Open windows or control outdoor air? for a bounded action, stop rule and professional handoff route.
