Two questions to check first
- Is outdoor air quality currently suitable for ventilation?
- Is outdoor air cooler or more comfortable than indoor air?
During heat, nighttime or early morning may be cooler. During smoke, dust or pollution episodes, local authorities may advise keeping outdoor air out even if the temperature feels better.
A practical decision pattern
- Cleaner and cooler outside: ventilation may be useful if it is otherwise safe.
- Polluted or smoky outside: follow local advisories and consider filtration or mechanical ventilation guidance.
- Hotter outside than inside: windows and shading may be more useful closed during the hottest hours.
- Uncertain data: avoid presenting a precise “best hour” as fact.
Safety comes first: window security, children, local fire guidance and building-specific ventilation requirements can override general suggestions.
Primary references: US EPA — Indoor air and ventilation and WHO — Heat and health.
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Download on the App StoreLast reviewed 2026-08-29. Wellness guidance only — not medical advice.