Closed — completely. This is one of the most common mistakes Dr. Dixit Kumar Thakur sees asthma patients make. Many people open windows during a dust storm thinking ventilation will help. In fact, outdoor PM10 during an andhi event can exceed 1,500 µg/m³ — thousands of times higher than indoor air with a HEPA purifier running. Keep all windows and doors shut, run your HEPA air purifier on maximum, and switch your air-conditioner to internal recirculation mode. Do not open windows for at least 60–90 minutes after a storm passes — fine particles remain suspended in outdoor air long after visibility improves.
