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Guide

How to keep indoor air clean during smog

Last reviewed 4 Nov 2026 · DanixSoft

Smog gets indoors. Keep windows and doors shut when the AQI is above 150, run a HEPA air purifier sized for the room you use most (especially the bedroom), avoid smoke indoors (frying, incense, cigarettes, burning wood or coal), and air the house briefly when the hourly forecast is lowest.

What helps most

  • One clean room: put the purifier where you sleep and keep that door shut.
  • Size it right: the purifier’s CADR (clean air delivery rate) should match the room; the purifier guide has a calculator.
  • Cut indoor sources: cooking smoke, incense, mosquito coils, smoking and heaters that burn fuel all add PM2.5.
  • Air out smartly: open windows for a short time when the hourly chart is lowest, often the early afternoon.

A low-cost option

A box fan with a good furnace-style filter taped to the back is a well-known do-it-yourself air cleaner (often called a Corsi-Rosenthal box when built with four filters). It is not as quiet or tidy as a purifier but can clean a room’s air well for little money. Use filters rated MERV 13 or higher and never leave a home-made unit running unattended for long periods.

Questions people ask

Should I open windows during smog?

Not when the AQI is above 150. Between 101 and 150, open them briefly at the cleanest hour. At 100 or below, air the house normally.

Do air purifiers work against smog?

A purifier with a true HEPA filter removes most fine particles from the air that passes through it. It works only in a closed room and only if its CADR suits the room size.