In many cities the worst air of the year is not in summer but in winter. Heating, traffic and cold, still weather combine, and fine particles (PM2.5) build up close to the ground. On those days the reflex is to keep every window shut — but a closed home collects moisture and stale air too. The answer is not "never open", it is open at the right hour.
On cold, calm nights the ground cools quickly and a layer of cold air forms near the surface, with warmer air above it (a temperature inversion). It acts like a lid: smoke from heating and exhaust from evening traffic stay trapped in the air you breathe. During the day the sun warms the ground, the lid lifts, air mixes higher up and particle levels fall. That is why, in most cities, the cleanest air of a winter day comes in the early-to-mid afternoon.
Rule of thumb: on smog days, air the home briefly in the afternoon and avoid opening in the evening and at night, when particle levels are usually highest.
For each city, the month with the highest average PM2.5 and, in that month, the hour with the dirtiest air and the cleanest hour between 08:00 and 20:00:
| City | Worst month | PM2.5 avg (µg/m³) | Dirtiest hour | Cleanest daytime hour |
|---|---|---|---|---|
| London | March | 12 | 00:00 (15) | 16:00 (9) |
| Manchester | March | 12 | 00:00 (15) | 16:00 (8) |
| New York | June | 20 | 21:00 (25) | 18:00 (17) |
| Chicago | June | 19 | 13:00 (21) | 08:00 (18) |
| Los Angeles | January | 26 | 00:00 (34) | 14:00 (20) |
| Toronto | June | 23 | 20:00 (29) | 08:00 (17) |
| Delhi | January | 121 | 22:00 (152) | 15:00 (78) |
| Mumbai | January | 68 | 14:00 (85) | 20:00 (64) |
| Beijing | December | 122 | 20:00 (183) | 14:00 (53) |
| Seoul | March | 34 | 09:00 (40) | 16:00 (27) |
| Bangkok | January | 54 | 20:00 (75) | 08:00 (43) |
| Mexico City | May | 28 | 01:00 (37) | 16:00 (20) |
Model estimates (CAMS) via Open-Meteo, hourly averages 2023–2025, local time; values in brackets in µg/m³. For reference, the WHO 24-hour guideline for PM2.5 is 15 µg/m³.
Skipping ventilation for days has its own costs: humidity rises, windows fog up and mould can start, while carbon dioxide from breathing makes rooms stuffy. Short, well-timed airing is almost always better than none. If you also struggle with condensation, see our guide on airing your home in winter.
OpenWindow.live checks outdoor air quality, temperature and humidity at your location in real time and turns red when the air outside is too polluted to open the windows.