Don't open the windows during these two times in winter!
Release date:
2025-09-01
Many people have the habit of opening their windows for ventilation right after waking up, but did you know? Actually, *when* you open the windows can make a big difference to your health. Experts say that in cities, sunrise and sunset are two peak pollution periods—times when it’s best not to open windows, as doing so could harm your health.
Remember! Don’t forget to open the windows for ventilation, no matter how cold it is.
Ge Ming, deputy head of the Environmental Health and Disinfection Control Division at the Nanjing Center for Disease Control and Prevention, explains that during winter, many people keep their doors and windows tightly closed due to the cold—something that is highly unwise. Under normal conditions, a person exhales about 22 liters of carbon dioxide per hour, while engaging in heavy physical activity can increase this amount to as much as 80 liters per hour. If indoor ventilation is poor, the carbon dioxide emitted by occupants will accumulate indoors, potentially compromising health. Therefore, it’s essential to regularly open windows to ensure proper air circulation.
Although most households now have range hoods, some people only use them when stir-frying, assuming there’s no need to turn them on when boiling water or simmering soups—since they believe those tasks produce little to no smoke. However, this practice allows harmful gas emissions from the stove to accumulate easily in the kitchen and even spread into living rooms and bedrooms, posing serious health risks and, in severe cases, potentially leading to poisoning. That’s why it’s crucial to either open windows for ventilation or keep the range hood running whenever you’re using gas appliances. In fact, using a range hood not only helps capture cooking fumes but also effectively promotes mechanical air exchange throughout the home.
Window opening times: around 10 a.m. and 3 p.m.
Emphasizing ventilation doesn’t mean leaving doors and windows wide open all day—instead, it’s about choosing the right times to ventilate. Research and testing have shown that in cities with high industrial activity, there are two peak pollution periods—one during the day and one at night—alongside two relatively cleaner low points throughout the 24-hour cycle. Two pollution peaks typically occur around sunrise and in the evening, making it unsuitable to open windows for ventilation during these times. In contrast, the two relatively cleaner periods are around 10 a.m. and 3 p.m., so you can consider opening windows depending on the situation.
Number of window openings: At least two times, each lasting no less than 15 minutes.
The duration and frequency of window ventilation within a single day can be adjusted based on factors such as the size of the home, the number of occupants, daily living habits, indoor pollution levels, and weather conditions. For instance, in a 100-cubic-meter space—under windless conditions with a 20°C temperature difference between indoors and outdoors—it typically takes just over ten minutes to achieve a full air exchange.
It’s cold in winter, but you should still open the windows for ventilation at least twice a day, each time for no less than 15 minutes.
The choice of ventilation also varies depending on different climate conditions. On rainy or snowy days, pollutants are naturally washed away, while windy conditions tend to disperse them more easily, resulting in better air quality—so it’s perfectly fine to open windows more frequently. However, on cloudy days, temperature inversions caused by warm air rising from the ground can persist for extended periods, making it difficult for pollutants to dissipate. In such cases, photochemical smog may even form, so it’s best to avoid opening windows or spending time outdoors. Instead, you can opt to place a compact, high-efficiency negative-ion generator indoors—it’s an excellent way to maintain fresh, circulating air while keeping your space comfortable.
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