If there’s no air, opening a window solves the problem.
The feeling of lacking fresh air is not only about high CO₂ levels. Other factors matter too: higher temperature, volatile organic compounds, excess humidity, and even dust and particles you can’t smell.
After a thunderstorm the air feels especially fresh. That’s thanks to ozone, the earthy smell of geosmin released from soil, and rain washing dust and pollutants out of the air.
Opening a window in summer can work well if the outside air is clean and insects aren’t a problem. But not in heavy traffic areas—unfiltered outdoor air can be even more harmful. In winter, drafts and heat loss add to the downsides.
Still, sometimes opening a window makes sense, even with a good ventilation system. Room- and demand-based ventilation switches off automatically when windows are open, saving energy and filters.
In winter or when outdoor air is polluted, it’s smarter to rely on demand-based ventilation. Ventilation air intakes are placed away from pollution sources on the street, while dust, insects, and allergens are filtered out. A well-built house with a proper system keeps insects outside, maintains healthy indoor air, and saves energy. Dehumidifiers can reduce summer humidity, while in winter the system helps keep humidity at a healthier level—since heating cold air otherwise drops relative humidity more than tenfold.