Growing up, my mom would open the bedroom windows every morning, regardless of the season, to 'air out the night germs'. It was a morning ritual of her childhood, and one that I have now embraced for good reason. Indoor air pollution is a health concern.
According to the U.S. Environmental Protection Agency, opening a window or opening a door to let outside air circulate inside is "one important factor in promoting good air quality" as it reduces the level of pollutants (cleaning products, dust mites, new furnishing off-gassing, mould, paint, etc.) in a home.
Opening your windows will also let bacteria in. I agree, it seems counterintuitive since we clean our homes to rid our living environment of pesky germs, but it's the beneficial bacteria that you want to welcome in -- those very microbes that research has found aid our bodies in digestion, bolster immunity and contribute to overall good health.
As I type this, I'm sitting close to an open window and I can feel the gentle breeze roll in.