When I was in school we opened the windows to let "fresh air" in even at -20 C... not all day ofc but every day. And rain? What's the problem with opening a wi

When I was in school we opened the windows to let "fresh air" in even at -20 C... not all day ofc but every day. And rain? What's the problem with opening a window when it's raining? Only if there's a rain storm... and not even then, depending on wind direction.