Too Little Circulation
It's not a new phenomenon, but many people in industrialized countries get very little fresh air. We often wake in heated or air-conditioned rooms, commute with car, subway and bus windows sealed tight, and spend days in hyper-sealed work spaces.
Safety and surrounding air quality factor in to whether we open windows and let natural air circulate, and privacy, noise and convenience issues determine whether we open windows in our homes at all. Many older homes have windows painted shut from lack of use, and some newer home windows are nailed down to prevent potential home invasion.
Sealed and shuttered windows create a safe feeling and can prevent drafts in colder climates, but they also pose a safety hazard when the need to escape from fire or other home emergencies arises. Building codes in the United States and elsewhere require exit planning, and older homes need upgrades to meet the minimum codes for safety. Likewise, concealed outer doors and windows -- sometimes considered safer due to being out of sight -- can become ideal spots for crime or illegal entry.