It is not uncommon for the air inside a home or office to be more toxic than the air outside. This is because of the presence of toxic materials and substances and the fact that homes and buildings are better insulated than ever before (which is a good thing from an energy standpoint).
Keeping windows open as often as possible allows fresh air in and keeps toxins flowing out. This is especially important when cleaning your home, and ironically, especially if you are using toxic cleaners.