The concentration of air pollutants is almost always lower indoors than outdoors.

Question

Quizplus · Accepted Answer

While it is true that some indoor environments may have lower concentrations of certain pollutants, such as those that are outdoor-sourced, indoor air can also have its own sources of pollutants (e.g. cleaning products, tobacco smoke, mold, etc.) which can result in higher concentrations of these specific pollutants indoors than outdoors. Ultimately, whether the concentration of air pollutants is higher indoors or outdoors depends on a variety of factors and can vary from one environment to the next.

The Concentration of Air Pollutants Is Almost Always Lower Indoors