It's not a necessity if you have the window open to move around the air, If you live in an urban area or someplace else with lots of air pollution, the outdoor

It's not a necessity if you have the window open to move around the air, If you live in an urban area or someplace else with lots of air pollution, the outdoor air might be a problem! Of course, I don't think an air filter like this would be very effective with the windows open.