You know those air vents on planes that twist open? Some people think it's better to keep them closed, so you don't get blasted in the face with re-circulated air filled with germs. According to one expert, the opposite is true.

ThinkStock

Dr. Mark Gendreau studies how infectious diseases spread on airplanes. In general, he says the air quality on planes is a lot better than you think.

First of all, it's not really re-circulated the way most people imagine it is. It's constantly being filtered and mixed with new air from outside the plane.

The filtration system is broken up into sections. So you're also not getting air from all over the cabin. You're really just exposed to the same air as the three or four rows around you.

Air coming out of those vents is actually pretty fresh. The main reason to leave your vent on is because that flow of air creates a barrier around you that keeps germs out.

Any germs that come your way get pushed to the floor instead of hanging around in front of your face. So you're less likely to breathe them in, and less likely to catch something.

Read more at Travel + Leisure.