[TAKPAK]

If you have the covers over the vents, such as Maxx-air, or Fantastic covers, you would be OK. But, if not, and it's just the "open and close" lid that comes with the fan, then I would not run with it open. They are not that substantial, and the wind would beat them to death, possibly breaking or ripping them completely off. I've even tried just "cracking" the vent cover open a bit, and going down the road, it eventually vibrated open just from the wind buffeting it.

If you want to run with them open, you should get the "permanent" mounted covers for them. If you do, make sure you get the ones that are made for the Fantastics, as they are a bit taller so the lid can open all the way.

As also mentioned, crack a couple of small windows open so fresh air can get in while the vent is open.