Just get an ESS with a built in fan. Why take the chance. It's your eyes dude.
here is a link where you can pick up both... but in all seriousness. Don't take the chance and spend the extra $40 for the ESS with built in fan. All the people that I've known who have them have no regrets at all. I know people who wear glasses too who pretty much jammed a fan into their face masks and attached a battery just to have it not fog up.
It's better to spend the money where it counts most in airsoft.... your safety.