despite all the talk about contamination and sanitizing, its not THAT easy to infect beer.
it does happen, but as you progress through your brewing career, you'll find that just because you take risks doesn't mean you'll get burned. you might...you might not.
you should be aerating in a fairly clean environment, not outside next to a pile of manure. If you're using an aquarium pump, I'd leave the lid loosely on top since you're probably running it 10-15 minutes.
but if you have pure O2, don't mess with covering it. you're only running it for maybe 60 seconds. too short to worry about.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10