I'd guess temperature control, but trying to eliminate airborne: shaking/stirring/splashing to introduce more air into the wort when it is at its most vulnerable is a bad idea. I'd stick with a dried yeast such as Nottingham as luckily, according to Danstar, aeration is unnecessary as the yeast has been produced with adequate reserves. Nottingham is also kosher and GMO free (if anyone cares about that sort of thing).
PDF: Nottingham data sheetOnce the yeast gets going the theory is it should have more chance of overwhelming anything that drifts in later (when you're lifting the lid to have a peek!). Still, a plus of something like 1469 West Yorkshire is it forms a reassuring sticky protective mat on top of the beer that then drops away sometime after ten days.