Nothing really. The airlocks are sealed, and you have co2 pushing out, and if you have co2 pushing out, then nothing can get in. A lot of people don't even use airlocks, just a piece of tinfoil, or use a baking pan over the bucket.
None whatsoever. I often use nothing more than a piece of aluminum foil over the carboy mouth. The positive pressure from the co2 production keeps stuff from getting in, so don't worry.