Why does ventilation stop mold in a AC unit?

Ventilation may or may not stop mold in an AC unit. By "ventilation," I assume you mean the introduction of fresh (outdoor) air. If this air is considerably drier than the air in the AC unit, it will help dehumidify what's in the unit, and the drier we can keep that air, the worse for mold.
The problem with AC equipment is that, when they're running, the cold supply air Relative Humidity is actually higher than the warmer return air. This provides a nice growing environment for mold.
Based on 34 years of professional experience and personal experience, there are at least two effective mold killers. The first is "food grade" hydrogen peroxide. It's way more effective than chlorine bleach. It comes at 35% strength, and I like to cut it to 17% with clean water, then apply it with a spray applicator. Be sure to suit up with protection like a mask, goggles, gloves, and a paper painter's suit if you're going to work with this stuff. It isn't poisonous like chlorine, but it is very corrosive, so be careful with it. Be aware that the peroxide you buy at the drugstore is only 5% strength, and pretty much useless for cleaning up mold.
The second abatement tool is the UV lamp. It is mounted in the supply air plenum over the AC coil so its light can shine onto the coil and into the drain pan to kill the mold. Be aware that UV tends to destroy plastic, so if the air conditioner drain pan is made of something other than metal, I usually wrap it in tinfoil to protect it from the UV rays.


The HVAC Veteran