A few times when I've put the cold A/C (its possible it happens also when I put the heat on), a white smoke has come out of the vents. I don't think that it smells. I've opened the windows when it happens. During the summer I've seen a puddle of water under my car. I was told that was just water condensing and not to worry about that. I also opened my front passenger door once and it looked like there was water pouring out from underneath my car by the front passenger door. Not sure if these are related.

I'm just worried that this is harmful smoke rather than water exaporating, etc.

 

On hot humid days the cold AC will produce H2O vapor. Kinda like frost. I usually brag about it. A testament of how cold your AC blows.

 

Could also be your heater core has a leak in it. Mine did and exhibited the same blowing of steam as you describe when the AC is on full. Though unless you're not running anti-freeze you'd probably smell it. More than likely you'd also have issues with the windows fogging up a lot when the AC isn't on.

Pull up the carpet in the front seats and look to see if you have coolant under the carpet on the floor pan. The heater core isn't a total PITA to change, but it's not super fun either.

 

I had to replace the heater core and had various fluid leaks including radiator fluid.