Humidity: Water is a by product of combustion, so an "indoor" ventless heater puts out HEAT, Water and Carbon Dioxide (if properly adjusted) and Carbon Monoxide (if not properly adjusted) and both of these gasses are not good for you.. Too much of the first chokes you and the second is poison.
A Properly VENTED heater, such as an RV furnace, puts the water, and gasses, outside, The flame heats a heat exchanger (Metal box/baffles) which then heat the air inside, True DRY heat, less like me you have leak where it can get into the air intake (fixing that as I type). (Oh, least anyone worry, it is a WATER leak.. I have identified it and dissabled the leak and will get replacement seals tomorrow).