Typical evaporation rates are about 10-20% depending on several factors.
That means you could have about .5 - 1 gallons of water evaporating into the air every time you brew. It's not a hazard but if it condenses it could be a bitch.
If your kettle is electric there a few dangers. But poor ventilation is a HUGE issue (Carbon Monoxide) if your using gas though.
In Process - Russian Imperial Stout, Nelson Sauvin Rye IPA, Mild No.3
In Kegs - Barley Wine, Apfelwein, Wild BlackBerry Wheat, Coffee Oatmeal Porter
Gone - so many :(