They must be used in places that are very well ventilated. If they are used in smaller and airtight spaces, they are easily fatal. This is the same reason a NG or LPG range must have a ventilator fan. Most of these are older units; my grandmother had one that was probably made in the 1920's or earlier, and my parents bought a portable one for emergency backup heat after a freak ice storm knocked out the power throughout Georgia for about a week, circa 1964. Code is pretty strict on these things today, particularly because of the airtightness that comes from vapor barriers, but every now and then you still hear about someone dying from CO poisioning, or asphyxiation (all O2 is used up as CO2 is produced) from one of them. The most airtight houses now have to have air-to-air heat exchangers to bring in fresh air while transferring the heat from the stale air to the fresh air.