Propane heaters that are vent-free are safe to use indoors. Such units pull oxygen from the room and emit heat and carbon dioxide. This heating process is extremely efficient, but it can be dangerous if there is not an adequate supply of oxygen in the area. That results in incomplete burning, which results in carbon monoxide emissions. Many of the best propane heaters feature oxygen depletion sensors that will shut the unit off as soon as oxygen levels in the room are too low, or if the pilot light goes out. Properly installed carbon monoxide detectors in your home can also help keep you safe.