Propane heaters should not be used in small enclosed spaces like closets or bathrooms due to the lack of proper ventilation requirements. Carbon monoxide poisoning is a serious risk when using propane heaters indoors, and proper ventilation is necessary even with indoor-rated heaters. Safety features such as oxygen depletion sensors (ODS) are designed to shut off the heater if oxygen levels go below 18%, but these features won't work above 4,000 feet. It is important to keep propane heaters away from potential fire hazards and to use battery-operated CO detectors for added safety. While propane heaters can be used safely with proper precautions, it is crucial to follow all safety guidelines and avoid using them in small enclosed spaces without proper ventilation.