As with all heaters that use fossil fuels, propane heaters generate more than enough heat to cause a fire, posing a risk to homeowners. Although propane produces only carbon dioxide and water vapor when under complete combustion, incomplete combustion can form dangerous gas.
Inefficient combustion creates carbon monoxide, an odorless gas that replace oxygen in the human body, potentially leading to a rapid loss of consciousness and death. Some outdoor heaters generate carbon monoxide levels low enough for use outdoors but dangerous for indoor use. Homeowners must ensure that their propane heaters are designed for indoor use before using them inside.
Indoor heaters are designed to generate only trace amount of carbon monoxide, but small problems can potentially lead to inefficient operation. While most propane heaters have safeguards protecting against this possibility, it's difficult to rule out any potential for dangerous malfunction. Unfortunately, it can be difficult to detect carbon monoxide poisoning symptoms before it's too late to respond appropriately. These types of poisonings, however, are exceedingly rare.
Propane gas is highly flammable when mixed with air, and it can be difficult to detect. While an operating heater burns off excess propane, a unit that is turned off but leaking propane can pose a potential risk. Proper tank safety reduces the likelihood of fires.