Properly maintained and vented space heaters fueled by propane have few safety risks. Units that are not vented, however, can be dangerous if used incorrectly or if the unit malfunctions.
When combusted efficiently, propane produces only trace amounts of dangerous chemicals. However, problems with combustion can cause the formation of carbon monoxide, a potentially deadly chemical. Most propane systems are designed with safeguards to prevent improper combustion, but older systems might not have such protections.
Special care must be given when using a converted unit. Most natural gas space heaters have conversion kits to allow them to operate on propane, but improper installation can lead to leaks and poor combustion, potentially causing carbon monoxide or smoke poisoning.