There are a couple of required safety features needed for ventfree heaters to be used indoors safely. A BURNER that is 99.9% efficient at burning all the gas is consumes so that very little CO or carbon monoxide is emitted. The heater also requires a ODS or Oxygen Depletion System that will shut off the heater if the Oxygen level drops to a still safe 18%. Because these heaters are portable, they also require a TIP SWITCH so if it they are knocked over the gas flow stops shutting off the flame and gas. Last there is a FLAME SENSOR safety shut-off system that will shut off all gas flow if the flame goes out.