Most portable space heaters are electric, but there are portable gas-powered space heaters available as well. Gas heaters require ventilation for safety, however, making them a poor choice for indoor heating. Typically, gas-powered portable heaters are only used in garages or sheds where there is plenty of airflow.
Gas space heaters require ventilation because incomplete combustion of fossil fuels creates a host of toxic gases, including carbon monoxide. This colorless, odorless gas can displace oxygen and cause suffocation and death.
Electric space heaters produce no toxic exhaust, but the large amount of current required to generate resistance heat can overwhelm the electrical system inside a home. These heaters can regularly draw as much as 1,500 watts, and so should be connected to circuits that are not used by other major appliances.