Thermacell Patio Shield Mosquito Repeller with 48 Hours of Mosquito Repellent provides legendary defense against mosquitoes, thanks to an invisible, 15-foot zone of protection. The Thermacell Patio Shield Mosquito Repeller comes with four fuel cartridges and 12 repellent mats that produce a total of 48 hours of freedom from mosquito bites. The Patio Shield Repeller uses an interior-heating element to activate the mosquito repellent contained in the blue repellent mat.