(Premier Pet) The citronella refills contain a gas mixture that is safe for people, animals and the environment. Most of us are familiar with its pleasant lemony scent. Citronella is an essential oil that is derived from the stem and leaves of lemon grass and has been used in medicine for a thousand years. The propellant gas is the ozone friendly R134a. Used in many medical inhalation products, it is harmless to pets and people. Hold refill cartridge upside down. Insert nozzle into refill valve of the device and press firmly to fill the unit.