Plug-in air freshener. This air freshener is powered by a small electric current which heats a concentrated gel. The evaporating gel emits a continuous fragrance until it is depleted. Some brands of air may freshener contain chemicals that are hazardous to health. These include phthalates which are suspected of influencing reproductive and foetal development in humans. Air fresheners are also known to contain carcinogenic chemicals benzene and formaldehyde. Other ingredients may also trigger an allergic reaction such as asthma, headaches or other health problems when exposed to air fresheners.