Freon gases have a sweet odor similar to ether or freshly mowed grass. The odor becomes noticeable at concentrations of higher than 20 percent. At 11 percent concentration and higher, Freon gases are toxic, so the effects can become debilitating before the odor is detected.
Freon is the trade name for a number of different gases with similar chemical formulas and characteristics. They are registered by the Du Pont chemical company and sold as refrigerants. Freon 22 and Freon 23 are the sole remaining chemicals under the Freon name that are still on the market, but Freon 22 is being phased out because of its environmental effects. Both gases have similar, ether-like odors.