Natural gas is almost odorless as it comes from the well or processing facility. If the gas is destined for use as a fuel in homes or industry, a chemical called a mercaptan is added to give the gas a distinctive odor so that people can easily smell it when its concentration in air reaches 1%. Gas and air mixed in this concentration are not hazardous, but a mixture containing 5% gas is explosive. The odorant makes leaks or other unburned discharges of the gas quite evident long before a real hazard exists. Odorants injected into the gas burned in homes or industry do not create odors while burning, nor do they leave troublesome residue. Odorants are usually not introduced into gas sent to petrochemical plants where the gas is used as a feedstock for producing other commodities such as plastics, since the mercaptan will frequently interfere with the chemical process.