Odorants are chemical additives that are mixed in with natural gas during the odorization process to add an artificial smell to the gas. These odorants are added as a safety precaution, as natural gas in its pure state is completely odourless. These chemicals add a characteristic "gas" smell that can be identified at concentrations of just 1%, reducing the risk that leaks will go undetected and gas will accumulate to hazardous levels. Generally, this odorization process occurs before distribution to customers.
Since natural gas is odorless and colourless, odorants are added that are deliberately distinctive and unpleasant so that the presence of gas in air is detectable. Odorants do not change any physical or chemical properties of the gas besides adding a smell. The odorization of natural gas is performed with a blend of synthetic chemicals that generally have a low molecular weight.
There are two main groups of odorants - sulfur-based odorants including mercaptans and sulfides and sulfur-free odorants. The second type of odorant has been developed more recently and hold potential as there are zero sulfur dioxide emissions after gas combustion. Odorants must meet all requirements to be used, including having a strong odor that is detectable as long as a leak exists, they must not produce toxic products, they must be chemically stable and cannot react with rust or piping material.
There are numerous different types of odorants that can be injected into the natural gas, and they all have varying physical and chemical properties that are better suited to certain environments. Small things such as the composition of the soil that the gas distribution pipeline runs through can help determine which odorant is best for the job. Odorants include tetrahydrothiophene, dimethyl sulfide, diethyl sulfide (which is very stable but not suited for use in odorant blends), methylethyl sulfide, ethyl mercaptan, propyl mercaptan, and butyl mercaptan. These are just a few of the different types, and although they are chemically complex they all serve the main purpose of adding a detectable smell to natural gas.
Flashlights, not a flame, should be used to investigate minor gas odors. When investigating these leaks, never use matches, candles, or electric switches. Older appliances and water heaters tend to have a small, continuously burning gas flame known as the pilot light. If this light is out and gas can be smelled, follow manufacturers' instructions on how to relight the pilot light after shutting off the gas at the appliances gas shutoff valve. Before relighting, wait five minutes to allow gas to disperse.