First, what is "gas"? Chemically, the term specifically refers to a colorless, odorless mixture of gases-predominantly methane-derived from fossilized organic matter. In everyday life, however, the term does not refer to a single specific gas but rather serves as a collective label for several types, primarily categorized into liquefied petroleum gas (LPG), natural gas, and coal gas.
While LPG and coal gas are manufactured products, natural gas occurs naturally. The gas used in most households is primarily LPG or natural gas. It is important to note that the gas inside the containers commonly referred to as "coal gas cylinders" is actually LPG, not coal gas.
LPG consists mainly of propane and butane and is considered a modern fuel. It offers advantages such as low pollution, high heat output, ease of transport, and simple storage, making it a staple in many households. However, it carries a significant risk: a leak during use can pose a severe danger to human health.
Both LPG and natural gas are colorless and odorless. However, due to their flammable and explosive nature, a chemical agent-such as the early-use ethanethiol or the currently used tetrahydrothiophene-is added to household gas supplies to impart a pungent odor resembling rotten eggs. Outdoors, methane (the main component of natural gas) dissipates quickly upon leaking because it is lighter than air, which helps to some extent in reducing the risk of explosion.