Denatured alcohol is an altered form of ethanol that has been made intentionally unpalatable or even poisonous. The purpose behind adding these denaturing substances to ethanol is to prevent people from consuming it. This can allow a government to levy fewer or no taxes on the denatured alcohol than it does on ethanol intended for human consumption. Many different agents can be used to denature alcohol, and each government has its own regulations about the acceptable formulas. One of the main qualifications for selecting a denaturing agent is that it should not be easily removed from the ethanol.
The ethyl alcohol commonly used in everything from mouthwash to cleaning agents is chemically identical to the type of alcohol found in whiskey, vodka, tequila and other spirits. Since many governments rely on the heavy taxation of these spirits, denaturing some alcohol for industrial and household use can allow the selective taxation of only the ethanol that is intended for human consumption. If the denaturing process was not enforced, people could purchase and consume cheap, pure ethanol that was intended for industrial use and effectively bypass various liquor laws and taxes.
Each government that requires the denaturing of alcohol has its own policies that serve to regulate the process. In the United States, the government has around 100 different ingredients that are allowed in denatured alcohol in addition to about 60 specific formulas. Each formula stipulates the types of denaturing agents that can be used in addition to the ratios they need to be added in. Companies are sometimes given some latitude in choosing which authorized denaturing agents to use, as various products can have different requirements.
One of the main denaturing agents that is commonly added to ethanol is methyl alcohol, which is why the term "methylated spirits" is sometimes used. Methanol, which is made from a variety of feedstocks such as wood and coal, is highly toxic. It also has a similar boiling point to ethanol, which makes the two spirits difficult to separate once they have been mixed into a solution. Other denaturing agents are chosen for similar characteristics, and it is typically a requirement that any denatured alcohol product should not be easily reduced into the component ingredients through processes such as precipitation, distillation and solvent extraction. Some countries also have requirements that denatured alcohol be dyed bright blue or purple to reduce the likelihood of accidental ingestion.