There are hundreds of ways ethanol is denatured. Denatured alcohol that is intended for use as a fuel or solvent typically contains 5% or more methanol. Methanol is flammable and has a boiling point close to that of ethanol. Methanol is absorbed across the skin and is highly toxic, so when I recommend people not use denatured alcohol for making perfume or bath products, my reason is that you usually don't know what has been added to the ethanol. Denatured alcohol for use in cosmetics often contains water and a bittering agent (Bitrex or Aversion which are denatonium benzoate or denatonium saccharide), but other chemicals are sometimes used. Other common additives include (but are not limited to) isopropanol, methyl ethyl ketone, methyl isobutyl ketone, pyridine, benzene, diethyl phthalate, and naphtha. In some countries, denatured alcohol must be colored blue or purple using an aniline dye, in order to distinguish it from consumption-grade ethanol.
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