Thiols, also named as mercaptan, are a class of organic compounds containing a sulfhydryl group (SH). They are similar to the alcohols and phenols but with the oxygen atom replaced by a sulfur atom. Actually, thiols are responsible for high volatility and unpleasant odor resembling that of garlic or rotten eggs. Thiols also contribute to the telltale odor skunks sprays in self-defense and the scent of freshly chopped onions. A concentration of 10 ppb can be recognized by human nose.