Cresol
Cresols (also "hydroxytoluene") are a group of aromatic organic compounds. They are widely-occurring phenols (sometimes called "phenolics") which may be either natural or manufactured. They are also categorized as methylphenols. Cresols commonly occur as either solids or liquids because their melting points are generally close to room temperature. Like other types of phenols, they are slowly oxidized by exposure to air, and the resulting impurities often give the samples a yellow to brownish red tint. Cresols have an odor characteristic to that of other simple phenols, reminiscent to some of a "coal tar" smell. The name "cresol" is  an adduct of phenol and their traditional source, creosote. Structure and production. In its chemical structure, a molecule of cresol has a methyl group substituted onto the ring of phenol. There are three forms (isomers) of cresol: "ortho"-cresol ("o"-cresol), "meta"-cresol ("m"-cresol), and "para"-cresol ("p"-cresol). These forms occur separately or as a mixture, which can also be called cresol or more specifically, tricresol. About half of the world's supply of cresols are extracted from coal tar. The rest is produced by hydrolysis of chlorotoluenes or the related sulfonates. Another method entails methylation of phenol with methanol over a solid acid  catalyst, often comprising magnesium oxide or alumina. Temperatures above 300 °C are typical. Anisole converts to cresols under these conditions. Applications. Cresols are precursors or synthetic intermediates to other compounds and materials, including plastics, pesticides, pharmaceuticals, and dyes. For cresol bactericides or disinfectants the mechanism of action is due to the destruction of bacterial cell membranes. Most recently, cresols have been used to create a breakthrough in manufacturing carbon nanotubes at scale that are separated and not twisted, without additional chemicals that change the surface properties of the nanotubes. Derivatives. Derivatives of "p"-cresol include: Derivatives of "o"-cresol include: Derivatives  of "m"-cresol include: Health effects. When cresols are inhaled, ingested, or applied to the skin, they can be very harmful. Effects observed in people include irritation and burning of skin, eyes, mouth, and throat; abdominal pain and vomiting; heart damage; anemia; liver and kidney damage; facial paralysis; coma; and death. Breathing high levels of cresols for a short time results in irritation of the nose and throat. Aside from these effects, very little is known about the effects of breathing cresols, for example, at lower levels over longer times. Ingesting high levels results in kidney problems, mouth and throat burns , abdominal pain, vomiting, and effects on the blood and nervous system. Skin contact with high levels of cresols can burn the skin and damage the kidneys, liver, blood, brain, and lungs. Short-term and long-term studies with animals have shown similar effects from exposure to cresols. No human or animal studies have shown harmful effects from cresols on reproduction. It is not known what the effects are from long-term ingestion or skin contact with low levels of cresols. The Occupational Safety and Health Administration has set a permissible exposure limit at 5 ppm (22 mg/m) over an  eight-hour time-weighted average, while the National Institute for Occupational Safety and Health recommends a limit of 2.3 ppm (10 mg/m).