In 1994, the US EPA issued a series of rules under the Toxic Substances Control Act to require more than 100 health effect studies in respect of 25 chemical air pollutants: biphenyl, carbonyl sulphide, chlorine, chlorobenzene, chloroprene, cresols (mixture or isomers), crumene, dibutyl, phthalate, diethanolamine, ethylbenzene, ethyl chloride, ethylene dichloride, ethylene glycol, hydrolic acid, hydrogen fluoride, maleic anhydride, methyl isobutyl ketone, methyl methacrylate, naphthalene, phenol and phthalic anhydride.