threat to honeybee health. the ecological impact of lindane's environmental persistence continues to be debated. the us epa determined in 2002 that the agency does not believe that lindane contaminates drinking water in excess of levels considered safe. u.s. geological survey teams concluded the same in 1999 and 2000. with regard to lindane medications, the epa conducted "down-the-drain" estimates of the amount of lindane reaching public water supplies and concluded that lindane levels from pharmaceutical sources were "extremely low" and not of concern. note that the epa has set the maximum contaminant level or "mcl" for lindane allowed in public water supplies and considered safe for drinking at 200 parts per trillion (ppt). by comparison, the state of california imposes a lower mcl for lindane of 19 ppt. however, the california standard is based on a dated 1988 national water criterion that was subsequently revised by the epa in 2003 to 980 ppt. the epa stated that the change resulted from "significant scientific advances made in the last two decades particularly in the areas of cancer and noncancer risk assessments." while the epa considered raising the mcl standard for lindane to 980 ppt at that time, the change was never implemented because states had little difficulty in maintaining lindane levels below the 200 ppt mcl limit already in place. today, the legally enforceable mcl standard for lindane is 200 ppt, while the national water criterion for lindane is 980 ppt. isomers lindane is the gamma isomer of hexachlorocyclohexane ("γ-hch"). in addition to the issue of lindane pollution, some concerns are related to the other isomers of hch, namely alpha-hch and beta-hch, which are notably more toxic than lindane, lack its insecticidal properties, and are byproducts of lindane production. in the 1940s and 1950s, lindane producers stockpiled these isomers in open heaps, which led to ground and water contamination. the international hch and pesticide forum has since been established to bring together experts to address the clean-up and containment of these sites. modern manufacturing standards for lindane involve the treatment and conversion of waste isomers to less toxic industrial chemicals, a process known as "cracking". today, only a few production plants remain active worldwide to accommodate public-health uses of lindane and declining agricultural needs. lindane has not been manufactured in the u.s. since the mid-1970s, but continues to be imported. α-hexachlorocyclohexane β-hexachlorocyclohexane