http://en.wikipedia.org/wiki/Trihalomethane#Water_pollutants
Trihalomethanes are formed as a by-product when chlorine or bromine are used to disinfect water for drinking (commonly known as disinfection by-products). They result from the reaction of chlorine and/or bromine with organic matter in the water being treated. The THMs produced may have adverse health effects at high concentrations, and many governments set limits on the amount permissible in drinking water. In the United States, the EPA limits the total concentration of chloroform, bromoform, bromodichloromethane, and dibromochloromethane to 80 parts per billion in treated water. This number is called "total trihalomethanes" (TTHM).
Chloroform is also formed in swimming pools which are disinfected with chlorine or hypochlorite in the haloform reaction with organic substances (urine, sweat and skin particles). The reaction to phosgene under the influence of UV is also possible. Some of the THMs are quite volatile and may easily vaporize into the air. This makes it possible to inhale while showering, for example. The EPA, however, has determined that this exposure is minimal compared to that from consumption.
http://en.wikipedia.org/wiki/Perchlorate#Environmental_presence
Low levels of perchlorate have been detected in both drinking water and groundwater in 35 states in the US according to the Environmental Protection Agency. In 2004, the chemical was also found in cow's milk in the area with an average level of 1.3 parts per billion ("ppb" or µg/L), which may have entered the cows through feeding on crops that had exposure to water containing perchlorates.[5] According to the Impact Area Groundwater Study Program, the chemical has been detected as high as 5 µg/L in Massachusetts, well over the state regulation of 2 µg/L[6].
In a statement issued after the discovery of perchlorate on Mars (see below), NASA declared, among other things, that perchlorates are found naturally on Earth at such places as Chile's hyper-arid Atacama Desert.[7]
In some places, perchlorate is detected because of contamination from industrial sites that use or manufacture it. In other places, there is no clear source of perchlorate. In those areas it may be naturally occurring, or could be present because of the use of Chilean fertilizers, which were imported to the U.S. by the hundreds of tons in the early 19th century. One recent area of research has even suggested that perchlorate can be created when lightning strikes a body of water, and perchlorates are created as a byproduct of chlorine generators used in swimming pool chlorination systems.[8]
Fireworks are also a source of perchlorate in lakes.[9]
As of April 2007, the EPA has not yet determined whether perchlorate is present at sufficient levels in the environment to require a nationwide regulation on how much should be allowed in drinking water.[10] In 2005, U.S. EPA issued a recommended Drinking Water Equivalent Level (DWEL) for perchlorate of 24.5 µg/L. In early 2006, EPA issued a “Cleanup Guidance” for this same amount. Both the DWEL and the Cleanup Guidance were based on a thorough review of the existing research by the National Academy of Science (NAS). This followed numerous other studies, including one which suggested human breast milk had an average of 10.5 µg/L of perchlorate.[11] Both the Pentagon and some environmental groups have voiced questions about the NAS report, but no credible science has emerged to challenge the NAS findings. In February 2008, U.S. Food and Drug Administration said that U.S. toddlers on average are being exposed to more than half of the U.S. EPA's safe dose from food alone[12].
The two production sites of PEPCON and Kerr McGee in Henderson, Nevada, which were the biggest producers till the explosion of PEPCON in 1988 and the closure of the Kerr McGee plant in 1998, leaked significant amounts of perchlorates into the Las Vegas Wash and from there into Lake Mead and the Colorado River.
The disposal of unused rocket motors and ammunition has led to contamination by perchlorates of several military installations.
Health effects
Perchlorate greatly impacts human health by interfering with iodide uptake into the thyroid gland. In adults, the thyroid gland helps regulate the metabolism by releasing hormones, while in children, the thyroid helps in proper development. Perchlorate is becoming a serious threat to human health and water resources.[13]
The NAS found that perchlorate only affects the thyroid gland. It is not stored in the body, it is not metabolized, and any effects of perchlorate on the thyroid gland are fully reversible once exposure stops[citation needed]. There has been some concern on perchlorates effects on fetuses, newborns and children, but several peer-reviewed studies on children and newborns also provide reason to believe that low levels of perchlorate do not pose a threat to these populations.[citation needed] On October 1, 2004, the American Thyroid Association (ATA) reported that perchlorate may not be as harmful to newborns, pregnant women and other adults as previously thought.[14]
A study involving healthy adult volunteers determined that at levels above 0.007 milligrams per kilogram per day (mg/kg-d), perchlorate can temporarily inhibit the thyroid gland’s ability to absorb iodine from the bloodstream ("iodide uptake inhibition", thus perchlorate is a known goitrogen).[15] The EPA converted this dose into a reference dose of 0.0007 mg/kg-d by dividing this level by the standard intraspecies uncertainty factor of 10. The agency then calculated a "drinking water equivalent level" of 24.5 ppb by assuming a person weighs 70 kilograms (154 pounds) and consumes 2 liters (68 ounces) of drinking water per day over a lifetime.[16] Thus, 25 ppb was set as the recommended drinking water standard (the DWEL). For that reason, most media reports call this the "safe" level of exposure. The NAS report also stated additional research would be helpful, but emphasized that the existing database on perchlorate was sufficient to make its reference dose recommendation and ensure it would be protective for everyone.[citation needed