Perchlorate

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The structure and dimensions of the perchlorate ion
A space-filling model of the perchlorate ion

Perchlorates are the salts derived from perchloric acid (HClO4). They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are also used as an oxidizer in rocket fuel and can be found in airbags, fireworks, and Chilean fertilizers. Both potassium perchlorate (KClO4) and ammonium perchlorate (NH4ClO4) are used extensively within the pyrotechnics industry, whereas ammonium perchlorate is a component of solid rocket fuel. Lithium perchlorate, which decomposes exothermically to give oxygen, is used in oxygen "candles" on spacecraft, submarines and in other esoteric situations where a reliable backup or supplementary oxygen supply is needed. Most perchlorate salts are soluble in water.

Scientific definition

The perchlorate ion is ClO4, and it has a molecular mass of 99.45 a.u.

A perchlorate (compound) is a compound containing this group, with chlorine in oxidation state +7.

Strength of oxidation

The perchlorate ion is the weakest oxidizer of the generalized chlorates. Lower oxidation numbers are progressively stronger oxidizers, and less stable. Perchlorate has the highest redox potential, but is a closed shell species, and as such, is sluggish to oxidize other species.

Stability

Most perchlorates, especially salts of electropositive metals such as sodium perchlorate or potassium perchlorate, are slow to react unless heated, the perchlorate ion being largely inert and not oxidizing at lower temperatures. This property is useful in many applications, such as flares, where the device should not explode, or even catch fire spontaneously.

Mixtures of perchlorates with organic compounds are more reactive; although generally they do not catch fire or explode unless heated, there are a number of exceptions.

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.[1] According to the Impact Area Groundwater Study Program [1], the chemical has been detected as high as 5 µg/L in Massachusetts, well over the state regulation of 1 µg/L.

In some places it is being detected because of contamination from industrial sites that use or manufacture perchlorate. 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.

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.[2] 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.[3] 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.

Health effects

A study involving healthy adult volunteers determined that at levels above 0.007 milligrams per kilogram per day (mg/kg-d), perchlorate can temporarily and reversibly inhibit the thyroid gland’s ability to absorb iodine from the bloodstream ("iodide uptake inhibition", thus perchlorate is a known goitrogen).[4] The EPA converted this dose into a "drinking water equivalent level" of 245 ppb by assuming a person weighs 70 kilograms (154 pounds) and consumes 2 liters (68 ounces) of drinking water per day over a lifetime.[5]

While the thyroid uses iodine to produce hormones, NAS says this process of iodide uptake inhibition is not an "adverse," or harmful, effect. There has been some speculation that exposure to extremely high doses of perchlorate, for several months or years could lead to hypothyroidism, but NAS found that iodide uptake inhibition was the only consistently documented health effect of perchlorate in humans.[citation needed] Furthermore, a 2006 CDC study found this to only be true in women, and because the study was not properly controlled for autoimmune thyroid disease (which causes identical effects on iodine uptake and which is much more common in women), improved analysis may show that the effect is extremely small or even nonexistent.[citation needed]

The NAS also found that perchlorate only affects the thyroid gland. There is no evidence that it causes brain damage, birth defects or cancer in humans.[citation needed] It is also not stored in the body, it is not metabolized, and any effects of perchlorate on the thyroid gland are fully reversible once exposure stops. 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. 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.[6]

The EPA and NAS divided the No Observed Effect Level (NOEL) for perchlorate of 0.007 mg/kg-d by the standard intraspecies uncertainty factor of 10 to derive a “reference dose” of 0.0007 mg/kg-d, and declared this would be protective of even the most sensitive subpopulations. Usually an additional 10-fold interspecies uncertainty factor is also used in the calculation of reference doses, but since the perchlorate NOEL was derived from a human study, rather than an animal study, this additional uncertainty factor was not used. Using the 70 kg body weight and 2 liter/day assumptions used above, this dose is converted to 25 ppb in drinking water. 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.

Recent research, however, has shown inhibition of iodide uptake in the thyroids of women at much lower levels, levels attainable from normally contaminated water and milk.[7]

Types of perchlorates

References

  1. Associated Press. "Toxic chemical found in California milk". MSNBC. June 22, 2004.
  2. EPA Press Release "EPA Issues Determination on 11 Contaminants" April 4, 2007
  3. McKee, Maggie. "Perchlorate found in breast milk across US". New Scientist. February 23, 2005
  4. Greer, M.A., Goodman, G., Pleuss, R.C., Greer, S.E. (2002). "Health effect assessment for environmental perchlorate contamination: The dose response for inhibition of thyroidal radioiodide uptake in humans" (free online). Environmental Health Perspectives. 110 (9): 927–937.
  5. US EPA Memorandum Jan 26, 2006
  6. "Various Levels of Perchlorate Exposure Found Not to Be Harmful to Newborns, Pregnant Women, and Other Adults" (Press release). American Thyroid Association. 1 Oct 2004.
  7. Benjamin C. Blount, James L. Pirkle, John D. Osterloh, Liza Valentin-Blasini, and Kathleen L. Caldwell (2006). "Urinary Perchlorate and Thyroid Hormone Levels in Adolescent and Adult Men and Women Living in the United States". Environmental Health Perspectives. 114 (12). doi:10.1289/ehp.9466.

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