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The term “chlorate” most commonly refers only to chlorine in the +5 oxidation state, or chlorate ion. Chlorate ion is a known byproduct of the drinking water disinfection process, forming when sodium hypochlorite or chlorine dioxide are used in the disinfection process

A number of compounds can react to release chlorate ion in water, including some in herbicides, fireworks and other explosives.

The most direct source of exposure to chlorate is through drinking water that has been disinfected with sodium hypochlorite or chlorine dioxide. The amount of chlorate in drinking water depends on a number of chemical reactions in both the formation of these disinfectants and the conditions in which they are used. As a result, concentrations of chlorate ion can vary at different points within the system and at different times during the year. Water providers can mitigate the production of chlorate ion during disinfection by observing best practices such as avoiding exposure to light sources, minimizing storage time, maintaining proper pH and properly handling chemicals involved in the process.

The U.S. Environmental Protection Agency’s current reference concentration indicates that ongoing exposure to chlorate ion at levels of more than 210 parts per billion per day can lead to an enlarged thyroid. As part of its Third Unregulated Contaminant Monitoring Rule, water systems across the United States tested for chlorate to determine how prevalent chlorate ion is in U.S. drinking water supplies and at what levels it appears. With this information, EPA will be better able to determine if chlorate ion in tap water presents a health concern.

Under the recent round of UCMR3 testing, many water systems nationwide tested for chlorate ion. Contact your public water system to learn more about chlorate testing and results. If your water system observed chlorate in this testing, it will publish the results in its next annual consumer confidence report, which is publicly available. The report is often available on the internet, but you can also contact your water provider to request a copy. You can usually find contact information for your public water system on your water bill.

There is not a federal drinking water standard for chlorate ion at this time, but the EPA has set a daily reference dose of 0.03 milligrams per kilogram of body weight (0.03 mg/kg/day). EPA defines a reference dose as “an estimate of a daily oral exposure to the human population that is likely to be without an appreciable risk of deleterious effects during a lifetime.”

When determining what substances merit regulation, EPA considers both health effects and occurrence data. If there is scientifically compelling evidence that shows a large number of U.S. drinking water systems have high amounts of chlorate ion above the current reference concentration of 210 ppb per day, it’s possible that EPA may decide to regulate it in the future.

If you get your drinking water from a private well and have concerns about chlorate, you can have your water tested by a certified laboratory. You can find information on how to sample for chlorate and where to send samples for analysis by contacting your state water laboratory certification officer. Contact information for your state can be found on labs on the UCMR3 laboratory list . Additional information about well water testing from EPA is available on their private drinking water well FAQ page

Currently, no home treatment device is available to remove chlorate ion once it has been formed in drinking water.
Bottled water quality can vary. Bottled water in the United States is regulated by the U.S. Food and Drug Administration and is required to meet standards equal to the EPA’s tap water standards. There are also individual state standards. However, in most cases, you must contact the bottled water manufacturer for information about chlorate ion levels.