EWG's Tap Water Database



Forest Water Supply Corporation

Chloroform, one of the total trihalomethanes (TTHMs), is formed when chlorine or other disinfectants are used to treat drinking water. Chloroform and other disinfection byproducts increase the risk of cancer and may cause problems during pregnancy. Read More.

In addition to tap water disinfection, chloroform pollution in the environment also comes from industrial discharges from pulp and paper mills, and from urban wastewater effluent. Human studies show that chloroform damages the kidneys, liver and central nervous system. In animals, chloroform causes infertility, birth defects and cancer.

Click here to read more about disinfection byproducts.





Samples exceeding legal limit (MCL)


Samples exceeding
health guidelines

Testing results - average by year

YearAverage resultSamples takenDetectionsRange of results
201055.4 ppb4429.7 ppb - 82.0 ppb
201136.9 ppb4420.9 ppb - 54.9 ppb
201244.5 ppb5532.4 ppb - 62.4 ppb
201394.0 ppb5582.0 ppb - 115.0 ppb
201488.3 ppb8860.7 ppb - 107.6 ppb
201586.9 ppb9944.1 ppb - 124.0 ppb

ppb = parts per billion.

State and national drinking water standards and health guidelines

Health Guideline 0.4 ppb

The health guideline of 0.4 ppb for chloroform was proposed in 2018 by the California Office of Environmental Health Hazard Assessment as a one-in-a-million lifetime risk of cancer. Values greater than one-in-a-million cancer risk level can result in increased cancer cases above one in a million people.

ppb = parts per billion.

EPA Maximum Contaminant
Level Goal (MCLG)70.0 ppb

A non-enforceable federal health guideline. For cancer-causing chemicals, these health goals are typically set to zero.

ppb = parts per billion.

All test results

Date Lab ID Result
2010-03-18100354300329.7 ppb
2010-06-07100621700153.5 ppb
2010-09-30101000500182.0 ppb
2010-12-08101240400156.3 ppb
2011-02-07110216000254.9 ppb
2011-06-02110611700120.9 ppb
2011-08-251108B2300139.2 ppb
2011-12-14111257000132.5 ppb
2012-01-24120155500534.7 ppb
2012-01-24120155500352.5 ppb
2012-06-21120676400140.4 ppb
2012-08-07120833800132.4 ppb
2012-10-11121044700162.4 ppb
2013-03-06130320300282.0 ppb
2013-04-02130413100185.5 ppb
2013-07-16Q1303849002115.0 ppb
2013-10-22Q130917100486.4 ppb
2013-10-22Q1309171003101.0 ppb
2014-03-06Q140723500460.7 ppb
2014-03-06Q140723500373.1 ppb
2014-05-30Q141860000296.1 ppb
2014-05-30Q1418600001107.1 ppb
2014-09-09Q144269200282.0 ppb
2014-09-09Q144269200184.7 ppb
2014-11-06Q145513600294.7 ppb
2014-11-06Q1455136001107.6 ppb
2015-02-05Q150470900344.1 ppb
2015-02-05Q150470900189.5 ppb
2015-02-05Q150470900266.7 ppb
2015-06-08Q1520776002123.0 ppb
2015-06-08Q1520776001124.0 ppb
2015-09-01Q1533590002109.0 ppb
2015-09-01Q153359000186.9 ppb
2015-12-04Q154841400251.8 ppb
2015-12-04Q154841400187.1 ppb