EWG's Tap Water Database — 2019 UPDATE



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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
201216.3 ppb36364.30 ppb - 45.4 ppb
201313.0 ppb889.10 ppb - 18.0 ppb
201410.7 ppb30303.20 ppb - 29.1 ppb
20159.39 ppb16167.10 ppb - 13.6 ppb
201611.6 ppb32322.90 ppb - 35.1 ppb
201712.4 ppb32322.50 ppb - 39.0 ppb

ppb = parts per billion

State and national drinking water standards and health guidelines

EWG Health Guideline 0.4 ppb

The EWG 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

All test results

Date Lab ID Result
2013-08-1323116501 TTHM18.0 ppb
2013-08-1323116601 TTHM14.0 ppb
2013-08-1323116801 TTHM13.0 ppb
2013-08-1323116701 TTHM12.8 ppb
2013-08-1323117001 TTHM12.8 ppb
2013-08-1323116901 TTHM12.6 ppb
2013-08-1323116301 TTHM11.9 ppb
2013-08-1323116401 TTHM9.10 ppb
2014-02-1124694501THM3.60 ppb
2014-02-1124694301THM3.50 ppb
2014-02-1124694401THM3.40 ppb
2014-02-1124694901THM3.30 ppb
2014-02-1124694701THM3.30 ppb
2014-02-1124694601THM3.20 ppb
2014-02-1124694801THM3.20 ppb
2014-05-1325610201THM29.1 ppb
2014-05-1325610601THM22.6 ppb
2014-05-1325610701THM19.6 ppb
2014-05-1325610301THM18.1 ppb
2014-05-1325610001THM16.1 ppb
2014-05-1325610401THM14.7 ppb
2014-05-1325610501THM14.0 ppb
2014-08-1326553201THM15.4 ppb
2014-08-1326553601THM12.1 ppb
2014-08-1326553701THM12.1 ppb
2014-08-1326553401THM11.3 ppb
2014-08-1326553301THM10.2 ppb
2014-08-1326553101THM10.1 ppb
2014-08-1326553001THM9.90 ppb
2014-08-1326553501THM9.30 ppb
2014-11-1127472401THM10.1 ppb
2014-11-1127472301THM9.80 ppb
2014-11-1127471701THM9.70 ppb
2014-11-1127471901THM9.50 ppb
2014-11-1127472201THM8.90 ppb
2014-11-1127472101THM8.40 ppb
2014-11-1127471801THM8.30 ppb
2014-11-1127472001THM7.70 ppb
2015-08-12460274-AMERICAN13.6 ppb
2015-08-12460274-AMERICAN12.5 ppb
2015-08-12460274-AMERICAN11.5 ppb
2015-08-12460274-AMERICAN9.10 ppb
2015-08-12460274-AMERICAN8.80 ppb
2015-08-12460274-AMERICAN7.90 ppb
2015-08-12460274-AMERICAN7.70 ppb
2015-08-12460274-AMERICAN7.10 ppb
2015-11-1000001_EXP-AMERIC10.5 ppb
2015-11-1000001_EXP-AMERIC9.90 ppb
2015-11-1000001_EXP-AMERIC9.80 ppb
2015-11-1000001_EXP-AMERIC9.40 ppb
2015-11-1000001_EXP-AMERIC8.80 ppb
2015-11-1000001_EXP-AMERIC8.10 ppb
2015-11-1000001_EXP-AMERIC8.10 ppb
2015-11-1000001_EXP-AMERIC7.40 ppb
2016-02-0931204101THM4.50 ppb
2016-02-0931204001THM4.40 ppb
2016-02-0931203901THM4.10 ppb
2016-02-0931204501THM3.90 ppb
2016-02-0931204601THM3.60 ppb
2016-02-0931204201THM3.20 ppb
2016-02-0931204401THM3.10 ppb
2016-02-0931204301THM2.90 ppb
2016-05-0931993501THM35.1 ppb
2016-05-0931993901THM28.4 ppb
2016-05-0931993601THM27.0 ppb
2016-05-0931993401THM24.3 ppb
2016-05-0931993301THM24.0 ppb
2016-05-0931993701THM23.5 ppb
2016-05-0931993801THM23.1 ppb
2016-05-0931993201THM19.5 ppb
2016-08-1533037401-THM11.7 ppb
2016-08-1533037001-THM11.7 ppb
2016-08-1533037101-THM11.5 ppb
2016-08-1533037201-THM11.2 ppb
2016-08-1533037601-THM10.00 ppb
2016-08-1533037301-THM9.90 ppb
2016-08-1533037501-THM9.90 ppb
2016-08-1533036901-THM9.80 ppb
2016-11-1633709301-THM7.80 ppb
2016-11-1633709801-THM6.40 ppb
2016-11-1633709501-THM6.10 ppb
2016-11-1633709401-THM6.00 ppb
2016-11-1633709601-THM6.00 ppb
2016-11-1633709901-THM5.90 ppb
2016-11-1633709701-THM5.70 ppb
2016-11-1633710001-THM5.60 ppb
2017-02-1334625101-THM4.20 ppb
2017-02-1334625201-THM4.20 ppb
2017-02-1334624601-THM3.40 ppb
2017-02-1334624701-THM3.40 ppb
2017-02-1334624901-THM3.10 ppb
2017-02-1334624801-THM2.80 ppb
2017-02-1334625001-THM2.60 ppb
2017-02-1334624501-THM2.50 ppb
2017-05-1035406101-THM39.0 ppb
2017-05-1035407101-THM33.1 ppb
2017-05-1035406901-THM30.6 ppb
2017-05-1035405901-THM29.7 ppb
2017-05-1035406301-THM28.7 ppb
2017-05-1035406701-THM28.3 ppb
2017-05-1035405701-THM27.0 ppb
2017-05-1035406501-THM26.5 ppb
2017-08-1436594801-THM12.1 ppb
2017-08-1436595101-THM12.0 ppb
2017-08-1436594401-THM10.6 ppb
2017-08-1436594901-THM10.4 ppb
2017-08-1436594601-THM10.3 ppb
2017-08-1436594501-THM9.90 ppb
2017-08-1436595001-THM9.80 ppb
2017-08-1436594701-THM9.60 ppb
2017-11-1337617101-THM7.40 ppb
2017-11-1337617001-THM7.30 ppb
2017-11-1337616801-THM5.50 ppb
2017-11-1337616901-THM5.20 ppb
2017-11-1337616401-THM4.70 ppb
2017-11-1337616501-THM4.50 ppb
2017-11-1337616701-THM4.50 ppb
2017-11-1337616601-THM4.20 ppb