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EWG's Tap Water Database — 2021 UPDATE

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Total trihalomethanes (TTHMs)

City of Houston Willow Chase

Trihalomethanes are cancer-causing contaminants that form during water treatment with chlorine and other disinfectants. The total trihalomethanes group includes four chemicals: chloroform, bromodichloromethane, dibromochloromethane and bromoform. Read More.

Studies conducted in the U.S. and Europe have found that drinking tap water that carries disinfection byproducts increases the risk of developing bladder cancer. In animal studies, all trihalomethanes cause liver, kidney and intestinal tumors. Some human epidemiological studies also reported an association between disinfection byproducts and an increased risk of problems during pregnancy, including spontaneous miscarriage, cardiovascular defects, neural tube defects and low birth weight.

The health guideline of 0.15 parts per billion, or ppb, for the group of four trihalomethanes, or THM4/TTHM, was defined in a peer-reviewed scientific study by EWG and represents a one-in-one-million lifetime cancer risk level.

Click here to read more about disinfection byproducts.

MORE ABOUT THIS CONTAMINANT

 

96

Samples

0

Samples exceeding legal limit (MCL)

18

Samples exceeding
health guidelines

Testing results - average by year

 
YearAverage resultSamples takenDetectionsRange of results
20141.20 ppb165ND - 10.00 ppb
20152.16 ppb166ND - 21.8 ppb
20160.225 ppb162ND - 2.20 ppb
20170.794 ppb161ND - 12.7 ppb
20180.0750 ppb161ND - 1.20 ppb
20190.575 ppb163ND - 5.80 ppb

ppb = parts per billion

State and national drinking water standards and health guidelines

EWG Health Guideline 0.15 ppb

The health guideline of 0.15 parts per billion, or ppb, for the group of four trihalomethanes, or THM4/TTHM, was defined in a peer-reviewed scientific study by EWG and represents a one-in-one-million lifetime cancer risk level.

EPA Maximum Contaminant
Level (MCL) 80 ppb

The legal limit for the group of four trihalomethanes, established in 1998, was based on the need for residual disinfectant levels in water served to customers and the cost of treatment. This limit does not fully protect against the risk of cancer due to exposure to trihalomethanes.

ppb = parts per billion

All test results

Date Lab ID Result
2014-03-06AC45957ND
2014-03-06AC45974ND
2014-03-06AC45967ND
2014-03-06AC459661.000 ppb
2014-05-20AC547122.70 ppb
2014-05-20AC54710ND
2014-05-20AC54711ND
2014-05-20AC547091.000 ppb
2014-08-14AC63793ND
2014-08-14AC637914.50 ppb
2014-08-14AC63794ND
2014-08-14AC63789ND
2014-12-18AC7544510.00 ppb
2014-12-18AC75434ND
2014-12-18AC75450ND
2014-12-18AC75449ND
2015-03-05AC82711ND
2015-03-05AC82710ND
2015-03-05AC82689ND
2015-03-05AC82709ND
2015-05-29AC939572.80 ppb
2015-05-29AC940081.30 ppb
2015-05-29AC9399821.8 ppb
2015-05-29AC940023.00 ppb
2015-09-17AD07544ND
2015-09-17AD07542ND
2015-09-17AD07559ND
2015-09-17AD07545ND
2015-12-08AD159564.60 ppb
2015-12-08AD15957ND
2015-12-08AD15959ND
2015-12-08AD159331.000 ppb
2016-03-03AD26844ND
2016-03-03AD26845ND
2016-03-03AD26843ND
2016-03-03AD26849ND
2016-06-03AD39413ND
2016-06-03AD393622.20 ppb
2016-06-03AD39368ND
2016-06-03AD393651.40 ppb
2016-09-22AD50717ND
2016-09-22AD50725ND
2016-09-22AD50714ND
2016-09-22AD50726ND
2016-11-21AD58771ND
2016-11-21AD58773ND
2016-11-21AD58770ND
2016-11-21AD58772ND
2017-03-02AD7308512.7 ppb
2017-03-02AD73087ND
2017-03-02AD73089ND
2017-03-02AD73086ND
2017-05-31AD86109ND
2017-05-31AD86106ND
2017-05-31AD86108ND
2017-05-31AD86107ND
2017-08-15AD93060ND
2017-08-15AD93059ND
2017-08-15AD93058ND
2017-08-15AD93061ND
2017-12-04AE06555ND
2017-12-04AE06556ND
2017-12-04AE06557ND
2017-12-04AE06558ND
2018-01-31AE11529ND
2018-01-31AE11527ND
2018-01-31AE11530ND
2018-01-31AE11531ND
2018-04-11AE22016ND
2018-04-11AE22023ND
2018-04-11AE22004ND
2018-04-11AE22022ND
2018-08-15AE37136ND
2018-08-15AE37137ND
2018-08-15AE37139ND
2018-08-15AE371381.20 ppb
2018-11-07AE45579ND
2018-11-07AE45581ND
2018-11-07AE45582ND
2018-11-07AE45580ND
2019-02-28AE56534ND
2019-02-28AE56533ND
2019-02-28AE56535ND
2019-02-28AE56536ND
2019-05-20AE67108ND
2019-05-20AE67114ND
2019-05-20AE671091.20 ppb
2019-05-20AE67107ND
2019-09-23AE84231ND
2019-09-23AE84230ND
2019-09-23AE84228ND
2019-09-23AE842262.20 ppb
2019-11-22AE90353ND
2019-11-22AE903505.80 ppb
2019-11-22AE90352ND
2019-11-22AE90359ND