Chlorine Byproducts
Trihalomethanes, chloroform, haloacetic acids
What is it?
Chlorine is added to municipal water supplies to kill pathogens, which it does effectively. The problem is that chlorine reacts with naturally occurring organic matter in water to form disinfection byproducts (DBPs), the most common being trihalomethanes (THMs) including chloroform. These DBPs are present in virtually all chlorinated tap water and are absorbed through drinking, cooking, bathing and particularly through skin during showering, where warm water vapour significantly enhances absorption.
What it does to your body
Cancer
Chloroform is classified as a possible human carcinogen; higher THM exposure is associated with bladder cancer in long-term epidemiological studies.
Reproductive harm
Several studies link high THM exposure in pregnancy to low birth weight and spontaneous miscarriage, particularly from bathing and showering exposure.
Respiratory irritation
Chlorine gas released in hot showers irritates airways, worsening symptoms for asthma sufferers and contributing to chronic respiratory inflammation.
Skin and gut microbiome disruption
Chlorine alters the balance of beneficial bacteria in the gut and on skin, potentially contributing to skin conditions and digestive issues.
How widespread is the problem?
The EWG tap water database has found THMs in water supplies serving over 200 million Americans. A 2019 UK study found chloroform in 100% of tap water samples tested. Shower exposure is estimated to account for a significant proportion of total THM intake due to the efficiency of dermal and inhalation absorption.
Where it hides in your home
Key research
IARC Chloroform Classification (Group 2B)
IARC classified chloroform as a possible human carcinogen, Group 2B, based on sufficient animal evidence and limited human evidence.
EWG Tap Water Database
EWG analysis of water utility testing data found THMs in water supplies serving over 200 million Americans, with many utilities exceeding health guidelines.
UK Drinking Water Inspectorate THM Report
DWI found chloroform present in 100% of UK tap water samples at varying concentrations, consistent with chlorination chemistry.
THMs are volatile and metabolised fairly quickly. However most people are re-exposed daily through tap water. Filtering drinking and bathing water is the most effective way to reduce chronic exposure to these compounds.