The same process that makes our water safe to drink may be exposing us to dangerous chemicals
Imagine pouring yourself a glass of water from the tap. It looks clean, tastes fine, and you trust it's safe to drink. But what if that water contained invisible chemical byproducts formed during disinfection—byproducts that scientists have found can damage DNA and increase cancer risk? This isn't a hypothetical scenario. Around 250 million Americans are potentially exposed to these chemicals daily through drinking water, beverages, and even food prepared with treated water 4 .
Without disinfection, waterborne diseases like cholera and polio would threaten public health; with it, we generate unintended chemical byproducts with concerning toxic properties.
Among the most studied of these disinfection by-products are haloacetic acids (HAAs) and haloacetonitriles (HANs)—compounds that have become the focus of intense scientific scrutiny due to their effects on mammalian cells 1 3 .
When chlorine-based disinfectants are added to water, they don't just kill harmful microorganisms. They also react with natural organic matter present in the source water, creating a variety of chemical byproducts 3 6 . The same reaction that destroys pathogens simultaneously generates these unintended compounds.
Natural Organic Matter
Disinfectant
DBPs
Consist of an acetic acid backbone with one or more halogen atoms (chlorine, bromine, or iodine) attached 4 .
Nitrogen-containing compounds that form when disinfectants react with nitrogen-containing organic matter 3 .
The specific types and amounts of these byproducts that form depend on several factors:
Type and amount in source water
Chlorine, chloramine, etc.
Presence of ions
Temperature, pH, contact time
In a pivotal 2010 study that systematically compared the toxicity of 12 different HAAs, researchers made a startling discovery: not all HAAs are created equal 1 . Using Chinese hamster ovary cells as a model mammalian system, scientists uncovered a clear hierarchy of toxicity.
The results revealed that iodoacetic acid (IAA) was the most potent HAA in both cytotoxicity (ability to kill cells) and genotoxicity (ability to damage DNA). Brominated HAAs generally fell in the middle of the toxicity spectrum, while chlorinated HAAs tended to be less toxic 1 .
This pattern led to an important general conclusion: iodinated HAAs > brominated HAAs > chlorinated HAAs in terms of toxic potency 1 .
| Rank | HAA Compound | Abbreviation | Relative Cytotoxicity | Relative Genotoxicity |
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| 1 | Iodoacetic acid | IAA |
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| 2 | Bromoacetic acid | BAA |
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| 3 | Bromochloroacetic acid | BCAA |
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| 4 | Chloroacetic acid | CAA |
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| 5 | Trichloroacetic acid | TCAA |
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| 6 | Dichloroacetic acid | DCAA |
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The National Toxicology Program later evaluated the carcinogenic potential of these compounds and designated six HAAs as reasonably anticipated to be human carcinogens: bromochloroacetic acid (BCA), bromodichloroacetic acid (BDCA), chlorodibromoacetic acid (CDBA), dibromoacetic acid (DBA), dichloroacetic acid (DCA), and tribromoacetic acid (TBA) 4 . These classifications were based on sufficient evidence from animal studies showing these compounds caused liver tumors, mesotheliomas, and other cancers in rodents 4 .
While HAAs have been studied for decades, recent research has revealed that haloacetonitriles (HANs) may pose an even greater concern despite typically occurring at lower concentrations 3 . These nitrogen-containing byproducts are formed during chlorination or chloramination of water containing nitrogenous organic matter 3 .
A 2024 study using normal tissue-derived human epithelial cells (RPE-1hTERT cells) provided crucial new insights into HAN toxicity 8 . Unlike many earlier studies that used cancer-derived cell lines, this research employed normal human cells, making the findings more relevant to human health.
| HAN Compound | Full Name | IC50 Value (μM) | Relative Toxicity |
|---|---|---|---|
| IAN | Iodoacetonitrile | 3.0 |
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| BAN | Bromoacetonitrile | 8.7 |
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| CAN | Chloroacetonitrile | 219.8 |
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The study tested both individual HANs and mixtures, revealing that iodoacetonitrile (IAN) was the most cytotoxic, followed by bromoacetonitrile (BAN), with chloroacetonitrile (CAN) being significantly less toxic 8 . The inhibitory concentration that reduced cell viability by 50% (IC50) was 3.0 μM for IAN, 8.7 μM for BAN, and 219.8 μM for CAN—demonstrating that iodinated HANs were dramatically more potent than their chlorinated counterparts 8 .
When researchers tested mixtures of HANs, they discovered that combinations exhibited primarily antagonistic effects—meaning the mixture was less toxic than expected based on the individual components 8 .
This finding challenges assumptions about mixture toxicity and suggests that current risk assessment approaches may need refinement 8 .
To understand how scientists determine the toxicity of these disinfection byproducts, let's examine the groundbreaking 2010 study that systematically compared 12 different HAAs 1 . This research provides an excellent example of how toxicological testing is conducted and why it's crucial for public health protection.
Researchers used Chinese hamster ovary (CHO) cells, a standard mammalian cell line widely employed in toxicological studies due to their well-characterized genetics and consistent responses 1 .
The team tested 12 different HAAs, including the five regulated by the EPA (BAA, DBAA, CAA, DCAA, TCAA) plus seven others (IAA, DiAA, BIAA, TBAA, CDBAA, BDCAA, BCAA) 1 .
To measure cell killing potential, scientists exposed CHO cells to varying concentrations of each HAA for extended periods, then measured survival rates. The specific metric used was chronic cytotoxicity, which assesses long-term cell damage rather than immediate effects 1 .
The researchers employed the single cell gel electrophoresis assay (Comet assay) to detect DNA damage—a key indicator of cancer-causing potential. This sensitive method can detect strand breaks and other DNA lesions at the individual cell level 1 .
The findings from this systematic comparison were revealing. The cytotoxicity ranking showed: IAA > BAA > TBAA > CDBAA > DIAA > DBAA > BDCAA > BCAA > CAA > BIAA > TCAA > DCAA 1 . Meanwhile, the genotoxicity ranking was: IAA > BAA > CAA > DBAA > DIAA > TBAA > BCAA > BIAA > CDBAA, with DCAA, TCAA, and BDCAA showing no genotoxic effects 1 .
Most Toxic
Iodoacetic AcidHighly Toxic
Bromoacetic AcidLeast Toxic
Dichloroacetic AcidThese results demonstrated that iodine-containing HAAs were consistently more toxic than their brominated or chlorinated counterparts. The study also highlighted that compounds with higher genotoxicity posed greater cancer risks, as DNA damage is a key mechanism in cancer development.
Perhaps most importantly, this research provided the water supply community with critical information for making decisions about disinfection methods, especially as utilities consider alternatives to chlorine that might generate different DBP profiles 1 .
Understanding how HAAs and HANs affect cells requires sophisticated laboratory techniques and specialized materials. Here's a look at the key tools and methods scientists use:
This analytical technique is the "gold standard" for detecting and measuring HAAs in water samples. Methods like EPA 552.2 and 552.3 use GC-ECD after extracting HAAs with methyl tert-butyl ether (MTBE) and derivatizing them with acidic methanol 2 7 .
EPA Method 557 uses this advanced technique, which allows for direct injection of samples without extensive pretreatment. This method offers higher sensitivity and avoids the need for derivatization 7 .
To assess DBP formation risk, scientists conduct Haloacetic Acid Formation Potential (HAAFP) tests. Water samples are dosed with chlorine and incubated for 7 days at 25°C in the dark, then analyzed for HAA content 2 .
The Comet assay (single cell gel electrophoresis) is widely used to detect DNA damage. This sensitive method can detect strand breaks at the individual cell level by measuring DNA migration in an electric field 1 .
The evidence is clear: disinfection byproducts like HAAs and HANs can damage mammalian cells at multiple levels, from impairing basic cell functions to causing DNA damage that may lead to cancer 1 4 5 . This creates a complex challenge for water utilities tasked with balancing microbial safety against chemical risks.
Advanced treatment processes can remove natural organic matter before disinfection, reducing DBP formation 4 .
Using ozone, chloramines, or other disinfectants can alter DBP profiles, though they may create different byproducts 1 .
Reverse osmosis or activated charcoal filtration at the tap can effectively remove most disinfection byproducts, including HAAs 4 .
As scientists identify more toxic DBPs, regulatory agencies may need to expand monitoring and regulation to include additional compounds beyond the current HAA5 7 .
Ongoing research continues to investigate less familiar byproducts and their health effects, particularly focusing on iodinated compounds that appear to be more toxic despite typically occurring at lower concentrations 1 8 . This work will be crucial for developing safer disinfection practices that protect against both microbial threats and chemical hazards.
As consumers, we can stay informed about our local water quality by reviewing annual water quality reports and considering appropriate filtration methods if concerned about DBP levels. The solution isn't to abandon disinfection—which remains one of the most important public health advances in history—but to continue refining the process to make our drinking water as safe as possible from all threats, both biological and chemical.