What Is Iodoacetic Acid?

Iodoacetic acid is an iodinated disinfection byproduct in the haloacetic acid family. It is not normally added to drinking water intentionally. Instead, it can form during water treatment when disinfectants such as chlorine, chloramine, or ozone-derived oxidants react with naturally occurring iodide and organic matter in source water. In drinking water chemistry, it is often discussed as an “iodo-HAA,” a smaller and more reactive subset of haloacetic acids than the regulated chlorinated and brominated haloacetic acids commonly reported as HAA5 or HAA9.

Iodoacetic acid is important because iodinated disinfection byproducts have repeatedly shown high biological reactivity in laboratory toxicity studies compared with many chlorinated DBPs. Among iodinated DBPs, iodoacetic acid is frequently highlighted because it can act as an alkylating agent, meaning it can react with biological molecules such as proteins and DNA under experimental conditions. This does not mean that every water system containing trace iodoacetic acid produces an immediate health effect, but it does make the compound a high-priority research and treatment-optimization concern.

In finished drinking water, iodoacetic acid is usually expected at trace levels, often in the nanogram-per-liter to low microgram-per-liter range where it is detected. Occurrence is strongly site-specific. Waters with elevated iodide, high natural organic matter, certain algal or wastewater-impacted precursors, and chloramination practices may be more favorable for formation. Because it is not included in many routine compliance DBP panels, iodoacetic acid can be missed unless a utility or investigator requests specialized iodinated DBP analysis.

Scientific Identity

Iodoacetic acid is a small organoiodine acid with the molecular formula C₂H₃IO₂ and CAS number 64-69-7. Its IUPAC name is 2-iodoacetic acid. Structurally, it is acetic acid in which one hydrogen on the methyl carbon has been replaced by iodine, producing ICH₂COOH. This carbon-iodine bond is central to its reactivity: iodine is a good leaving group, which makes the molecule capable of reacting with nucleophilic sites in biological and environmental organic matter.

At typical drinking water pH, iodoacetic acid is largely present as the iodoacetate anion rather than as the fully protonated acid. Its pKa is near the acidic range, so at pH 6.5 to 8.5 it behaves as a charged, polar, water-soluble compound. This matters for treatment: it is not a volatile contaminant like trihalomethanes, and it is not efficiently removed by aeration. Its polarity also makes it harder to remove by simple adsorption than larger, more hydrophobic organic chemicals.

Iodoacetic acid is a chemical contaminant, not a microorganism and not a radionuclide. Its presence reflects a chemical reaction history in the water: iodide in the source water was oxidized to reactive iodine species, and those species reacted with dissolved organic precursors. The compound is therefore best managed by controlling precursor chemistry, disinfectant conditions, and distribution-system reactions rather than by treating it as a naturally occurring raw-water contaminant.

How Iodoacetic Acid Enters Drinking Water

Iodoacetic acid enters drinking water primarily by forming during disinfection. Many source waters contain small amounts of iodide from seawater intrusion, coastal aquifers, brines, certain geologic formations, industrial discharges, oil and gas produced water, wastewater influence, or natural mineral dissolution. Iodide itself is not iodoacetic acid. The key step is oxidation: chlorine, ozone, and other oxidants can convert iodide into reactive iodine species such as hypoiodous acid, which can then iodinate natural organic matter.

Natural organic matter provides the carbon backbone for iodoacetic acid formation. Humic substances, algal organic matter, wastewater-derived organic nitrogen compounds, amino acids, and other low-molecular-weight organic precursors can participate in reactions that produce haloacetic acids. The specific yield of iodoacetic acid depends on source-water iodide concentration, dissolved organic carbon character, pH, disinfectant type, disinfectant dose, contact time, ammonia conditions, and whether the system uses free chlorine or chloramine.

Chloramination can be especially relevant for iodinated DBPs. Chloramine is generally less powerful than free chlorine in destroying some DBP precursors, and it can allow reactive iodine intermediates to persist long enough to form iodinated organic byproducts. Ozonation can complicate the picture: ozone may oxidize iodide toward iodate, which is less likely to form iodinated organic DBPs, but intermediate iodine species can still contribute to iodinated DBP formation, especially if ozonation is followed by chloramination or if organic precursors remain.

Distribution systems also matter. Iodoacetic acid can form after water leaves the treatment plant if residual disinfectant, iodide or reactive iodine, and organic precursors remain in contact. Long water age, warm temperatures, dead-end mains, storage tanks, and changes in disinfectant residual can increase opportunities for continued DBP formation. Therefore, a finished-water sample at the plant may not always represent the maximum concentration experienced by consumers at distant points in the distribution network.

Occurrence and Exposure

Iodoacetic acid has been reported in research surveys of disinfected drinking water, but it is less commonly monitored than regulated haloacetic acids. It is most likely to be investigated in systems with known iodide in the source water, use of chloramine, coastal or estuarine influence, wastewater impact, or unusual iodinated DBP occurrence. Concentrations, when found, are typically much lower than the total concentration of regulated HAA5, but toxicity concerns are not based solely on concentration; molecular potency is a major reason iodoacetic acid receives attention.

Exposure is mainly through ingestion of drinking water and beverages prepared with that water. Because iodoacetic acid is polar and nonvolatile, inhalation during showering is expected to be much less important than it is for volatile trihalomethanes. Dermal uptake is also generally considered less significant than ingestion, although the full exposure picture depends on concentration, water use, and individual behavior. Boiling is not a reliable solution because the compound is not removed like a volatile gas, and boiling can concentrate nonvolatile dissolved substances as water evaporates.

Private wells that are not disinfected generally do not form iodoacetic acid unless the homeowner or a treatment system applies an oxidizing disinfectant. However, wells with iodide and organic matter can become susceptible if chlorination, shock chlorination, ozone, or other oxidizing treatment is installed. Public water systems are more likely to encounter iodoacetic acid because they maintain disinfectant residuals to protect against microbial pathogens throughout the distribution system.

Health Effects and Risk

Iodoacetic acid is classified here as a high-risk disinfection byproduct because of its strong biological activity in toxicology research, not because a universal drinking water limit has been established. Laboratory studies have identified iodoacetic acid as one of the more cytotoxic and genotoxic haloacetic acids tested in mammalian cell systems. Its chemical structure allows it to react with cellular nucleophiles, and this alkylating behavior is a plausible basis for concern about DNA damage and disruption of normal cellular function.

Public-health interpretation requires caution. Most available evidence for iodoacetic acid is from laboratory and mechanistic studies, while epidemiological studies usually evaluate mixtures of DBPs rather than one compound at a time. People are exposed to complex DBP mixtures that may include haloacetic acids, trihalomethanes, haloacetonitriles, nitrosamines, and iodinated trihalomethanes. The health relevance of iodoacetic acid depends on its concentration, mixture context, exposure duration, and the susceptibility of the exposed population.

The main concern is chronic exposure to trace levels in disinfected water, particularly where iodinated DBP formation is favored. Sensitive groups may include pregnant people, infants, people with compromised health, and communities served by systems with high organic matter and iodide. However, microbial safety remains the first priority: reducing disinfectant to avoid DBPs without maintaining pathogen control can create a much more immediate health hazard. The goal is not to eliminate disinfection, but to optimize treatment so that pathogen protection is maintained while iodoacetic acid and related DBPs are minimized.

Testing and Monitoring

Iodoacetic acid is not reliably captured by every routine haloacetic acid compliance test. Standard regulatory monitoring in many jurisdictions focuses on HAA5, HAA9, or other defined groups that may not include iodoacetic acid. Utilities or researchers seeking iodoacetic acid data should request a laboratory method specifically validated for iodinated haloacetic acids. Analytical approaches may include liquid chromatography with tandem mass spectrometry, ion chromatography-mass spectrometry, or gas chromatography methods after derivatization, depending on laboratory capability.

Sampling requires careful handling because haloacetic acids and iodinated DBPs can change after collection if disinfectant residual remains active. Samples are commonly quenched with an appropriate reagent, stored cold, protected from excessive holding times, and shipped promptly to the laboratory. The exact preservative and protocol should match the analytical method because improper quenching can either allow additional formation or degrade target compounds.

For a water utility, useful monitoring pairs iodoacetic acid with supporting water-quality indicators: iodide, iodate, total organic carbon, dissolved organic carbon character, UV254, bromide, ammonia, pH, temperature, disinfectant residual, water age, and conventional DBP groups. Single-point testing can miss seasonal peaks. Higher-risk periods may include warmer months, algal events, source-water changes, drought concentration of salts, seawater intrusion, or switching from free chlorine to chloramine.

Treatment Methods

Treatment for iodoacetic acid is most effective when it prevents formation rather than trying to remove the finished byproduct after it appears. Because iodoacetic acid is small, polar, and usually ionized at drinking water pH, it can be difficult to remove by simple household treatment unless the device is specifically designed and tested for haloacetic acid reduction. Utilities generally manage it through precursor removal, disinfectant strategy, and distribution-system control.

Activated carbon is best viewed as both a treatment and prevention tool. At a municipal scale, GAC or BAC ahead of final disinfection can remove the organic matter that would otherwise react with iodine species. This approach is often more reliable than trying to adsorb iodoacetic acid after it has formed. At the household scale, point-of-use carbon may be appropriate for drinking and cooking water when a product has credible performance data for haloacetic acids or relevant DBP reduction. Point-of-entry treatment is more complex and usually unnecessary unless there is a documented whole-house DBP issue; it should be designed to avoid disinfectant removal that encourages microbial regrowth in plumbing.

Treatment optimization is essential because actions that reduce one DBP class may increase another. For example, switching from free chlorine to chloramine can lower trihalomethanes in some systems but may increase iodinated DBP or nitrosamine concerns if iodide, ammonia, and organic precursors are present. Ozone can reduce some precursor problems but may create bromate in bromide-containing waters and may influence iodine chemistry. The best approach is site-specific: characterize source water, remove organic precursors early, maintain adequate but not excessive disinfectant exposure, limit water age, and monitor multiple DBP classes rather than relying on HAA5 alone.

Regulations and Guidelines

Iodoacetic acid is not commonly regulated as an individual drinking water contaminant with a numeric maximum contaminant level. In the United States, federal DBP compliance under the Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules includes limits for total trihalomethanes and HAA5, but iodoacetic acid is not one of the five HAA5 compounds. Some broader monitoring programs and research projects evaluate additional unregulated DBPs, but routine compliance data may not report iodoacetic acid unless specifically required or voluntarily tested.

The World Health Organization and national authorities provide guidance for many disinfectants and DBPs, but guideline values for iodinated haloacetic acids are less developed than for regulated trihalomethanes and common haloacetic acids. Where iodoacetic acid is addressed, it is often in the context of emerging DBP research, occurrence studies, or treatment optimization rather than a standalone enforceable standard. Local or national requirements can vary, especially in jurisdictions that monitor expanded DBP lists or have special rules for systems affected by high iodide, desalinated water blending, or chloramination.

Because limits vary by country or jurisdiction and may not exist for iodoacetic acid specifically, consumers should not assume that a compliant HAA5 result means iodinated DBPs are absent. Compliance with existing DBP rules remains important, but iodoacetic acid risk assessment requires targeted testing and expert interpretation. Utilities concerned about iodoacetic acid typically evaluate it as part of a broader DBP control strategy that includes iodide chemistry, natural organic matter removal, disinfectant selection, and distribution-system water age.