What Is Chlorate?

Chlorate is the anionic form of chloric acid, represented as ClO₃^–. In drinking water, it is mainly discussed as an oxychlorine disinfection byproduct rather than as a naturally occurring mineral contaminant. It is not the same as chloride, the common salt-related ion, and it is also distinct from chlorite, another oxychlorine byproduct associated especially with chlorine dioxide. Chlorate is highly soluble, chemically persistent under normal finished-water conditions, and difficult to remove once it has already formed.

In public water systems, chlorate is most often linked to the use, storage, and aging of sodium hypochlorite solution, the liquid bleach commonly used for drinking water disinfection. Hypochlorite slowly decomposes to chlorate, and this decomposition is accelerated by high temperature, long storage time, high solution strength, exposure to light, and contamination with metals. As a result, chlorate can enter water not only because of reactions occurring in the pipe network, but also because it is already present in the disinfectant chemical before that chemical is dosed.

Chlorate may also be associated with chlorine dioxide treatment. Chlorine dioxide can form both chlorite and chlorate depending on generation efficiency, feed chemical purity, reaction conditions, and distribution system chemistry. In some treatment configurations, ozonation and advanced oxidation processes can also influence oxychlorine byproduct formation, especially when chlorine-containing species are present.

The public health importance of chlorate comes from its toxicity profile and the fact that it is not easily controlled by simple household carbon filters after formation. The best control strategy is usually upstream: preventing formation, minimizing chlorate in disinfectant stock, optimizing disinfectant dose, controlling contact time and pH where relevant, and using treatment steps such as activated carbon or enhanced organic matter removal to reduce precursor demand and unnecessary oxidant exposure.

Scientific Identity

Chlorate is an oxychlorine anion containing chlorine in a high oxidation state. Its formula is ClO₃^–, and in water it is generally present as a dissolved inorganic ion paired with cations such as sodium, potassium, calcium, or magnesium. Because drinking water pH is far above the acidity at which chloric acid would dominate, the relevant form in distribution water is the chlorate ion rather than undissociated chloric acid.

Chemically, chlorate is more oxidized than chlorite, ClO₂^–, but less oxidized than perchlorate, ClO₄^–. This family relationship matters because these compounds can be discussed together in regulatory and toxicological assessments, but they do not behave identically. Chlorate is not volatile, so it does not transfer readily from water to indoor air during showering. It is also not hydrophobic, so it is not well removed by simple adsorption onto conventional granular activated carbon in the same way that many organic disinfection byproducts can be.

Chlorate is typically measured as an inorganic anion in the microgram-per-liter to milligram-per-liter range. Its concentration can reflect a combination of source-water chemistry, disinfectant choice, chemical storage practices, treatment plant operations, and distribution system residence time. Because it is stable enough to persist after formation, a finished-water sample may represent both chlorate introduced from chemical feed solutions and chlorate formed during treatment.

How Chlorate Enters Drinking Water

The most important pathway for chlorate in many drinking water systems is degradation of sodium hypochlorite. Commercial hypochlorite solutions are not static chemicals; they decompose during storage. Higher-strength bleach, warm storage rooms, long storage periods, sunlight exposure, and transition-metal contamination can significantly increase chlorate formation in the chemical tank. When the hypochlorite is later pumped into the treatment stream, the chlorate impurity is dosed directly into finished water.

Chlorine dioxide treatment is another major pathway. Chlorine dioxide is used by some systems to control taste and odor, oxidize iron and manganese, reduce some algal metabolites, or manage certain disinfection byproduct tradeoffs. However, chlorine dioxide generation and decay can produce chlorite and chlorate. Poor generator efficiency, incorrect chemical ratios, high residuals, or inadequate process control can shift byproduct formation upward.

Chlorate may also form when disinfectants react with natural organic matter, reduced inorganic species, or other oxidant-demanding substances. While chlorate is not one of the classic carbon-based disinfection byproducts such as trihalomethanes or haloacetic acids, its occurrence is still strongly tied to oxidant management. High disinfectant doses used to overcome dirty filters, high organic carbon, ammonia, biofilm demand, or long distribution residence times can indirectly increase chlorate exposure.

In distribution systems, chlorate levels can be influenced by booster chlorination, storage tanks, long dead-end mains, and seasonal temperature. Warm water and extended residence time increase disinfectant decay and can encourage utilities to add more disinfectant to maintain a residual. If that disinfectant is aged hypochlorite or if operational control is poor, chlorate can increase at downstream taps.

Occurrence and Exposure

People are exposed to chlorate primarily by drinking and preparing food or infant formula with treated tap water that contains chlorate. Because chlorate is not volatile, inhalation during bathing is generally a much less important route than ingestion. Boiling water is not an effective control measure; it can concentrate nonvolatile dissolved ions if water volume is reduced.

Chlorate occurrence is most often associated with disinfected public water supplies rather than pristine untreated groundwater. Small and medium-sized systems can be vulnerable when they buy hypochlorite in bulk but use it slowly, especially in warm climates or during low-demand seasons. Remote facilities, schools, campgrounds, and seasonal systems may also experience higher risk if disinfectant stock is stored for long periods.

Large utilities can also have chlorate concerns, particularly where hypochlorite is generated or stored onsite, where chlorine dioxide is used, or where distribution system retention times are long. In some systems, chlorate varies seasonally, with higher levels during summer because both disinfectant decomposition and water age problems can worsen at higher temperature.

Exposure is not limited to tap water alone. Chlorate can be present in foods and beverages when chlorinated water is used in processing, washing, irrigation, or sanitation. However, a drinking water profile focuses on the direct contribution from finished water and the operational choices that utilities can control. For sensitive groups, including infants consuming formula made with tap water, the drinking water contribution can be especially relevant because intake per body weight is higher.

Health Effects and Risk

Chlorate is considered a health-relevant disinfection byproduct because it can affect oxygen-carrying blood chemistry and thyroid-related physiology at sufficient exposure levels. Toxicological concerns include oxidative effects on red blood cells, including potential formation of methemoglobin, and interference with iodide uptake in the thyroid. These mechanisms are important because oxygen transport and thyroid hormone production are critical for normal development and metabolism.

Infants, pregnant people, individuals with anemia or glucose-6-phosphate dehydrogenase deficiency, and people with low iodine intake may be more sensitive to effects associated with chlorate exposure. The thyroid-related concern is not identical to perchlorate, but both compounds are often discussed in relation to iodide uptake and thyroid hormone homeostasis. Adequate dietary iodine can reduce vulnerability to thyroid-disrupting anions, but it does not eliminate the need for water system control.

Short-term high exposure is generally more concerning for blood effects, while repeated exposure may raise concern for thyroid-related endpoints. Drinking water levels encountered in regulated systems are usually far below concentrations associated with acute poisoning, but chronic low-level exposure matters because water is consumed every day and because chlorate can occur together with other disinfection byproducts.

Risk evaluation should consider both concentration and context. A chlorate detection in a single sample may reflect an operational problem such as aged hypochlorite rather than a permanent source-water issue. Conversely, repeated detections across multiple sampling locations may indicate a systemwide disinfectant management problem. The best response is not to stop disinfection; microbial pathogens pose an immediate and serious risk. The correct public health approach is to maintain effective disinfection while reducing avoidable chlorate formation.

Testing and Monitoring

Chlorate testing requires laboratory analysis; it cannot be reliably identified by taste, odor, color, or standard household chlorine test strips. The most common analytical approach is ion chromatography, often used for oxyhalide anions such as chlorite, chlorate, bromate, and sometimes perchlorate. Laboratories may use EPA, ISO, Standard Methods, or other validated procedures depending on the jurisdiction and monitoring program.

Sampling should be designed around how chlorate forms. For a system using sodium hypochlorite, useful sampling locations include the finished water leaving the plant, storage tank outlets, remote distribution points, and locations after booster chlorination. Testing the hypochlorite stock solution itself can be extremely informative because it can reveal whether chlorate is being introduced before the disinfectant reaches the water.

For systems using chlorine dioxide, monitoring should include both chlorite and chlorate because generator efficiency and decay chemistry can shift the balance between the two. Operational parameters such as chlorine dioxide dose, residual, pH, temperature, contact time, and feed chemical ratios should be reviewed alongside laboratory results. A single chlorate number without operational context is often insufficient to identify the cause.

Homeowners on private wells generally do not encounter chlorate unless they add chlorine-based disinfectants, have a treatment device using oxidants, or store treated water. For municipal customers, the most relevant information usually comes from utility monitoring data, consumer confidence reports where applicable, or special disinfection byproduct testing. If a household has a medical reason to minimize chlorate exposure, laboratory testing of tap water is preferable to relying on filter claims.

Treatment Methods

Chlorate treatment is best understood as a formation-control problem. Once chlorate is present as a dissolved inorganic anion, it is much harder to remove than chlorine, many organic disinfection byproducts, or taste-and-odor compounds. The most effective approach is usually to prevent chlorate from forming in disinfectant stock and to reduce unnecessary oxidant demand in the treatment process.

Activated carbon deserves careful interpretation for chlorate. It can be an important part of a chlorate control strategy when used upstream to remove organic matter, reduce chlorine demand, dechlorinate process streams, or support biologically active filtration that stabilizes finished water. It may also reduce the need for high disinfectant doses by improving overall treatment performance. However, chlorate itself is a small, highly soluble inorganic anion, so conventional carbon adsorption is not a dependable direct removal mechanism. A carbon filter installed after chlorate has already formed may leave much of the chlorate in the water.

Point-of-use treatment can be appropriate when a household wants an added barrier for drinking and cooking water, especially if laboratory testing confirms elevated chlorate at the tap. Reverse osmosis is generally more defensible than ordinary carbon for direct reduction. Point-of-entry treatment for chlorate is less common because whole-house removal of a nonvolatile ingestion contaminant is expensive and usually unnecessary. For public systems, the most protective and cost-effective solution is almost always utility-level treatment optimization, chemical storage control, and distribution system management.

Regulations and Guidelines

Chlorate regulation varies by country and jurisdiction. In the United States, chlorate has been monitored in national occurrence programs, including unregulated contaminant monitoring, but it does not have a nationwide federal Maximum Contaminant Level under the Safe Drinking Water Act. Utilities may still monitor chlorate as part of special studies, state requirements, treatment changes, or broader disinfection byproduct control efforts.

The World Health Organization has published guideline context for chlorate in drinking water, recognizing it as an oxychlorine disinfection byproduct with health-based relevance. WHO guideline values and health-based values should be checked against the most current edition because updates can occur as toxicological data and exposure assumptions are revised.

Some national and regional drinking water frameworks include specific values or operational requirements for chlorate, particularly where chlorine dioxide or hypochlorite is widely used. The European Union drinking water framework includes chlorate and chlorite in its parametric approach, but implementation details, transitional provisions, and enforcement practices can vary among member states. Canada, Australia, and individual states or provinces may use their own health-based values, aesthetic considerations, or operational guidance.

Because legal limits vary, consumers should not assume that a chlorate result is interpreted the same way everywhere. A water result should be compared with the applicable local standard, health advisory, or guideline used by the responsible authority. In all jurisdictions, chlorate control must be balanced with microbial safety. Reducing disinfectant so aggressively that pathogens survive would create a more immediate public health hazard than properly managed disinfection byproducts.

Related Contaminants

Frequently Asked Questions

Is chlorate the same as chlorine in drinking water?

No. Chlorine is an active disinfectant residual used to kill or control microbes. Chlorate is an oxychlorine byproduct that can form from disinfectant decomposition or treatment reactions. A water sample can have low free chlorine but still contain chlorate if the chlorate was introduced from aged hypochlorite or formed earlier in treatment.

Can a refrigerator or pitcher carbon filter remove chlorate?

Usually not reliably. Carbon filters can reduce chlorine taste and some organic chemicals, but chlorate is a dissolved inorganic anion that is not strongly adsorbed by ordinary activated carbon. A filter should only be relied on for chlorate if it has specific certified performance data for chlorate reduction.

Does boiling water remove chlorate?

No. Chlorate is nonvolatile and remains in the water during boiling. If boiling reduces the water volume, the chlorate concentration can increase slightly. Boiling is useful for microbial emergencies when advised by authorities, but it is not a chlorate treatment method.

Why would chlorate be higher in summer?

Warm temperatures accelerate hypochlorite decomposition in storage tanks and can increase disinfectant decay in distribution systems. Utilities may also need more disinfectant during warm periods to maintain residuals. These factors can raise chlorate concentrations if chemical age, dose, and water residence time are not well controlled.

Should a utility stop disinfecting water if chlorate is detected?

No. Disinfection prevents waterborne disease and must be maintained. The proper response is to optimize treatment: use fresher hypochlorite, improve storage conditions, adjust disinfectant dose, manage chlorine dioxide generation if used, reduce organic matter and oxidant demand, and control distribution system water age.