Disinfection Byproducts in Tap Water: What THMs and HAAs Are and Why They Exist

Most people assume that once water is treated, it is chemically neutral. In reality, water treatment is a process of tradeoffs. The same methods that make water microbiologically safe can also create new chemical compounds along the way.

Disinfection byproducts are one of the clearest examples of this dynamic. They are not contaminants that enter water from pollution or infrastructure failure. They are formed during treatment itself, as a result of deliberate decisions made to control pathogens and prevent infectious disease.

Understanding what disinfection byproducts are, how they form, and how exposure occurs helps clarify a common point of confusion in water discussions: safety is not the absence of chemicals, but the management of risk over time.

What Are Disinfection Byproducts in Tap Water (THMs and HAAs)?

Disinfection byproducts in tap water form when disinfectants like chlorine react with naturally occurring organic matter during treatment, creating compounds. Organic matter can include naturally occurring plant material, decaying leaves, or other carbon-based substances found in source water. The two most common types of compounds are:

Trihalomethanes (THMs)

THMs form when chlorine reacts with organic matter in source water. Common examples include chloroform and bromodichloromethane. THMs are volatile, meaning they can move from water into air under certain conditions such as showering or cooking.

Haloacetic Acids (HAAs)

HAAs are another group of byproducts created during disinfection. Unlike THMs, HAAs are less volatile and tend to remain dissolved in water rather than becoming airborne.

Both THMs and HAAs are regulated in public drinking water systems, but regulation does not eliminate their presence.

Why Chlorination Is Still Used

Chlorination has been one of the most effective public health interventions in modern history. Before widespread water disinfection, waterborne diseases such as cholera, typhoid, and dysentery caused widespread illness and death.

Chlorine is used because it:

• Kills bacteria, viruses, and parasites effectively

• Continues to disinfect water as it moves through distribution systems

• Is relatively inexpensive and reliable

The formation of disinfection byproducts is not a failure of treatment. It is a known consequence of using strong disinfectants in water that contains organic material.

This is why discussions about disinfection byproducts must start with context. The goal of treatment is to reduce acute infectious risk, not to eliminate every chemical reaction that occurs along the way.

How People Are Exposed to Disinfection Byproducts

Drinking water is only one exposure pathway.

Drinking and Cooking

THMs and HAAs can be ingested directly through drinking water or indirectly through cooking, especially when water is used frequently over time.

Bathing and Showering

Because some THMs are volatile, they can enter the air during hot showers or baths. In enclosed spaces, inhalation can contribute to overall exposure.

Skin Contact

Certain disinfection byproducts can also be absorbed through the skin during bathing, although this pathway is generally less studied than ingestion or inhalation.

This multi-pathway exposure is one reason water quality cannot be evaluated solely by what is consumed from a glass.

What the Health Research Actually Shows

Research on disinfection byproducts focuses primarily on long-term exposure rather than short-term toxicity.

Studies have explored associations between chronic exposure and:

• Increased cancer risk

• Reproductive and developmental effects

• Potential impacts on liver and kidney function

Importantly, much of this research is observational. Associations do not automatically establish causation, and results vary depending on exposure level, duration, and study design.

What is consistent across the literature is that risk, if present, is related to cumulative exposure over time, not occasional contact.

Why Disinfection Byproducts Are Regulated but Still Present

Disinfection byproducts are regulated because they are known to form during treatment and because long-term exposure has raised concern in epidemiological studies.

However, regulation operates within constraints:

• Source water quality varies widely

• Removing organic matter completely is not feasible

• Stronger disinfection reduces pathogens but can increase byproduct formation

• Weaker disinfection reduces byproducts but increases infection risk

Regulatory limits reflect negotiated tradeoffs between microbial safety and chemical exposure, not an assumption that byproducts are harmless.

How to Think About Risk Without Overreacting

Disinfection byproducts illustrate a broader principle in environmental health: many exposures are real, measurable, and biologically relevant, but not all require the same level of response.

A practical way to think about risk involves:

• Frequency of exposure

• Duration across years or decades

• Contribution relative to other exposures

• Individual sensitivity and life stage

For many people, disinfection byproducts represent a small portion of total chemical exposure. For others, particularly those with high water use or specific vulnerabilities, they may warrant closer attention.

The key is understanding exposure pathways rather than assuming that treatment byproducts are either harmless or catastrophic.

Disinfection Byproducts in the Context of Water Contaminants

Disinfection byproducts are one piece of the broader water exposure picture. Unlike contaminants that enter water through pollution or infrastructure, these compounds exist because water is treated at all.

This distinction matters. It highlights why water quality discussions often involve tradeoffs rather than simple solutions.

For a broader explanation of how contaminants enter water systems and how exposure occurs across drinking, cooking, and bathing, see the Water Contaminants page.

Conclusion: Understanding the Tradeoffs in Water Treatment

Disinfection byproducts exist because water treatment prioritizes microbial safety. They are not accidental pollutants, but predictable chemical outcomes of a process designed to prevent disease.

Understanding what THMs and HAAs are, how exposure happens, and why regulation allows their presence helps replace confusion with context. This does not minimize their relevance. It clarifies how and where they fit within the larger picture of water exposure.

As with most environmental health topics, informed decisions depend less on eliminating every exposure and more on understanding which exposures matter most over time.