ATSDR Updates Chemical Mixtures Interaction Profiles: Why They Matter For Risk Assessment And Regulation

The U.S. Agency for Toxic Substances and Disease Registry (ATSDR) recently released two new Interaction Profiles for Toxic Chemical Mixtures, continuing a long-running federal effort to better understand the health effects of exposures to combinations of hazardous substances rather than individual chemicals in isolation.

The two new profiles evaluate mixtures commonly encountered in environmental and indoor air contexts:

• Chloroform, 1,1-dichloroethylene (1,1-DCE), trichloroethylene (TCE), and vinyl chloride; and

• Carbon monoxide, formaldehyde, methylene chloride, nitrogen dioxide, and tetrachloroethylene.

While the profiles themselves do not impose regulatory requirements, they provide toxicological analyses that federal and state agencies may use when evaluating health risks and developing exposure guidelines. The release continues ATSDR's work under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) to assess priority chemical mixtures frequently encountered at hazardous waste sites and in other environmental settings.

ATSDR's interaction profiles are part of a broader federal initiative to address a longstanding challenge in environmental health science by assessing real-world exposure to multiple substances. Most regulatory frameworks evaluate chemicals one at a time, such as under the Toxic Substances Control Act (TSCA), even though real-world exposures generally occur to more than one substance simultaneously. Understanding the risks of our total exposure to the environmental chemical milieu is an enormous challenge because of the variability inherent to mixtures in the environment and responses to those exposures by biological systems. To address this gap, ATSDR established its chemical mixtures program in the 1990s under its CERCLA mandate leveraging the U.S. Environmental Protection Agency's (EPA) mixtures guidance as a foundational reference. The program develops interaction profiles for combinations of chemicals that have been characterized as commonly co-occurring in environmental media such as groundwater, soil vapor, and indoor air.

Each interaction profile compiles toxicological evidence to determine how chemicals in a mixture may interact biologically. These interactions may include:

• Additivity, where combined effects equal the sum of individual effects;

• Synergism, where the combined effect exceeds the expected additive effect; or

• Antagonism, where the combined toxicity is less than expected.

The profiles typically include quantitative parameters such as Minimal Risk Levels (MRL) and Target-Organ Toxicity Doses (TTD) that risk assessors can use to estimate cumulative health risks at sites that have similar environmental chemical mixture profiles. Interaction profiles can then be considered, “along with biomedical judgment, community-specific health outcome data, and community health concerns, to determine the health impacts and public health actions for a site contaminated with multiple chemicals or other stressors of concern.”

ATSDR released 14 interaction profiles in 2024, and the two newly issued profiles continue ATSDR's effort to expand its mixture toxicology database.

Chlorinated Solvent Mixture

The first new profile examines a mixture consisting of:

• Chloroform;

• 1,1-dichloroethylene (1,1-DCE);

• Trichloroethylene (TCE); and

• Vinyl chloride.

ATSDR selected this mixture because its components frequently co-occur in groundwater and drinking water near hazardous waste sites. ATSDR identified inhalation and oral exposure over intermediate or chronic durations as the most relevant exposure pathways. Because no studies evaluated the mixture directly, ATSDR relied on component-based approaches that assume additivity when assessing potential risks. Through weight-of-evidence analysis, ATSDR identified a likely shared mechanism of joint toxicity involving the cytochrome P450 2E1 (CYP2E1) liver enzyme, which is highly conserved in mammals and metabolizes several chlorinated solvents.

The profile identified several non-cancer endpoints of concern, including impacts on:

• The liver;

• Developmental processes;

• The kidneys;

• The nervous system;

• The immune system; and

• The respiratory system.

Three components of the mixture — chloroform, TCE, and vinyl chloride — are also associated with cancer risks.

Indoor Air Mixture

The second profile evaluates a mixture of:

• Carbon monoxide;

• Formaldehyde;

• Methylene chloride (dichloromethane);

• Nitrogen dioxide; and

• Tetrachloroethylene (perchloroethylene).

ATSDR selected this mixture based on the potential for co-exposure in indoor air environments, where concentrations of these chemicals may exceed those measured outdoors. As with the chlorinated solvent mixture, ATSDR found no studies evaluating the combined mixture directly and therefore applied component-based approaches to estimate risk. ATSDR found no single endpoint common to all components, but several overlapping health effects were identified, including impacts on:

• Blood and oxygen transport;

• Respiratory function;

• The nervous system; and

• The liver.

Some components of the mixture are also associated with cancer.

ATSDR concluded that available data are insufficient to characterize the precise modes of joint action for many of the chemicals. In the absence of mixture-specific evidence, ATSDR concluded that dose additivity assumptions remain appropriate for protective risk assessment, given the overlap in toxicological targets across the components.

Of the sixteen interaction profiles published to ATSDR's website, the majority find that additivity, and therefore a component-based approach, is an appropriate interaction assumption for the specific mixtures assessed. There are exceptions to the additivity mechanism across ATSDR's interaction profiles, including synergism due to neurotoxicity (e.g., cholinesterase activity) or mutagenic potential and antagonism due to competitive binding for specific mixtures.

Although the interaction profiles themselves do not establish regulatory requirements, they illustrate a broader scientific and policy challenge: regulatory programs generally evaluate chemical risks one substance at a time, while environmental exposures rarely occur in isolation. There are global efforts underway to understand our “exposome” — the totality of exposures to chemicals in the environment throughout our lifetime — so as to characterize and control risk. This tension is particularly relevant in several current regulatory contexts.

For example, several of the chemicals evaluated in the new profiles — including trichloroethylene, methylene chloride, and perchloroethylene — are the subject of ongoing or recent regulatory activity under TSCA. TSCA risk evaluations typically focus on individual chemicals and specific conditions of use, rather than cumulative exposure to multiple substances with overlapping toxicological effects. There are exceptions to this, including the recent bolus of phthalate final risk evaluations and the supporting cumulative risk assessment for exposure to six phthalates.

Federal and state agencies are increasingly considering cumulative risk assessment frameworks, particularly when evaluating exposures near hazardous waste sites, in indoor air environments, or in communities experiencing multiple sources of chemical exposure. ATSDR's interaction profiles provide one of the few systematic federal efforts to evaluate these mixture effects and to develop practical approaches for assessing cumulative toxicity. A critical application of the ATSDR interaction profile effort outside of CERCLA may be as a model for the target-organ toxicity approach, where apical endpoints (e.g., liver toxicity) drive the hazard index approach. A key factor in assessing cumulative risk is ensuring that the basis of shared hazard is established using appropriate, comparable biological endpoints.

In the European Union, a mixture assessment factor (MAF) has been proposed for inclusion in Regulation (EC) No 1907/2006 under the Chemicals Strategy for Sustainability within the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH). Under the proposal, a default factor would be applied to single substance safety assessments, as a pragmatic means for protecting against combined exposure to chemicals from different sources without identifying or characterizing interactions among particular substances. In practice, if applying the MAF leads to a conclusion that a condition of use is not safe, additional risk management would be required. This indiscriminatory approach is counter to the ATSDR approach which focuses on specific mixtures that commonly co-occur and evaluating evidence for joint toxic action.

ATSDR's continued work on interaction profiles highlights an issue that is receiving increasing attention among regulators worldwide: people are exposed to mixtures of chemicals, yet regulatory frameworks have traditionally evaluated substances individually. As governments explore ways to address cumulative exposures, approaches to chemical mixtures are evolving and vary significantly across jurisdictions. In this context, ATSDR's interaction profiles serve as an important reminder that advancing the science of mixture toxicology is essential to informing sound policy. As regulators consider how best to address potential risks from chemical mixtures, stakeholders should continue to advocate for decisions grounded in robust scientific evidence and transparent risk assessment methodologies, rather than precautionary assumptions that may not accurately reflect real-world exposures or toxicological interactions.

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