1. (a) For purposes of this section, the following terms have the following meanings:

   1. (1) AGENCY. The term as defined in Section 41-22-3.

   2. (2) BEST AVAILABLE SCIENCE. Science that:

      1. a. Is reliable, unbiased, subject to independent verification, and applies to the agency’s rule;
      2. b. Maximizes the quality, objectivity, relevance, completeness, and integrity of information, including statistical information; human, animal, and other relevant scientific studies; and, if applicable, human health risk-based assessments; and

      3. c. Involves the use of scientifically defensible and quality-assured supporting studies conducted in accordance with generally accepted scientific or technical practices utilizing data collected by generally accepted methods or best available methods and that are:

         1. 1. Site-specific studies, including area-wide or statewide studies;
         2. 2. Studies published in a refereed journal; or
         3. 3. External peer-reviewed studies contained in a federal government report published for a purpose other than development of a rule.

   3. (3) GENERALLY ACCEPTED SCIENTIFIC OR TECHNICAL PRACTICES. Scientific methods, principles, or protocols that:

      1. a. Are broadly acknowledged and routinely applied by the relevant scientific community;
      2. b. Are consistent with refereed journal literature or established technical standards; and
      3. c. Have been tested for reliability and validity.

   4. (4) MANIFEST BODILY HARM. A physical disease or injury that is:

      1. a. Presently existing and diagnosable;
      2. b. Not based solely on the presence or detection of a substance in the human body; and
      3. c. Not based solely on an increased risk of disease.

   5. (5) REFEREED JOURNAL. A publication that:

      1. a. Uses an editorial board or critical review panel of subject matter experts in the relevant scientific or technical disciplines who critically and objectively assess the methodology and analysis of submitted scientific studies in a nonpartisan fashion and provides editorial services prior to publication; and
      2. b. Takes meaningful steps to avoid biases in the publication’s scientific review process.
   6. (6) WEIGHT OF SCIENTIFIC EVIDENCE. An approach to scientific evaluation in which each piece of relevant information is considered based on its quality and relevance, which, at a minimum, includes consideration of study design, fitness for purpose, replicability, peer review, and transparency and reliability of data; and the information is transparently integrated with other relevant information to inform the scientific evaluation prior to making a judgment about the scientific evaluation.
2. (b) An agency may not adopt a new rule or amend an existing rule that establishes new or changes existing numeric criteria or numeric limitations applicable to a chemical substance, mixture, contaminant, pollutant, hazardous substance, solid waste, hazardous constituent, or hazardous waste that relates to drinking water, water pollution control, hazardous substances, contaminated site remediation, air quality, solid waste handling, or hazardous waste handling if the new rule or amendment would result in criteria or limitations that are more stringent or extensive in scope, coverage, or effect than any federal law or regulation setting a standard regarding the same or a substantially similar topic.
3. (c) Notwithstanding subsection (b), if there is no federal law or regulation establishing a standard regarding the same or a substantially similar topic, an agency may not adopt a new rule or amend an existing rule that establishes numeric criteria or numeric limitations applicable to a chemical substance, mixture, contaminant, pollutant, hazardous substance, solid waste, hazardous constituent, or hazardous waste that relates to drinking water, water pollution control, hazardous substances, contaminated site remediation, air quality, solid waste handling, or hazardous waste handling unless, for each individual chemical substance, mixture, contaminant, pollutant, hazardous substance, solid waste, hazardous constituent, or hazardous waste to which the rule applies, the scientific and technical information relied on to support the standard established by the rule is based on the best available science and the weight of scientific evidence.

4. (d)

   1. (1) For any rule adopted pursuant to subsection (c) to protect human health, safety, or welfare, the best available science and the weight of scientific evidence shall establish a direct causal link between exposure at or above the numeric criteria or numeric limitations provided by the rule and manifest bodily harm in humans, based on generally accepted scientific or technical practices.
   2. (2) In the absence of data from voluntary scientific studies on humans, best available science and the weight of scientific evidence may be based on tests performed on experimental animal species or human and animal cells establishing a direct causal link, based on generally accepted scientific or technical practices, between exposure at or above the numeric criteria or numeric limitations provided by the rule and manifest bodily harm in humans, provided that the harm may be extrapolated to humans based on the best available science and the weight of scientific evidence.

5. (e)

   1. (1) No agency shall establish as the default or be required to use values from the United States Environmental Protection Agency’s Integrated Risk Information System in the development of numeric water quality criteria.
   2. (2) An agency that has adopted a rule establishing use of the United States Environmental Protection Agency’s Integrated Risk Information System as the default in any numeric water quality criteria shall revise all rules to comply with this subsection not later than November 19, 2026.

6. (f) This section does not apply to any of the following:

   1. (1) A rule that is required by federal law or regulation, including a rule adopted to comply with, and in furtherance of, federal agency authorization or primacy requirements, so long as an agency does not have discretion or flexibility under some or all of the federal requirements or authorization.
   2. (2) A rule that is less stringent or extensive in scope, coverage, or effect than any federal law or regulation setting a standard regarding the same or a substantially similar topic or that is substantively equivalent to a federal law or regulation.
   3. (3) A rule that would repeal or modify an existing rule to be less stringent or extensive in scope, coverage, or effect.
   4. (4) An emergency rule adopted pursuant to Section 41-22-5.

(Act 2026-81, §1.)