EPA must use the best available science − by law − but what does that mean?

Science is essential as the U.S. Environmental Protection Agency carries out its mission to protect human health and the environment.
In fact, laws passed by Congress require the EPA to use the 'best available science' in many decisions about regulations, permits, cleaning up contaminated sites and responding to emergencies.
For example, the Clean Air Act requires the EPA to rely on science for setting emission standards and health-based air quality standards. The Safe Drinking Water Act requires the EPA to consider the best available peer-reviewed science when setting health-based standards. The Clean Water Act requires the agency to develop surface water quality criteria that reflect the latest science. The Toxic Substances Control Act requires the EPA to use the best available science to assess risk of chemicals to human health and the environment.
But what exactly does 'best available science' mean?
That's an important question as the Trump administration launches an effort to roll back clean air and water regulations at the same time it is preparing to replace all the members of two crucial EPA science advisory boards and considering eliminating the Office of Research and Development – the scientific research arm of the EPA.
Some basic definitions for best available science can be found in laws, court rulings and other sources, including the EPA's own policies.
The science must be reliable, unbiased, objective and value-neutral, meaning it is not influenced by personal views. Best available science is the result of the scientific process and hypothesis testing by scientists. And it is based on current knowledge from relevant technical expertise and must be credible.
The EPA's scientific integrity policy includes 'processes and practices to ensure that the best available science is presented to agency decision-makers and informs the agency's work.' Those include processes to ensure data quality and information quality and procedures for independent reviews by scientific experts outside of government.
I have seen the importance of these processes and procedures personally. In addition to being an academic researcher who works on air pollution, I am a former member of the EPA's Science Advisory Board, former chair of the EPA's Clean Air Scientific Advisory Committee, and from 2022 to 2024 served as assistant administrator of the EPA's Office of Research and Development and the EPA science adviser.
The EPA Science Advisory Board plays an important role in ensuring that the EPA uses the best available science. It is tasked with reviewing the scientific and technological basis of EPA actions.
The 1978 Environmental Research, Development, and Demonstration Authorization Act ordered EPA to establish the board. The Science Advisory Board's members must be 'qualified by education, training, and experience to evaluate scientific and technical information on matters referred to the Board.' But those members can be replaced by new administrations, as the Trump administration is planning to do now.
During the first Trump administration, the EPA replaced several independent scientists on its advisory boards in a manner that deviated from established practice, according to the Government Accountability Office, and brought in scientists connected with the industries the EPA regulates. I was one of the independent scientists replaced, and I and others launched an independent review panel to continue to deliver expert advice.
No matter who serves on the EPA's advisory boards, the agency is required by law to follow the best available science. Failing to do so sets the stage for lawsuits.
The same law that established the Science Advisory Board is also a legal basis for the Office of Research and Development, the agency's scientific research arm and the EPA's primary source for gathering and developing the best available science for decision-makers.
During my time at the EPA, the Office of Research and Development's work informed regulatory decisions involving air, water, land and chemicals. It informed enforcement actions, as well as cleanup and emergency response efforts in EPA's regions.
State agencies and tribal nations also look to the EPA for expertise on the best available science, since they typically do not have resources to develop this science themselves.
Federal courts have also ordered the EPA to use the best available science, and they have recognized the importance of reviews by external experts.
In 2024, for example, the U.S. Court of Appeals for the District of Columbia Circuit denied an industry petition to review an EPA standard involving ethylene oxide, a pollutant emitted by some chemical and industrial facilities that has been associated with several types of cancer.
The court accorded an 'extreme degree of deference' to the EPA's evaluation of scientific data within its area of expertise. The court listed key elements of the EPA's best available science, including 'an extensive, eighteen-year process that began in 1998, involved rounds of public comment and peer review by EPA's Science Advisory Board ('SAB'), and concluded in 2016 when EPA issued a comprehensive report on the subject.'
The District of Columbia Circuit in 2013 also affirmed the central role of science to inform revisions of National Ambient Air Quality Standards, which set limits for six common air pollutants.
In that case, Mississippi v. EPA, the court noted that the EPA must receive advice from its Clean Air Scientific Advisory Committee, or CASAC. The court advised that, while the agency can deviate from the committee's scientific advice, 'EPA must be precise in describing the basis for its disagreement with CASAC.'
The Trump administration in 2025 dismissed all members of CASAC and said it planned to replace them.
Requiring the agency to use the best available science helps ensure that decisions are based on evidence, and that the reasoning behind them is the result of well-accepted scientific processes and free from biases, including stakeholder or political interference.
The scientific challenges facing the EPA are increasing in complexity. Responding to them effectively for the health of the population and the environment requires expertise and robust scientific processes.
This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: H. Christopher Frey, North Carolina State University
Read more:
America's clean air rules boost health and economy − charts show what EPA's deregulation plans ignore
How a lone judge can block a Trump order nationwide – and why, from DACA to DOGE, this judicial check on presidents' power is shaping how the government works
As federal environmental priorities shift, sovereign Native American nations have their own plans
H. Christopher Frey does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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