Science, Policy & Politics: How We Regulate Mercury

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The Mercury and Air Toxics Standards (MATS) rule has been a policy and regulatory ping-pong match for over 20 years. Obscured in the regulatory back and forth is a tremendous environmental success story. Yet the regulatory ping-pong match continues. The latest round is that the current Administration’s EPA reaffirmed an Obama Administration legal finding that regulating power plants for mercury is appropriate and necessary.   

The Long and Winding Road

The regulatory history of mercury in the air began with the passage of the Clean Air Act (CAA) Amendments in 1990, requiring the EPA to set emission standards for hazardous pollutants in dozens of industries. For the power plant industry, the nation’s largest source of mercury, EPA first had to determine that it was “appropriate and necessary” to proceed with the regulations.    

  • Ten years later, in 2000, EPA determined that necessity, adding coal- and oil-fired electric power plants to the list of categories that must be regulated.
  • The EPA reversed itself in 2004, finding that it was neither appropriate nor necessary.
  • In 2008, the US Court of Appeals determined that EPA was wrong, and the EPA consequently relisted power plants.
  • In 2011, the EPA issued the MATS rule. MATS established numerical emission limits for mercury, arsenic, chromium, nickel, and other pollutants and outlined widely available and economically feasible technologies to meet the limits.
  • The EPA reaffirmed this decision in 2012, concluding that these compounds posed risks to human health and the environment and that costs should not be considered when determining whether power plants should be regulated
  • In 2014, a group of non-profit organizations, corporations, and 23 states filed suit to challenge the EPA's refusal to consider costs when regulating power plants. The US Court of Appeals for the DC Circuit upheld EPA’s decision not to consider costs. In 2015, the US Supreme Court ruled (5-4) that EPA had erred in not taking costs into consideration.
  • In 2016, the EPA issued a finding that it remained “appropriate and necessary” to regulate mercury and other hazardous air pollutants from power plants after consideration of costs. The EPA reversed that decision in 2020. However, the mercury standards (MATS) remained in place. The EPA stated that the action was only about responding to the US Supreme Court ruling on cost-benefit analysis and not about the existing MATS rule.   
  • In January of this year, the EPA proposed to revoke its 2020 decision, allowing regulation of mercury and other hazardous air pollutants from power plants. The existing MATS rule would remain in effect and were not proposed to be changed.

Why is Mercury Reduction So Important?

Mercury poisoning has been noted throughout history.

The phrase “Mad as a Hatter” is derived from the neurological symptoms of mercury poisoning. In the 19th century, mercury was used to manufacture felt hats to produce a better quality of felt. When the felt was dried, a fine dust was formed containing mercury, which the workers inhaled in large amounts. This caused symptoms of mercury poisoning such as excessive salivation, excessive shyness and social phobia (erethism), and shaking of the limbs, which became known as hatter’s shakes.

In more recent times, there have several incidents of mercury poisoning that have caused widespread deaths and illnesses. One tragic incident was the release of mercury into Minamata Bay in Japan from a chemical factory from 1932 to 1968. Bacteria converted the mercury in the Bay to an organic form, methylmercury, which affects the central nervous system and causes developmental effects, such as mental retardation, blindness, and cerebral palsy, in infants born to women who ingested high levels. The methylmercury accumulated in the fish in the Bay, which was eaten by the local population, resulted in mercury poisoning, termed “Minamata Disease.” Thousands of people suffered from Minamata Disease, and approximately 1,700 died.   

Another incident occurred in Iraq in 1971. Grain treated with methylmercury, used as a fungicide and never intended for human consumption, was imported and used as food by Iraqi residents in rural areas. People suffered from the same central nervous system effects as seen with Minamata Disease. The recorded death toll was 459 but is thought to be at least ten times greater than that, and over 6,000 people were hospitalized.

All of these incidents have very high levels of mercury exposure. Today’s concern over mercury exposure from power plants involves long-term exposure to low levels of mercury, for which the adverse effects are not well documented.    

The ironic twist

The MATS rule mandated economically feasible technologies to reduce the emissions of mercury and other toxic air pollutants from power plants. The MATS rule has been in place and implemented since 2011. The result -  between 2007 and 2019, there has been a 91% reduction in mercury emissions from power plants and a 73% reduction from all sources!

So why does everyone continue to use significant resources to fight a successful rule?

While the reduction of mercury has been a success story, the regulatory process has been a convoluted back- and forth of lawsuits and reversals by EPA on many of their rules.    

This has nothing to do with the science of mercury toxicity; it has everything to do with policy issues that have become intertwined with politics. Science is stuck in the middle of the ping-pong match.    

One major issue prompted by the MATS rule is the role of cost-benefit analysis in rulemaking. Since 1981, new proposed regulations need to quantify the costs and the benefits in a Regulatory Impact Analysis submitted to the White House’s Office of Management and Budget (OMB). Over the years, the politics of the moment have played a significant role in determining the length and type of OMB review. What gets included in the cost-benefit analysis differs greatly depending on the current political winds.

For example, the EPA’s 2012 and 2016 cost-benefit analysis concluded that the cost of compliance was reasonable considering the public health benefits accrued. The EPA’s 2020 reversal maintained those prior cost-benefit analyses were flawed because most of the benefits were due not to the removal of mercury but a predicted drop in particulate matter or soot. The EPA argued in 2020 that the benefits must be calculated solely on reductions of mercury and other hazardous air pollutants. This is not a science issue, but one of regulatory policy -  that differs depending on your perspective on the appropriate role of government regulation in achieving public benefit.


The current fighting over the MATS rule is not over the rule itself but the use of cost-benefit analysis in regulations. One school of thought believes that the underlying legislative statutes strictly determine the costs and benefits. Another school of thought believes that government should be given wide latitude to reflect a changing environment without going back to Congress to amend the underlying legislation.      

This issue is being litigated beyond simply the scope of the MATS rule and other environmental problems. The current Administration is fearful that leaving the previous Administration’s findings might limit future actions under environmental regulation. The current Administration wants to ensure they have as much latitude as possible. These issues are likely to be settled in a series of Supreme Court cases over the next few years.

As someone who spent many years working in the government, I will only say that this regulatory ping-pong eats up enormous Agency resources. Every Administration is convinced that they have the “correct answer.” The continued back and forth is detrimental to what could be accomplished if those resources were used to work on other important issues.