Recently, commentators, regulators, and activists have placed acute focus on how carbon emissions from commercial jet aviation contribute to climate change. But strangely neglected has been a secondary environmental threat from the gas-guzzling airliners’ smaller, awkward cousins, the “general aviation” fleet of piston-driven propeller aircraft of the kind frequently spotted on Sunday afternoons at the local airstrip. Many Americans might be surprised to learn, in a country where leaded automotive gasoline has been illegal for almost thirty years, that more than 170,000 active aircraft still—legally—burn heavily leaded fuel, accounting for up to seventy percent of atmospheric lead pollution today. Last year, half a century after it began regulating the use of lead in automotive gasoline, and after decades of rulemaking petitions by environmental interest groups, the EPA issued an Endangerment Finding for leaded aviation fuel (known as “avgas”) under Section 231 of the Clean Air Act.

Aircraft emissions are regulated by EPA pursuant to a two-step process. The agency decides whether, in its judgment, the air pollution at issue “may reasonably be anticipated to endanger public health or welfare,” and whether emissions “from any class or classes of aircraft engines” contribute to such pollution. As with regulations under other parts of the Act, once the agency makes these findings, it is subject to a duty to propose and promulgate regulations.

Lead is added to aviation fuel in an organic compound, tetraethyl lead (TEL). It functions to prevent “engine knock,” or premature ignition, a problem with high-performance piston engines of the kind used to power small airplanes, by boosting a fuel’s “octane rating.” Lead and lead compounds have also been known to be toxic since as early as the Second Century B.C. Indeed, the hazards of lead exposure, particularly for children, are so well-documented that it is effectively a matter of common knowledge.

The recent Endangerment Finding builds a wealth of research, including the EPA’s studies of airport emissions beginning in the early 2010s linking higher atmospheric lead concentrations and blood lead levels with proximity to small airports. Those with the highest exposure risk are the 5.2 million Americans who live within 500 meters of an airport runway. As one might expect, these airport-adjacent populations are disproportionately minorities, children, and people with income below two times the federal poverty level. When we consider that half of the lead emissions from small planes are attributable to recreational flight—largely a luxury pastime of the well-to-do—the environmental justice implications of the continued use of leaded avgas, and the EPA’s foot-dragging even to make a finding of endangerment, are significant.

Environmental groups have been pressuring the EPA for almost twenty years to issue an endangerment finding of aviation lead emissions, including by filing formal petitions for rulemaking and even suing the agency. So, why has it taken so long for the EPA to formally reach the obvious and preordained conclusion that lead emissions from avgas are dangerous?

The answer is twofold. Firstly, while mandatory regulation provisions like those of Sections 231 and 202, especially when paired with the public right to petition for rulemaking, are designed to reduce the EPA’s discretion to refuse to regulate where there is clear evidence of harm from a pollutant, in practice this arrangement would appear to shift the fight to the endangerment finding stage. Such a shift arguably disserves the public interest because the EPA faces internal and external pressure to withhold its opinion about the dangers of substances in our air until it is prepared to commit to a regulatory action. This could easily mislead the public into a false sense of safety and thereby discourage public pressure for regulation until the agency has committed to regulating.

Secondly, in the case of leaded avgas, the EPA cannot unilaterally regulate or ban the use of such fuels; it requires cooperation from the FAA, which regulates fuel for aircraft safety. While safe technological alternatives to leaded avgas already exist, manufacturers have so far lacked the regulatory and market incentive to develop them at affordable prices. Facing resistance from aviation interest groups and from important public services that rely on piston aircraft (such as medical, survey, and police aircraft), the EPA continues to face a contentious regulatory fight over aviation emissions. Even with a finalized Endangerment Finding, we should expect it to take several years before we see regulations curbing the largest source of lead pollution in our air.