These are some of our most ambitious editorial projects. Enjoy!

Our emails are made to shine in your inbox, with something fresh every morning, afternoon, and weekend.

For decades, airplanes taking off from Reid-Hillview Airport in the heart of Silicon Valley filled up their tanks with a heavy metal most Americans assumed was banished long ago: lead.

Reid-Hillview was not unusual in this respect. Virtually every small airplane in the US burns fuel containing tetraethyl lead (TEL), an additive introduced in the 1920s to boost octane levels. It’s so toxic, a splash on the skin can be lethal. But in January, Santa Clara County, which owns Reid-Hillview, imposed the first ban in the US on refueling with leaded aviation gas, sparking a fight with the Federal Aviation Administration (FAA) and forcing some plane owners to refuel their airplanes elsewhere. County officials have since reaffirmed the ban, and now other airports are considering following suit.

Prompted by scientific research showing higher than average lead levels in children living near Reid-Hillview, Quartz has done the first investigation combining lead emissions data at the country’s top civil aviation airports with flight paths for small, piston-engine aircraft over affected neighborhoods. The analysis of over 350 million aircraft locations from planes taking off and landing in the US shows the extent to which lead-fuel burning aircraft regularly expose homes, parks, schools, and playgrounds across the country. The US Environmental Protection Agency (EPA) estimates 16 million people—and 3 million children—live within a kilometer of these airports.

Though it has now banned the substance, Reid-Hillview is among the best illustrations of how the last major source of leaded fuel in the US continues to contaminate communities. As many as 24,000 people live within one kilometer of the airport’s runways in the densely packed suburbs of San Jose, not far from the headquarters campuses of Google and PayPal. Three large parks and five schools are close by. One, Donald J. Meyer Elementary School, even shares its fence line with the airport, where more than 200,000 takeoffs and landings happen each year.

Jump to the searchable library of maps showing the areas surrounding the top lead-emitting airports in the US.

While old paint, water pipes, and contaminated soil remain the most visible legacies of lead in the US, aviation gasoline is now the largest source of airborne emissions—which makes civil aviation airports the epicenter of lead exposure for a new generation of Americans. Each year, America’s fleet of 170,000 piston-engine aircraft burns about 186 million gallons of leaded fuel. That releases about 468 tons of lead into the atmosphere, the US Environmental Protection Agency estimates.

As engines burn aviation gasoline, lead is ejected into the air as tiny particles, many as small as four nanometers across. More than 80% of an aircraft’s lead emissions occur as the plane taxis, takes off, and lands. These particles settle on surfaces, and make their way into the lungs and bloodstreams of people nearby. For millions living within a kilometer of one of the country’s 13,100 civil aviation airports, lead is not something they can avoid.

In 2021, Santa Clara County released a study of blood lead levels of children living near the airport. The results were unequivocal: the closer a child lived to the airport, the higher the lead levels, even after accounting for other variables. On average, the blood lead levels of children living within half a mile of the airport rose by 0.2 micrograms per deciliter, equivalent to half the surge in children’s lead levels seen during the Flint, Michigan, water crisis, in which as many as 12,000 were exposed to lead in their drinking water. The children living downwind of Reid-Hillview within half a mile of the airport were found to have blood lead levels nearing those in Flint.

But unlike Flint, where lead levels surged over 18 months, the exposure to airborne lead for children living near airports persists, year after year, during the most vulnerable period of their lives. While the levels found around Reid-Hillview are below the 3.5-microgram threshold signaling “elevated” levels, according to guidelines from the US Centers for Disease Control and Prevention (CDC), they can nevertheless leave a lasting impact on children’s brains. As the CDC itself says (pdf), no safe amount of lead exists. Even at the lowest recordable levels, researchers have found evidence of long-term mental harm, ranging from lower IQs to higher rates of ADHD and aggression, with social spillovers that depress lifetime incomes and graduation rates.

“Aviation gasoline exposure is a daily, unabated barrage of lead,” says Sammy Zahran, the professor of demography and epidemiology at Colorado State University who conducted the study at Reid-Hillview for Santa Clara County. “We can say that the observed increase here will have a detrimental effect on the cognitive performance of children living nearby.”

The initial exposure to lead, in relative terms, is the most harmful. So even low doses cause disproportionate damage. “That means the greatest benefits are in the last mile of regulation,” says Zahran. “There are cognitive gains that await the country if we can figure out how to get that last piece of lead out of the lived environment. And this is the last piece of lead.”

To reveal the scope of the problem, Quartz created maps for 95 of the top lead-emitting airports in the US, and the location of nearby schools, parks, and dense residential neighborhoods.

Reid-Hillview ranks 34th among US airports for annual lead emissions, according to the EPA’s annual National Emissions Inventory. The biggest annual emitter, at nearly one ton of lead, is Phoenix Deer Valley in Arizona, with about 400,000 takeoffs and landings each year (pdf).

Each map shows the flight paths from 2016 to 2022 of piston-engine aircraft flying at altitudes below 10,000 ft,  illustrating potential lead exposure to communities. (At greater altitudes, emissions are lower and more dispersed.) The maps are searchable by city and state.

These maps illustrate where initial emissions are likely to be highest. Because lead pollution disburses with the wind, anyone within a 1.5 km radius of the runways may be exposed over the long term. But essentially three factors dictate the amount of lead exposure: the volume of air traffic (and thus lead emissions), one’s proximity to the airport, and the prevailing winds. The worst-case scenario for residents? Living alongside a busy airport, downwind of the runway. Often it’s lower-income families living in these areas. To determine individual lead risks, more detailed studies, such as the one at Reid-Hillview, would be needed.

Without an unleaded alternative for the entire fleet approved by the FAA, most airports have defaulted to carrying leaded fuel for the minority of older aircraft that still demand it. (The average age of civil aircraft in the US is around 50 years old.) These aircraft have internal combustion engines that use a piston and crankshaft, similar to those in automobiles, to drive the propeller, relying on the higher octane provided by lead additives to generate enough power. By contrast, jet engines, which power the vast majority of commercial flights, burn an unleaded version of kerosene called Jet A.

There are solutions. But nearly 50 years after the US began to ban leaded automotive gasoline, leaded avgas still has not been officially designated a danger to human health. The EPA categorized leaded fuel as a “significant risk” (pdf) in 1973, with clear evidence of harm to human health. But both it and the FAA have repeatedly failed to regulate leaded avgas after years of lawsuits and missed deadlines.

An EPA process known as an endangerment finding is underway yet again. If it succeeds, it will force the FAA to ensure an alternative exists for the small but meaningful share of the country’s 170,000 piston aircraft that still need the higher octane tetraethyl lead provides.

To learn more, read our investigation of why a universal unleaded aviation fuel, more than half a century in the making, has never arrived.

Quartz analyzed 350 million aircraft transponder data points provided by ADS-B Exchange to map the flight paths of airplanes flying at the top 100 lead-emitting airports from 2016 to 2022. Aircraft were tracked between 50 and 10,000 ft of the FAA-listed elevation. Emissions are most likely to affect nearby communities at these altitudes. Areas near the end of runways are of particular concern because that’s where fuel consumption can be the highest as planes increase throttle for takeoff. The color scale of aircraft traffic is based on the number of aircraft that sent transponder locations originating from that spot.

The dataset we analyzed only contained location broadcasts sent at the top of a minute, so exact counts of airplane traffic are not possible to calculate. Furthermore, ADSB Exchange’s network of antennae has changed and grown in recent years, making numerical figures somewhat reflective of the site’s network rather than the frequency of flights.

The locations of schools, parks, and playgrounds are provided by OpenStreetMap, and in some cases may be incomplete or out of date. Census figures are aggregated from the block level figures released in the 2020 redistricting data files. We provide ranges of the populations to account for Census blocks that overlap our boundaries (up to 1  km from the end of a runway). The low figure is a scenario where no person who lives inside a boundary-overlapping Census Block also lives inside the boundary. The high figure is the population if every person in the Block lived inside the boundary.

Read more stories in this series:

Leaded airplane fuel is poisoning a new generation of American children

Living with the risk of childhood lead exposure: A day in the life

50 years of research shows there is no safe level of childhood lead exposure

📬 A periodic dispatch from the annual session of the United Nations General Assembly in NYC.

By providing your email, you agree to the Quartz Privacy Policy.