The United Nations Environment Program recently announced that has finally come to an end, but it’s not exactly true. A couple hundred thousand planes are still flying with leaded gasoline. Here’s why.

On August 30, the UNEP that fuel stations in Algeria finally stopped dispensing leaded gasoline. The country was the last in the world to hang on to the old fuel and its banishment means that leaded fuel is off of roads worldwide. However, missing in the UNEP’s announcement is that leaded fuel is alive and well up above, powering general aviation aircraft.

Leaded fuel by the Environmental Protection Agency in the U.S. in 1996. Since then, you’ll really only find it in some off-road vehicles, old boats, old farming equipment and yep, in the little Piper buzzing above your house.

There are 230,000 aircraft in the world that still run on leaded fuel, 167,000 of them in the U.S. alone. Everything from trainer Cessnas to small commercial aircraft.

One of the first things you learn when starting to fly is how to properly fuel your aircraft. Most aviation gas (avgas) will come in 100 octane low-lead (100LL).

Other grades include leaded 100/130 and you may see a variety of unleaded fuels. It’s important to use the fuel specified for your engine. Putting the wrong fuel in a plane has a similar impact as a car, where lower octane fuel can reduce performance and cause damage from detonation. However, your car isn’t being used to carry people thousands of feet into the sky.

Avgas uses Tetraethyl Lead (TEL) to boost the octane level of the fuel. As the Federal Aviation Administration , many piston aviation engines are high compression and high displacement, with high octane requirements to maintain stability, reliability and performance. TEL is very good at the job of boosting octane. Leaded fuel is especially useful in turbocharged engines.

TEL is also incredibly toxic. Breathing in lead has detrimental impacts to the human body ranging from nervous system and immune system damage to learning problems and a lower IQ in children. Lead is also an environmental pollutant that can be absorbed by plants and the ground.

As by Popular Science, planes burn between 150 and 175 million gallons of leaded fuel each year, and avgas is the largest contributor to the remaining lead emissions in the country.

Lead is also dirty for the plane itself, leaving behind deposits, and corroding parts.

Efforts to replace leaded fuel began decades ago, but really heated up this millennium. The FAA has established the for the EPA, the world’s fuel producers and the aviation industry to find a universal solution out of leaded avgas. All of them work towards a goal of allowing most of the 230,000 planes in the sky to fly, but . That solution has yet to be found.

The Popular Science report also notes that avgas makes up a fraction of a percent of gas produced for cars, so large fuel producers have little incentive to solve the problem.

A lack of a universal unleaded fuel isn’t the only issue. Over a decade ago, the EPA began researching the health effects of leaded avgas with the goal of with potential regulatory changes in 2018. That hasn’t happened and the EPA still isn’t pushing the issue.

There are some pockets of hope. As the Experimental Aircraft Association , a large variety of aircraft frames and engines can run on unleaded fuel now, many without major modification.

Additionally, a 2017 Santa Clara University study that UL94, essentially 100LL avgas without lead, could be used in more than half of general aviation aircraft. Most recently, General Aviation Modifications Inc certified a direct unleaded replacement for 100LL called G100, AVweb. The fuel is over a decade in the making, but there may still be a long time before you’ll see it at your local airport. GAMI is still ironing out certification for numerous engines.

This all sounds great! But as the Santa Clara study notes, availability of unleaded fuel at airports is very spotty. So even if you have a plane that can run unleaded fuel, you may still have to burn 100LL, anyway.

Still, progress is being made and the day lead gets out of aviation gas is coming. But until then, planes will continue to fly burning leaded fuel.