Back in 1972, the United Nations established June 5 as World Environment Day in an effort to raise awareness of the various harmful ways humans affect the planet. This year, air pollution is the theme.

Airlines are keenly aware of their role in damaging the atmosphere, given that they account for about 2% of annual global carbon emissions.

So it’s probably not a surprise that, this month, a few of them are showing how they’re trying to shrink their footprint.

One of them has an experiment lined up for Wednesday morning, the very day chosen by the UN: United has scheduled a flight to California that’s going to go all green.

The world’s second-largest airline plans to reduce cabin waste to zero, fly on a biofuel mixture and “erase” the remaining emissions with carbon offsets—all on a one-off flight from Chicago to Los Angeles.

The company calls the trip by one of its Boeing 737-900ERs a “flight for the planet,” asserting that it’s the first time an airline has ever combined all three efforts on a single commercial flight.

When it comes to garbage at least, airlines would like to see their flights produce zero waste someday, migrating to compostable and fully recyclable service ware such as cups and food wrappers.

United Continental Holdings Inc. said its average flight generates 65 pounds of trash. On the Wednesday trip, zero cabin trash is unlikely, but the carrier wants to see how close it can get, said Aaron Stash, United’s senior manager of environmental sustainability.

Part of that effort will include giving everyone a plated meal with fully recyclable or compostable dishes and utensils, instead of the usual for-sale snack boxes.

“When we talk about doing this, one of the first things we would jump to is to ‘just make it reusable—let’s wash it on the ground because that’s what we do in the premium cabin,’” Stash said. But there’s a problem with that approach: Reusable service items like forks, dishes and cups add a large weight penalty that negates fuel savings

“As an airline, we admit that we emit.”

The industry has taken some decidedly modest green steps over the years. Recently, many U.S. carriers, including American Airlines Group Inc., Alaska Air Group Inc., Delta Air Lines Inc. and United, have replaced their single-use plastic stir sticks and straws with those made from bamboo, birch and other sustainable materials.

But bigger moves are being made. On Tuesday, Delta announced that it will eliminate the plastic wrap on its in-flight amenity kits. 

Also this week, JetBlue Airways Corp. said it would offset the carbon output of its customers during June, the equivalent of 700,000 metric tons of carbon dioxide.

“As an airline, we admit that we emit,” Sophia Mendelsohn, JetBlue’s head of sustainability and environmental social governance, said in a statement. “Communally, the traveling public’s and airline’s first-line defense is flying efficiently and avoiding unnecessary emissions.”

United has some longer-term goals in mind as well. In December, Fulcrum BioEnergy Inc., announced plans to open a processing facility in Gary, Indiana, to convert 700,000 tons of municipal trash from Chicago into 33 million gallons of biofuel, which can be blended with traditional jet fuel.

Four years ago, United invested $30 million in Fulcrum, part of a deal to acquire almost 1 billion gallons over a decade.

United began filling planes in Los Angeles with a biofuel mix in 2016. The airline is exploring O’Hare International in Chicago as a potential second hub for its use. 

A United spokesman said no decision on the second venue has been made. The flight scheduled for Wednesday is seen “as a test case for O’Hare specifically in the biofuels sector,” Stash said.