The Indianapolis Motor Speedway recycles 20 tons of cans, plastic and cardboard in May, plus all its motor oil, brake fluid and transmission fluid. Its golf course is certified as environmentally friendly by Audubon International. Its trash gets converted to energy.

Still, any time hundreds of thousands of people come together there are repercussions. Each May, the IMS generates a million pounds of trash and tons of carbon dioxide.

And don’t forget the balloons — 30,000 of them.

As concerns over the environment mount, many major sporting events have turned their attention to going green.

Here is a look at what the IMS does for the planet — and what opportunities remain.

The Speedway’s most obvious, most visible byproduct is trash — bits of chicken bones, half-eaten hot dogs, ketchup-smeared napkins . . . — 500 tons of the stuff during the month.

It looks nasty, but good comes of it.

The rubbish is hauled by Waste Management of Indiana to a landfill outside Danville.

There, as it decomposes, it produces methane gas. Waste Management converts that to electricity and sells it through the Wabash Valley Power electricity co-op to nearby homeowners.

The landfill provides enough power to supply the electricity needs for 5,000 homes in Hendricks County.

A far smaller amount of refuse is recycled — only 20 tons of aluminum cans, plastics and cardboard. But John Goss, the Indiana Wildlife Federation’s executive director, says IMS’ recycling effort is “significant” and that track officials “are doing a much better job as reducing what goes to the incinerator.”

The biggest hit to the atmosphere comes not from the Speedway but from its fans through the carbon dioxide emitted by their automobiles.

The IMS declines to release attendance figures, but conservative estimates put the number at 300,000 to 350,000 on race day alone. Depending on the weather, another 100,000 people may visit the track during Community Day, Carburetion Day and the four days of time trials.

For safety reasons, the track forbids bicycles on the grounds.

Buses are available on race day to shuttle fans to the Speedway from Indianapolis International Airport and from Downtown. But typically only about 15,000 people choose that option.

That leaves cars, lots of cars. More than 100,000 of them descend on the Speedway on race day, says Jon Smithers, a veteran Indiana State Police officer. The cars cluster in huge traffic jams.

The state police advise race fans to allow two hours to get to the race track, even if they’re coming from inside Marion County.

The result: 2,000 tons of carbon dioxide goes into the air — “and that’s a conservative estimate,” says Kevin Gurney, a professor of earth and atmospheric sciences at Purdue University.

The national average carbon dioxide output, per person in the U.S., is six tons, Gurney says.

It’s invisible, and no one will notice it, “but it’s one more little contribution to the whole global bank of CO2 in the atmosphere,” says Larry Nies, a Purdue environmental engineer.

“But I wouldn’t want to pick on the Speedway for this,” Nies adds. “To me, it serves as a great example of how Indianapolis would be better served by better public transportation.”

Goss urges more regional mass transportation — “more buses from Chicago, Cincinnati” and other nearby cities that could serve as hubs for race fans. “I see it as a partnership,” Goss says. “It would be great if the track, the community and the bus companies expanded on that.”

IMS spokesman Fred Nation notes that on race day, some 300 charter buses already descend on the Speedway from other cities and towns.

“We would love to have more,” Nation says, “but all we can do is provide the tools, which include special parking places,” one adjacent to Turn Four, the other on 16th Street, right across from the track’s main entrance.

“If we have more charter buses, we’ll adjust in their favor,” Nation says. “If we reach capacity (in the fourth turn and on 16th street), we’ll make room available in our other perimeter lots.”

How another big sporting event tries to combat greenhouse gases: The National Football League plants trees at Super Bowl host cities — 1,850 of them this year in Tampa.

Since 2007, Indy Racing League race cars have run on ethanol. It’s biodegradable, and its emissions have a far smaller impact on air quality than other fuels. (NASCAR still burns gasoline — it switched from leaded to unleaded only last year.)

Last winter there was a flap — more patriotism-fueled than environmentalism-fueled — over IMS’ announcement it would acquire its ethanol from a Brazilian concern.

The Brazilians agreed to blend their sugar cane-based ethanol with American corn-based ethanol.

Environmentally, there’s a catch: sugar cane ethanol is more efficient to produce but has to be shipped more than 3,000 miles from Brazil to Indianapolis. Midwest-made, corn-based ethanol is so energy-intensive to produce, at least with today’s technology, that some scientists see no advantage to its use.

Gurney suggests some alternatives, such as a biodiesel based on soy “or better yet, recycled kitchen oil — a great marketing tool!”

The IMS in many ways is long on tradition. Since the 1940s, pre-500 protocol calls for the release of 30,000 brightly colored helium balloons. It coincides with the singing of “Back Home Again in Indiana.”

Race fans are mesmerized, but the balloons have long rankled environmentalists, who see them as litter.

The Speedway has heard it for at least two decades and has switched to latex balloons, which are biodegradable.

“But any time you disperse massive amounts of waste randomly with no way to get it back, it’s just pollution,” says Wallace Nichols, a marine biologist who runs the nonprofit Ocean Revolution. “There’s no way to responsibly do a balloon release.”

Nichols suggests the release of a bird, one that had been injured then nursed back to health by humans — “a golden eagle, or a bald eagle, or if not a hawk — something the crowd would be into.”

In the Speedway’s estimation, the crowd is into balloons, and on race day, balloons there’ll be.