Since the United Nations program began, 46 percent of all credits have been awarded to the 19 coolant factories, in Argentina, China, India, Mexico and South Korea. Two Russian plants receive carbon credits for destroying HFC-23 under a related United Nations program.

“I was a climate negotiator, and no one had this in mind,” said David Doniger of the Natural Resources Defense Council. “It turns out you get nearly 100 times more from credits than it costs to do it. It turned the economics of the business on its head.”

Destroying the waste gas is cheap and simple, but it is hard to know exactly how much any one company has earned from doing so, since the market price for carbon credits has varied considerably with demand — from about $9 to nearly $40 per credit — and they can be sold at a discount through futures contracts.

The production of coolants was so driven by the lure of carbon credits for waste gas that in the first few years more than half of the plants operated only until they had produced the maximum amount of gas eligible for the carbon credit subsidy, then shut down until the next year, United Nations reports said. The plants also used inefficient manufacturing processes to generate as much waste gas as possible, said Samuel LaBudde of the Environmental Investigation Agency, an organization based in Washington that has long spearheaded a campaign against what he called “an incredibly perverse subsidy.”

Michael Wara, a law professor at Stanford University, has calculated that in years when carbon credits were trading at high prices and coolant was dirt-cheap because of the oversupply, companies were earning nearly twice as much from the credits as from producing the coolant itself.

The United Nations, recognizing the temptation for companies to jump into the lucrative business, has refused since 2007 to award carbon credits to any new factories destroying the waste gas. And last November, it announced that in contract renewals, factories could claim credits for waste gas equivalent only to 1 percent of their coolant production, down from 3 percent. The United Nations believes that eliminates the incentive to overproduce, said Mr. Hession, the former Clean Development Mechanism board chairman.

Even with these adjustments, credits for destroying waste gas this year remain the most common type in the United Nations system, which rewards companies for reducing all types of warming emissions. Eighteen percent of credits in 2012 will go to the 19 coolant plants, compared with 12 percent to 2,372 wind power plants and 0.2 percent for 312 solar projects for the carbon dioxide emissions avoided by the clean energy they produce.