Fast-food giant McDonald's Corp. said yesterday it would phase out its use of foam packaging, including the clamshell-like hamburger containers that have been criticized for clogging landfills and harming the Earth's protective ozone layer.

Environmental advocates praised the decision, saying it would spur other users of polystyrene packaging to reduce or eliminate the material. With 8,400 outlets around the country, McDonald's uses about 2 percent of all the polystyrene produced in the United States, according to the Polystyrene Packaging Council, an industry trade group.

The company has been under pressure from consumers, environmental groups and government agencies to limit its use of the foam material, which is primarily used to hold its Big Mac and Quarter Pounder hamburgers. Several cities and states have passed laws banning the sale of fast food in the plastic boxes.

By churning out 10 million of these boxes a day -- more than 3.5 billion a year -- McDonald's has helped to make the packaging a symbol of a throwaway society.

The fast-food chain joins a long list of companies that have recently changed their practices as a result of pressure from environmentalists.

Last spring, Star-Kist Seafood Co. said it would stop buying tuna trapped by nets that kill dolphins. Star-Kist's move was quickly followed by others. Procter & Gamble Co. has changed the packaging on a variety of its products to reduce waste material, and grocery stores have become active participants in helping customers recycle cans and bottles.

McDonald's, which began using polystyrene packaging 15 years ago, has long maintained that the material is ideal for retaining the heat and moisture in its sandwiches and hamburgers.

But polystyrene is manufactured with chemicals, called chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), that studies show cause deterioration in the ozone layer and air pollution and are possibly carcinogenic. Polystyrene also does not break down as quickly as many other materials when discarded. McDonald's and other companies began switching about two years ago to polystyrene made with HCFCs, which is considered less harmful to the ozone layer than CFCs.

McDonald's had considered as recently as last week a plan to recycle its foam packaging materials at all of its U.S. restaurants. But the company, which has been working with the independent Environmental Defense Fund since August to improve its environmental record, said it could reduce the amount of waste its restaurants generate by up to 90 percent if it eliminated the foam materials altogether.

McDonald's is experimenting with a variety of paper-based materials -- such as paperboard and clay- and polyurethane-based products -- to replace foam, said Shelby Yastrow, senior vice president of the company. Company officials said they are studying how much of the new materials will be recyclable. The phasing out of polystyrene, which will begin in about 60 days, will not affect its prices, McDonald's said.

The decision was praised by environmentalists who hope the move will spur others to follow McDonald's lead.

"Anytime you reduce the amount of waste materials going into landfills, that's an environmental plus," said John Bell, a representative of the environmental group Friends of the Earth, although he questioned whether substitute materials such as plastic-coated papers would present other waste-disposal problems.

Packaging and chemical industry officials, however, criticized the decision as being motivated more by public pressure than environmental concerns.

Jerry Johnson, executive director of the Polystyrene Packaging Council, called McDonald's decision "unfortunate" because it could motivate other users of foam to switch to alternatives. Johnson said polystyrene can be safely burned or recycled into a variety of durable products, such as building materials, coat hangers and cassette cases. "The environmentalists have focused on polystyrene as a symbol of a wasteful material and they are willing to rely on half-truths and innuendo to make it go away," said Johnson.

A spokesman for Burger King Corp., the second-largest hamburger chain, said the company uses polystyrene only as material for coffee cups and has already begun to phase out its use. Wendy's International Inc., another fast-food company, said it does not use polystyrene to package its sandwiches.

Staff writer Kara Swisher contributed to this report.

Food service and food packaging use about 1 billion pounds of polystyrene a year, according to National Polystyrene Recycling Co., a plastics industry joint venture. That includes groceries and other supermarket products used in homes as well as packaging used by fast-food restaurants.

Polystyrene foam packaging makes up 1.12 percent of landfill volume in the United States, according to the Foodservice and Packaging Institute Inc. Plastics make up about 18 percent of landfill volume, according to industry estimates.

The polystyrene containers used by fast-food restaurants make up about 0.25 percent of U.S. landfill volume, and McDonald's Corp. uses about 10 percent of those containers -- or about 0.025 percent of landfill volume in the United States, according to the Polystyrene Packaging Council.