CHICAGO - Silver tooth fillings have been placed in the mouths of Americans since before the Civil War, an inexpensive, durable and reliable material that helped form the foundation of modern dentistry.

For nearly as long, they've been a source of controversy.

Because toxic mercury makes up about half of a silver filling, its use has been the subject of hundreds of scientific and academic studies examining its safety. The results have done little to settle the dispute.

What's not up for debate is that silver fillings, commonly called dental amalgam, contribute mercury pollution to the environment.

Global consensus on that issue has led some anti-mercury crusaders, who long have sought to ban the material from dental products because of health concerns, to shift their focus to a fight they think they have a better chance of winning.

"The approach to getting to the end of amalgam is the environment," said Charlie Brown, president of the World Alliance for Mercury-Free Dentistry, a multinational group that lobbies to ban mercury in dentistry.

Recent developments suggest momentum is building against silver fillings based on environmental concerns:

More than 140 nations agreed in January to a U.N. treaty calling for decreasing use of dental amalgam. One major U.S. dental school announced it is putting less emphasis on teaching students about preparation and placement of silver fillings.

Two large, nonprofit Catholic hospital chains are waging proxy battles with the two leading American makers of dental amalgam. The hospital chains' investment arms are seeking a shareholder vote that would mandate each company detail plans to phase out mercury.

Norway, Denmark, Sweden, Japan and Finland have banned amalgam or restricted its use in the last five years. Other countries, including Canada and Germany, recommend keeping amalgam out of the mouths of certain people, including pregnant women, children and those with kidney impairment, as a precaution.

The Food and Drug Administration and the American Dental Association, which represents more than 150,000 dentists, maintain that silver fillings should remain an option, saying they are safe and effective and often outperform other restorative materials.

The ADA acknowledged that mercury waste is a problem but noted that as of 2010, about half of dentists were filtering out more waste mercury through the installation of relatively inexpensive equipment.

About half the mercury entering municipal wastewater treatment plants, about 3.7 tons annually, comes from dental amalgam waste, according to a 2010 EPA estimate.

Treatment plants capture about 90 percent of amalgam.