When patients go to Dr. Robert Muller’s dentist office for a check-up and he finds a cavity that needs filling, they usually ask for the composite filling that matches the shade of their teeth.

“They’ll choose it because of what it looks like,” said Muller, of New Milford.

For cosmetic reasons alone, the mercury-silver amalgam fillings that left shiny black patches on a past generation’s teeth are fading from view.

“I have never had a patient come into my office and say ‘You know those big ugly silver fillings? I want one,’ ” said Dr. Nicholas Borrello of Stony Hill Dental Care in Bethel.

Now, if environmental activists have their way, those silver blots will go the way of elk-horn dentures.

They want the state Department of Environmental Protection to ban the use of mercury in dental fillings. The DEP will hold a hearing on the issue Thursday in Hartford. If the state accepts the argument, Connecticut would be the first state to require dentists to stop using mercury in fillings

Suporters say the issue isn’t the health of people walking around with mercury fillings in their mouth. It’s about getting mercury out of the environment, wherever it may be.

“Mercury is a toxic metal. It’s being phased out wherever it’s found,” said Charles Brown of Consumers for Dental Choice in Washington, D.C. “But some of it is in children’s mouth, and they won’t regulate that.”

“Our goal is the virtual elimination of the mercury humans put in the environment,” said Kathleen Bailey, chairwoman of the state chapter of the Coalition for the Enforcement of Zero Mercury Laws. “Think of the irony of this. If there’s a small amount of mercury spilled inside a school, there’s a lock-down of that school. But that same amount of mercury can be put in a molar filling and it’s all right.”

Dentistry experts have a different view.

“There is absolutely no health concern caused by silver-mercury amalgam fillings — and it’s not from a lack of looking,” said Dr. Robert Kelly, professor of dentistry at the University of Connecticut School of Medicine in Farmington. “It should be a non-issue.”

But it isn’t. Those opposed to mercury fillings argue those fillings should be subject to state laws governing the use of mercury. The law states that by 2004, products that contain more than 250 parts per million of mercury cannot be sold in the state. Silver-mercury amalgam fillings, they say, contain 500,000 ppm of mercury.

Robert Kaliszweski, the DEP ombudsman, said the hearing would not look at any larger issues — such as whether the mercury in fillings creates any health or environmental risk. Instead, he said, the focus will be narrow — whether mercury fillings should be subject to limits under the law, or be exempt from it.

The move comes as evidence grows that mercury, even in small doses, is a neurotoxin that can cause birth defects as well as speech, vision, memory and motor skill problems. It’s effects are especially pronounced on pregnant women, infants and small children. The DEP and the state Department of Public Health warns people — especially pregnant women and children — not to regularly eat fish from Connecticut streams and rivers because of high mercury levels. Canned tuna fish is also on the suspect fish list.

It’s also an element that once it is in the environment tends to stay there.

“The World Health Organization has said there is no safe level of mercury pollution,” said Brown of Consumers for Dental Choice.

For that reason, Connecticut has been one of the toughest-minded states in the country when it comes to mercury.

However, because of the increasing concern about mercury as a whole, environmental groups want to get rid of all sources of mercury in the environment. One source is dental fillings.

“Mercury for dental fillings accounts for 34 tons of mercury in the environment, nationally,” said Michael Bender of the Mercury Policy Project, a Vermont-based environmental group. “There is about 58 tons from power plants. Granted, the dental mercury doesn’t go instantly into the environment. But it does get released.”

Brown of Consumers for Dental Choice said Connecticut dentists use about 700 pounds of mercury a year for fillings.

“Even a pound of mercury is a scary amount,” Brown said. “There are 45 grams in a pound and one gram will pollute a lake. Not one gram is necessary.”

The average filling uses one-third to one-half gram of mercury; a big filling could use as much as a gram.

Those who want mercury fillings banned say no matter what, it will eventually end up in the environment. It can slowly vaporizes and enter a body, where it is absorbed or excreted. The Association of Metropolitan Sewerage Agencies said in 2001 the mercury from dental uses accounted for 40 percent of all mercury found in the waste at treatment plants.

Also, fillings last 10 to 15 years and eventually chip away as the patch cracks and wears out and gets into your body in bits. If someone is cremated it will be burned up and enter the atmosphere, only to get carried by precipitation back to earth.

UConn’s Kelly and Dr. Rod Mackert, professor of dentistry and oral rehabilitation at Medical College of Georgia — both of whom will testify at Thursday’s hearing — admit crematoriums may release mercury into the environment. But Mackert said it is a very small amount, simply because the fillings aren’t mercury anymore; when dentists prepare an amalgam filling, they mix mercury with a powdered alloy of silver, tin and copper. It creates a metal mixture that is very stable.

“They’ve studied the mercury vapors coming from filling materials compared to liquid mercury,” he said. “The liquid mercury had a million times more vapors.”

Mercury from fillings, he said, account for less mercury than the background levels found naturally in the environment.

Mackert and Kelly said many health organizations — from the U.S. Food and Drug Administration to the Centers for Disease Control and Prevention, to the National Institutes of Health to the World Health Organization — have studied whether mercury fillings cause any health problems. All clear it for use.

Kelly also saidbecause of cosmetic reasons, plastic composite fillings are now the material of choice. In 1990, he said, 70 percent of the fillings used by American dentists were mercury-silver amalgams; by 2003, it was down to 30 percent.

However, Kelly said, mercury fillings last longer. They’re better for using in large filling in the back teeth.

“The plastic fillings need dry conditions and they take longer to put in,” he said. “Mercury amalgam is a very robust material, and it’s easier to get it right.”

Borrello of Bethel — who only uses plastic composite fillings — admits it takes “a little more talent and a little more time and a little more skill” to use plastic rather than mercury. He said plastics fillings now are as durable as mercury, look better and can be used for any filling on any tooth.

“I would never say mercury shouldn’t be used. Dentists have to decide what’s best for them,” he said. “But in my opinion, it’s a 150-year old technology. I don’t see any need for it today.”

As dentists learn how to use plastic fillings, Borrello said, and the materials used in those fillings continues to improve, dentists will simply drop mercury in favor of something better-looking.

“I haven’t used it in 10 years,” he said. “And I think, in 10 more years, nobody will.”