BOSTON — Massachusetts and federal officials worked Tuesday to decontaminate an Atlantic City clam boat anchored in isolation off Massachusetts after it dredged up old munitions laced with mustard gas, severely sickening a crewman from Atlantic County.

The Coast Guard was trying to locate the two military shells, which the crew tossed overboard in about 60 feet of water about 45 miles south of Long Island, said Coast Guard Petty Officer James Rhodes. He acknowledged finding the shells will be difficult.

The military used the ocean as a dumping ground for munitions after World War II through 1970 and tons of old chemical weapons beneath U.S. waters still present a danger to fishermen.

The two shells came aboard the 145-foot clam boat ESS Pursuit, whose home port is Barney’s Dock at Gardner’s Basin on Rhode Island Avenue, in a haul of ocean quahogs. The area is in a known munitions dumping zone and is on charts, Rhodes said.

James Meyers, who works at Barney’s Dock and is one of the owners of the ESS Pursuit, said the boat has been working off New England recently on beds of ocean quahogs and bringing them to New Bedford. He said it is an area they have worked before.

The Coast Guard has not identified the crewman who was hurt and Meyers said he only knows him by his first name, Constantine. Meyers said he is a Russian who has worked on the boats for several years. He said the injuries are “pretty serious” and everybody is concerned about him. Meyers said Constantine had been living near Estell Manor, Atlantic County, and is married with one child.

“He’s a good worker, a deckhand, about 28 years old. We’re hoping to God this boy is all right. He’s a good boy,” Meyers said.

Meyers said clam boats sometimes dredge up old munitions and crewmen are told to throw them back over immediately. He said the crew is trained to recognize the different munitions that could come in with the clams.

“He threw it over and that’s when the end came off and he got the residue on him. That’s all I know,” Meyers said.

“God bless the military. We all respect them, but years ago the military threw this stuff overboard and we pay the consequences,” Meyers said.

Other clammers at the dock were concerned as the news made the rounds on Tuesday.

“I heard it a few hours ago. I heard it was Constantine. I just know him as a Russian kid that works on one of the boats,” said clammer David Tarr.

A National Guard team boarded the vessel Tuesday to test for contamination, while the Coast Guard worked to secure the ship in waters off New Bedford so that it can be moored and decontaminated. The captain and first mate have declined to leave the 145-foot dragger, fearing it could run aground, the Coast Guard said.

The boat had returned to New Bedford early Monday after Constantine, one of six crewmen, reported blistering and shortness of breath. The clams were also unloaded.

Hours later, another crewman was brought ashore after he reported feeling lightheaded. He was examined and released. Two other crewmen left the boat late Monday, with one reporting nose and eye irritation.

The most seriously injured crewman, Constantine, had painful blisters about three-quarters of an inch high on an arm and a leg, said Dr. Edward Boyer, a toxicologist who is treating the man at UMass Memorial Medical Center in Worcester.

Officials weren’t sure at first what the crew was exposed to, but Boyer said blood and urine tests confirmed exposure to mustard gas, used most frequently during World War I.

“It literally pulls the top of the skin off the layer underneath it,” Boyer said. But Boyer said his patient was “handling it very well.”

It is not the first such incident. On July 31, 1989, three crewmen, two from Wildwood, Cape May County, suffered severe burn injuries after pulling up a gas canister in their scallop dredge. The boat was fishing about 40 miles off Sea Girt, Monmouth County, at the time. The crew noticed the canister looked old and pitted but nothing grew on it. They threw it back but soon suffered skin irritation that led to large blisters.

The Defense Department began using the ocean as a dumping ground for chemical and conventional munitions after World War II. The military says it stopped in 1970, and two years later Congress banned waste disposal in oceans, including chemical weapons.

Officials say it’s impossible to know exactly how much and what type of weapons have been dumped in the ocean because of incomplete records. A 2001 Army report found 74 past instances of ocean disposal — 32 off U.S. shores and 42 off foreign coasts.

For example, in 1967 the Army dumped 4,577 one-ton containers of a mustard agent and 7,380 sarin rockets off the New Jersey shore, according to Army records.

There are several military dumping grounds off New Jersey used from the end of World War 1 until Congress banned the practice. An ocean area off Atlantic City was used during operation CHASE, which stands for “Cut Holes And Sink ‘Em.”

While much of the dumping was “loose dumped” from barges, the CHASE operation included the scuttling of entire ships containing weapons, often encased in concrete. There were three CHASE sites off Atlantic City, CHASE 8, CHASE 11 and CHASE 12.

Robert Schoelkopf of the Marine Mammal Stranding Center in Brigantine has fought for more disclosure about where these sites are and what was dumped there. Schoelkopf at one time was concerned the munitions were killing dolphins, though it was later found a canine virus that washed into the ocean from land was too blame.

An independent study in 1969 by the National Academy of Sciences, at the request of the Army, said there will be leakage even of the materials encased in concrete. Although it said “toxicity of the sea should be highly localized.”

Only some of the ocean dumps were mapped and chemical munitions have been found in areas they weren’t supposed to have been dumped, such as off Hawaii, said Craig Williams of the Chemical Weapons Working Group, a Kentucky-based organization.

The tons of chemical weapons underwater are an extremely unlikely source of weapons for terrorists, Williams said, given the difficulty in locating them, uncertainty about the hazards they present and the fact that more dangerous chemicals are more readily available.

Mustard gas can be deadly if it’s used as an aerosol and inhaled, causing blisters and other problems in the lungs. It was designed to incapacitate soldiers and suck up men and resources to take victims off the battlefield and care for them, said Steve Bird, an associate professor at the University of Massachusetts Medical School and a chemical weapons expert.

The gas was used most frequently during World War I, but has been used sporadically since, including during the Iran-Iraq War in the 1980s, he said. The chemical retains its potency over time, though some of its components break down, he said.

“The stuff still works just the same, and is still as toxic as it was before,” Bird said.