Maryland is about to become the first state to ban the use of additives containing arsenic in chicken feed, a practice already prohibited by Canada and the European Union.

The state’s House of Delegates and Senate approved the legislation last week and placed it before Gov. Martin O’Malley on Monday. The governor could sign it soon, said Del. Tom Hucker (D-Montgomery), who sponsored the House legislation.

“He congratulated me. He likes the bill. And he’s communicated with his staff about the bill,” Hucker said.

Arsenic occurs naturally in the environment but can also be a toxic carcinogen that contributes to diabetes and heart disease. Last year, the Food and Drug Administration tested 100 chickens by giving them feed that contained the additive roxarsone, an arsenic-based drug used to fight parasites in animals. Half the chickens later showed trace amounts of inorganic arsenic, a known carcinogen, in their livers.

The finding prompted Pfizer to suspend sales of roxarsone, which also makes the meat appear pinker and more plump by promoting growth in chickens’ blood vessels. Perdue Farms stopped using the additive years ago, and McDonald’s does not allow its suppliers to use it.

The United States produced 8.5 billion broiler chickens in 2009, according to the Agriculture Department. Georgia was the nation’s largest producer of broilers, turning out 1.3 billion. Maryland was 10th, with nearly 300 million that year, or about 1.4 billion pounds, generating 40 percent of the state’s farm revenue, according to the DelMarva Poultry Industry trade group.

Growers in Maryland, particularly on the Eastern Shore, continued to use stockpiles of the feed after Pfizer suspended it, feeding about 3 million chickens per year, according to Hucker and one of the bill’s supporters, Food & Water Watch.

The chickens produced about a billion pounds of waste, often spread as fertilizer. Hucker said unknown levels of arsenic have seeped into the state’s soil since the product was first used in 1946 or have washed into waters that run into the Chesapeake Bay.

“We know arsenic causes cancer, heart disease and diabetes,” Hucker said. “We’ll never know how much is caused by arsenic in chicken, but we do know it’s highly avoidable.”

“I would think it’s a huge marketing opportunity for Maryland chicken growers to let consumers know only Maryland chickens are guaranteed to be free of arsenic,” said Hucker, who is in discussions with lawmakers in other states who want to pass similar legislation.

Saying Maryland poultry is arsenic-free “doesn’t amount to a hill of beans,” said Del. Charles J. Otto (R-Somerset), who opposed the legislation. “You don’t know where your chicken comes from.”

Otto argued that arsenic occurs naturally and shows up in extremely low amounts in chickens.

“It’s not an environmental threat or human health threat,” he said. Tying it to disease “is a scare tactic,” Otto said.

But scientists have seen growing evidence that organic arsenic can become toxic. In the FDA test, roxarsone widely used by farmers was fed to the chickens.

“So we think the danger to public health and the environment was such that Maryland needed to take a major first step in eliminating this poison from our chicken,” said Jorge Aguilar, southern region director for Food & Water Watch.