NEW YORK — Dr. James Melius, an international expert on workplace medicine who advised the sponsors of a federal law that authorized billions of dollars for the medical care of first responders and others after the World Trade Center attacks in 2001, died on Jan. 1 at his home in Copake Falls, N.Y. He was 69.

His son Jeremy said the cause was cardiac arrest.

In the years before passage of the law in 2010, Mr. Melius gave forceful testimony to congressional committees that drew on his decades of work for government agencies and labor organizations.

He urged that Congress pass a comprehensive program to relieve the economic hardships of those who had developed — and would develop — respiratory diseases, cancer, and other illnesses caused by exposure to toxins like asbestos, alkaline dust, and benzene after the terrorist attack of Sept. 11, 2001.

He also lobbied to help workers who had lost their jobs and medical insurance and had found their medical claims being challenged by workers’ compensation.

When he testified on Capitol Hill in 2009, Dr. Melius cited the case of Leon Heyward, an inspector for the city’s Department of Consumer Affairs, who had helped evacuate disabled co-workers from ground zero and later learned he had sarcoidosis, a respiratory disease.

“His disease got worse,’’ Dr. Melius said in his 2009 testimony. “He had to stop working. He was denied workers’ compensation. He struggled to get by and needed to move to a smaller apartment. He later developed lymphoma and died last year.’’

An autopsy later ruled Heyward’s death a 9/11-related homicide, Dr. Melius said.

Dr. Melius also helped devise the medical treatment and monitoring program that the legislation created. It also applies to those affected by the Sept. 11 attack on the Pentagon and the crash of a hijacked plane that day in Shanksville, Pa.

“He was a rock to lean on,’’ US Representative Carolyn B. Maloney, a New York Democrat who was the lead House sponsor of the law, said in a telephone interview.

“He was the principal force for building the science that 9/11 caused these illnesses and deaths.’’

The law was named for James Zadroga, a New York City police detective whose death in 2006 from respiratory failure was linked to the dust he inhaled during rescue and recovery work at the World Trade Center.

Dr. Philip J. Landrigan, the dean for global health at the Icahn School of Medicine at Mount Sinai in New York, said Dr. Melius had created innovative health and safety programs in the construction trades as well as assisting states and Canadian provinces on legislation that presumes that the dangerous work of firefighters is a major factor when they are disabled or killed by heart and lung disease and cancer.

“I think it’s fair to say that Jim Melius was the senior medical officer in the entire American labor movement,’’ said Landrigan, a longtime colleague who collaborated with him on the 9/11 medical program at Mount Sinai.