Doctors have released a study timed to the anniversary of the September 11, 2001 attacks against the United States.

On that day, militants seized four passenger airplanes and flew them into U.S. targets. Nearly 3,000 people were killed in what Americans now call the 9/11 attacks.

Two of the airplanes hit the World Trade Center buildings in New York City. The strikes led to their collapse and fires that burned for days. Huge clouds of dust were released into the air.

The new study tells about firefighters who arrived early or spent a lot of time at the World Trade Center site after the 9/11 attacks. It found they seem to have a somewhat higher risk of developing heart problems than firefighters who arrived later or stayed less.

The study might influence any effort to expand the list of health problems eligible for payment to victims of the attacks. The money comes from a program set up by the U.S. Congress after the disaster.

The Associated Press noted limitations of the study. Researchers said they cannot prove that dust or anything else from the disaster caused increased heart risk. The study also does not compare the New York firefighters to the general population or to other emergency workers or construction industry workers.

FILE - In this June 11, 2019, file image made from video, retired New York Police Detective and 9/11 responder, Luis Alvarez, speaks during a hearing by the House Judiciary Committee as it considers permanent authorization of the Victim Compensation. Alvarez died in 2019.

However, the study does suggest that working at the World Trade Center site raised the risk for some firefighters more than others. It found that those who arrived by noon that day had a 44 percent greater chance of suffering heart problems compared to those who arrived hours or days later.

In addition, the risk of heart problems increased by 33 percent for those who worked in the area for six or more months.

However, the increased risk was evident in a fairly small number of cases. The study found that only about five percent of the firefighters studied had cardiovascular disease.

Risk increases with time

David Prezant is chief medical officer of the Fire Department of the City of New York. He said, "This is a modest increase, not an epidemic." But he added, "This risk increases over time; it doesn't disappear."

The study followed the health of 9,796 male firefighters for 16 years after the disaster. Female firefighters were not included because there were only 25 and their health risk was likely different from the men, the study said.

Researchers examined medical records of the male firefighters before the attacks. They identified 489 heart problems since September 11, 2001. That number included 120 heart attacks and 300 treatments for blocked arteries. The study found that heart-related risks were higher for the 1,600 firefighters who arrived at the trade center by noon.

The researchers noted there are good records of when the firefighters arrived. But there is less information about how long each worked in the area of the disaster.

Judith Graber is a researcher at the Rutgers School of Public Health in New Jersey. She has studied other workers involved in the 9/11 effort. She praised the study, saying the important thing is the additional evidence suggesting increased risk.

Prezant said other studies have suggested a greater risk of heart problems among 9/11 workers. He noted such problems are not covered under the program that treats emergency workers or the victim compensation fund.

U.S. lawmakers set up the fund to help individuals who suffered physical harm and family members of those killed in the attacks.

More than 51,000 people have asked for money from the fund. The Associated Press reported that $5.5 billion has already been paid out. On July 29, lawmakers passed a bill that provides money for the fund until the year 2092.

The results of the study on firefighters were published in JAMA Network Open.

The Associated Press reported this story. Mario Ritter Jr. adapted the report for VOA Learning English. George Grow was the editor.

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