Nearly half of the more than 1,000 screened rescue workers who responded to the Sept. 11, 2001 attacks suffer from new or exacerbated respiratory, mental and other health problems, according to a government report released on Thursday.

The report, from the U.S. Centers for Disease Control and Prevention, is the second released in two days to find that firefighters, police officers and volunteers show persistent effects from environmental toxins and psychological stress.

On Wednesday, a similar study from the Government Accountability Office, the investigative arm of Congress, noted that many rescue workers suffer wheezing, shortness of breath, sinusitis, asthma and a syndrome called “WTC cough.”

The latest health study, conducted at Mount Sinai School of Medicine, showed that nearly half of the 1,138 people screened had problems that either began or worsened after being exposed to the dust, airborne toxins and pollutants unleashed by the collapsed buildings.

“These preliminary findings of the WTC Screening Program demonstrate that large numbers of workers and volunteers suffered persistent, substantial effects on their respiratory and psychological health as a result of their efforts,” said Dr. Stephen Levin, co-director of the World Trade Center Worker and Volunteer Medical Screening Program.

Of those screened, 51 percent suffered mental health problems and their risk of post-traumatic stress disorder was four times the rate of the disorder in the general male population, the report said.

The analysis is part of a broader study of about 12,000 people being evaluated at Mount Sinai.

The CDC also released results of the first phase of an investigation, conducted with the Mailman School of Public Health at Columbia University, into the evacuation process at the World Trade Center.

The report looked at the factors influencing decisions people made as to whether to leave the twin towers once the attacks had begun. Some were delayed due to concern about getting permission from their bosses while others stayed to shut down computers and collect personal items.

Structural damage to the building, such as debris on stairs or partially collapsed interior walls, blocked exits, the report said, and heavy congestion on certain stairways caused some to back up to seek an alternative route down. In addition, there was a lack of back-up public address systems or other communications systems.

The CDC said that only 21 percent of the workers and volunteers participating in the screening program had appropriate respiratory protection between Sept. 11 and Sept. 14, 2001, the days when the impact of dust, diesel exhaust, pulverized cement, glass fibers and asbestos was considered the greatest.

The CDC plans to continue medical screening for five years.