A short test of the 'Tribute in Light' illuminates the sky over the World Trade Center site Wednesday, Sept. 10, 2008 in New York. The Tribute in Light will be turned on for the evening of Thursday, Sept. 11, in honor of those who lost their lives seven years ago in the terrorist attacks on the United States.

New data from a public health registry that tracks the health effects of 9/11 suggest that as many as 70,000 people may have developed post-traumatic stress disorder as a result of the terrorist attacks.

The estimate, released Wednesday by New York City's Department of Health, is based on an analysis of the health of 71,437 people who enrolled in the World Trade Center Health Registry. They agreed to be tracked for up to 20 years after the Sept. 11, 2001, attacks, and the study was based on answers they volunteered about their health two and three years after the attack.

Of the estimated 400,000 people believed to have been heavily exposed to pollution from the disaster, data suggests that 35,000 to 70,000 people developed PTSD and 3,800 to 12,600 may have developed asthma, city health officials said.

They include rescue and recovery workers, lower Manhattan residents, area workers, commuters and passers-by.

Overall, half of the respondents said they had been in the dust cloud from the collapsing towers; 70 percent witnessed a traumatic sight, such as a plane hitting the tower or falling bodies; and 13 percent sustained an injury that day.

"The consensus among physicians is that when it comes to physical health, the vast majority of people felt symptoms in the first year," said Lorna Thorpe, the deputy commissioner for epidemiology at the New York City Health Department. "A small proportion of people, however, developed symptoms years later. And in some cases, it's hard to tell whether they're World Trade Center-related or a result of allergies or existing conditions."

The post-traumatic stress disorder rate was highest among injured, low-income and Hispanic study volunteers. In general, minorities and low-income respondents experienced higher rates of mental and physical problems, as did women.

The study was conducted by the city health department and the federal Centers for Disease Control's Agency for Toxic Substances and Disease Registry. It was released in the Journal of Urban Health.