Children exposed to toxic dust from the destruction of the World Trade Center have higher rates of respiratory problems including asthma.
According to the department's World Trade Center Health Registry, half of the 3,100 children enrolled in the registry developed at least one new or worsened respiratory symptom, such as a cough, after the Sept. 11, 2001, destruction of the twin towers.
About 45 percent of the children were caught in the dust cloud when the towers collapsed, and those children were twice as likely to be diagnosed with asthma after 2001 as those not caught in the dust, the survey found.
The registry was begun in 2003 in an effort to track the health of residents, trade center workers, and rescue and recovery teams who may have been exposed to the dust and debris from the collapse.
Although the Health Department canvassed lower Manhattan neighborhoods to encourage enrollment, only a fraction of all those who lived, worked or went to school near the trade center completed the survey.
Deputy Health Commissioner Lorna Thorpe said the 3,100 children in the registry are between 20 and 30 percent of all the children who would have been eligible.
She acknowledged that there is a selection bias - parents whose children developed health problems after 2001 were more likely to complete the survey than parents whose children did not develop health problems.
"The selection biases occur in a lot of studies, and we are aware that they exist," she said.
Thorpe said the asthma numbers are "concerning," but further research is needed to learn the extent to which they can be attributed to selection bias.
The mental health portion of the survey showed that only 3 percent of the children surveyed had symptoms suggestive of post-traumatic stress disorder, a level that is not above that in children elsewhere. As with asthma, children who were caught in the dust cloud experienced higher levels.