Forty-nine years ago, in November 1953, New York City was stricken with a six-day siege of air pollution so fierce that it killed or contributed to the deaths of 25 to 30 residents a day. That was before scientists really understood what was darkening the skies and choking people on the street. In some respects, experts say, that terrible week resembles another one — the week of Sept. 11, 2001. Despite intensive data-gathering, researchers still haven’t fully grasped the human or environmental impacts of the attacks — but some public-health advocates are optimistic that, like the early smog crises that galvanized the clean-air movement, the destruction of the World Trade Center will advance environmental knowledge and protection. To date, pollution research in general has focused on chronic, daily exposure; very little is known about the consequences of brief, intense pollution encounters. David Rosner, a professor of history and public health at Columbia University, said Sept. 11 would help fill that knowledge gap: “We are constantly building worlds that end up being dangerous, and all the turning points in public health are the times when we realize that.”