New York has released more than 170,000 pages of previously sealed documents about the state of the air after the attacks of 11 September 2001. What they contain confirms what survivors have been saying for decades: the city authorities knew the pollution at Ground Zero was dangerous while publicly assuring people the air was safe to breathe.

Mayor Zohran Mamdani presented the documents at a press conference. "As the years pass and the number of victims grows, we pay the price of 9/11 every time we lose another New Yorker too early," he said.

The numbers behind the disaster

Nearly 3,000 people died in the attacks themselves. Thousands of others - among them rescuers and workers who cleared the site - died later of cancers and illnesses caused by toxic exposure. According to the 9/11 Memorial and Museum, at least 140,000 people from all 50 states are enrolled in the World Trade Center health programme, and 49,000 of them have been diagnosed with a cancer linked to the attacks.

The documents did not come out of goodwill. The organisation 9/11 Health Watch filed a lawsuit to force them into the open. Among them are internal notes from October and November 2001 in which city officials warned of elevated levels of toxins, including asbestos and hazardous chemicals.

Who knew what, and when they said it

The former head of the Environmental Protection Agency, Christine Todd Whitman, maintains that she said the air was "safe to breathe" on the basis of the scientific analyses available at the time. But a 2002 letter from Congressman Jerry Nadler to then-mayor Michael Bloomberg's office states that the Agency "abdicated its responsibility" for monitoring indoor air quality around Ground Zero.

The firmest evidence that somebody was calculating risk rather than safety is a legal memo sent to Deputy Mayor Robert Harding: it estimated the city's exposure to possible lawsuits from around 35,000 people. That is not the figure of an institution that believes the air is clean. That is the figure of an institution working out what it will cost when this comes out.

It took twenty-five years and a lawsuit to publish what was written down in the very first weeks. Who answers for it now, when the documents are public and half the people they concern are already gone?