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When Thomas Phelan and Keith Young died within a day of each other in March, in both cases it was as a result of cancer. But the underlying cause of the firefighters’ deaths (aged 45 and 53) was the event they both witnessed up close 17 years earlier – the 9/11 attack on New York.

The names of Phelan and Young will not be added to the official tally of nearly 3,000 people killed in the attack. However, according to records maintained by the Uniformed Firefighters’ Association of Greater New York City (UFANYC), theirs were the 172nd and 173rd deaths of firefighters to have occurred due to 9/11-related illnesses, and the sixth and seventh so far this year.

About 400,000 people are believed to have been exposed to toxic contaminants or suffered injury or trauma in Lower Manhattan on that September day, according to the US Government’s Center for Disease Control and Prevention (CDCP). The UFANYC has said that roughly one in eight firefighters who were at Ground Zero have since come down with cancer.

According to the CDCP, just under 70,000 people who helped during 9/11 have applied for medical aid after the disaster, as have about 14,300 people who were in New York City at the time. Among the main illnesses treated are chronic coughs, asthma, cancers and depression.

The New York Committee for Occupational Safety and Health (NYCOSH) says that about 6,000 of the 9/11 first responders are now living with cancer, with thousands more suffering breathing problems or mental health issues. Many, it said, had “suffered severe exposure to numerous WTC-derived contaminants”.