They included rescue and recovery workers, neighbouring office staff and passers-by.

A follow-up study of more than 46,000 people caught up in the terrorist attacks of September 11, 2001, found one in 10 had been diagnosed with asthma five or six years after the disaster. None of these 4,600 individuals had a previous history of the disease.

There was a strong association between exposure to the choking dust cloud generated by the collapse of the towers and asthma, the study found.