David Shukman reports on the thousands who have become ill from the toxic dust that blanketed Lower Manhattan after the Twin Towers collapsed on Sept 11th. The buildings released a cocktail of deadly carcinogens including, asbestos, lead, mercury and PCBs.

Frontline responders such as fire-fighters, police and emergency medical workers breathed in the contamination for several weeks as they toiled at Ground Zero. The fires burned for a hundred days and many of the emergency workers toiled without respirators or proper protection amid the dust and debris.

Now officials say more than 18,000 people have received medical treatment in the last 12 months for World Trade Center related conditions - many of them serious. The head of the federal programme overseeing victims compensation says he expects more people to die because of their exposure.

Nearly three thousand people perished on the day, but the suffering resulting from the attack is far from over.