People caught up in the London bombings are to have their health monitored to keep a check on any long-term effects.
There are 839 names on the Health Protection Agency's (HPA) register and experts think it could double.
People who were directly exposed to blood, smoke, fumes or the blasts on 7 July are being tracked down through hospitals and police information.
The main problem is likely to be the post-traumatic stress and the long-term mental health of survivors, according to BBC.
Dr Katri Jalava, an HPA scientist, said: "It is often the case that the effects of incidents like this are still felt years afterward."
The London 707 register follows similar projects in New York and Madrid, where mental health was the main long-term concern after the attacks.
More information is being collected on people who contacted NHS Direct and those treated by ambulance services and occupational health units.
Some were splashed by chemicals like oil, others by blood, potentially exposing them to diseases.
"People need to be followed up, looking out for particular symptoms, such as toxic symptoms, but that is quite unlikely to occur in this case really," said Dr Jalava.
"So the psychological effects are of most concern."
The HPA is also gathering as much information as possible on those affected.
They already know that 50% came from inside London, and, where they have details of injuries - 44% inhaled fumes, 7% smelled fumes and 22% were exposed to loud noise.