Terrorist attacks have widespread effects on people's mental health even when they are not directly involved or are far away at the time, experts say.
Many New Yorkers suffered post-traumatic stress disorder after 9/11
A team from University College, London, reviewed existing studies into the effects of attacks.
They found that after an attack in an urban area, 11 to 13% of the general population may suffer post-traumatic stress during the following six weeks.
Experts said the study showed it was vital to assess the impact of attacks.
In the review, Chris Brewin, professor of clinical psychology at University College London found that 30-40% of people directly affected by terrorist action are likely to develop post-traumatic stress disorder (PTSD) and at least 20% still experience symptoms two years later.
These include intrusive memories and nightmares, sleep disturbance, and irritability.
People suffering from PTSD also tend to be jumpy and on edge.
In the wider population, several studies have shown that rates of "substantial stress" is extremely high in the first few days after an incident.
However the numbers start to decline in the first two weeks, and by two months have fallen by two-thirds.
Analysis after the 9/11 attacks found that 5% of people living in New York were found to still be experiencing problems functioning on a day-to-day basis more than six months afterwards.
And 28% of children studied had agoraphobia, separation anxiety or PTSD.
Children have higher levels of traumatic responses to terrorist attacks because they are less able to judge when the immediate danger has passed.
Data from other studies looking at the effect of terrorist attack on the wider population suggested the level of PTSD fell from 11-13% in the first six weeks to around 3% two months afterwards.
Professor Brewin said intense media coverage, such as the repeated images of the 9/11 attacks, could increase general levels of stress.
He added that, although treatment is available, it can be hard to contact those who might need it.
Professor Brewin is currently trying to track down people who were involved in the London bombings for a study on how the incidents affected them.
"It's very common, but people don't know if it's normal. Most people hang on and assume they will soon get better.
Dr Jonathan Bisson, senior lecturer of psychiatry at Cardiff University, said people were quite resilient, but it was important to research rates of stress after such events.
"The focus now is more on how to pick up people as it's very difficult to make contact.
"One of the biggest challenges is to increase awareness within society to what people's reactions might be, what the natural course is and when and how to get help," he said.
Anyone affected by the London bombings who would like to take part in the research can call 020 7530 6847.