Wynn Schwartz, a professor of psychology at the Massachusetts School of Professional Psychology, told Boston.com he was a block away when the first bomb went off and he could smell the gunpowder and, "I'm still pretty upset by it."
Schwartz said the extent to which people are negatively affected by the trauma depended on how close they were to the events -- whether they witnessed them firsthand or had a loved one who was injured or killed -- and their brain's individual coping skills.
However, there is often no way to predict how a person might cope with a traumatic event, but it's always useful to have people to share their feelings with -- a close friend, loved one or professional counselor.
But friends and loved ones should not pressure anyone to talk about the events until a person is ready, but they can make sure those affected are eating and sleeping.
Nonetheless, those not personally affected might feel acute emotions for a few days, their emotional impact will most likely be short-lived, Schwartz said.
Couple mistakenly served bag of cash at McDonald's drive-thru
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'