Lots of unknowns: definition, not necessary to have a blow to the head, whiplash can cause it, brain being jarred, shaking violently inside the skull. Brain swelling causes pressure, leading to more damage. Statistics on traumatic brain injuries: 1. 3 million individuals yearly in the usa, half because of mvas- motor vehicle accidents, then, sports, falls, violence. Industrial accidents: head injuries are the most common form of brain damage in people younger than 40, two important factors, age- children (and older adults) are more vulnerable, sex- males between 15 and 30 are more vulnerable. Early symptoms: headache, dizziness, vague memory, disorientation, nausea, was assumed to be benign . In some cases, lasting more than six months: Prolonged symptoms: headaches, light and noise sensitivity, difficulty concentrating, irritability, apathy- no motivation, cognitive (eg. problem solving and memory) Long-term consequences: not clear and it depends , some findings, cumulative effects- multiple concussions.