Car accidents can cause a serious head injury that is not readily apparent.

You’re driving to work with a colleague, taking your usual route, mindful of the other cars and pedestrians that make up daily traffic in your neighborhood. Suddenly, you’re broad-sided by a speeding sports car that came out of nowhere . . . or, stopped at a red light, you are unexpectedly rear-ended by a SUV.

You hit the steering wheel, and your passenger hits the windshield.

Everyone’s first instinct when a motor vehicle accident happens is to ask, “Are you all right?” You check for obvious injuries: Is anyone unconscious? Can you move your arms and legs? Are you or your passenger bleeding?

When no one appears injured, you breathe a sigh of relief: Everyone’s okay.

But sometimes everyone is not okay. You or your passenger may have suffered a traumatic brain injury (TBI) that is not obvious, and immediate treatment may make a difference.

While federal agencies such as the National Highway Traffic Safety Administration compile statistics on motor vehicle accidents and the number of people killed or injured, there is no national database that specifically quantifies head injuries and resulting traumatic brain injuries. Here are some things we do know, however, from government statistics, neuroscientists, and consumer safety studies:

• Millions of people suffer head injuries each year, most of which are minor because of the protection the skull provides the brain. The symptoms of these minor injuries usually – but not always - disappear on their own.

• Motor vehicle accidents are the leading cause of all head injuries. These accidents cause about 28% of traumatic brain injuries; sports injuries cause another estimated 20%. Other common causes are household falls and physical assaults.

• Traffic accidents account for half (49%) of the 500,000 head injuries serious enough to require hospitalization each year. Altogether, traumatic brain injuries cause about a third (34%) of all injury deaths in the United States.

• There is increasing scientific evidence that brain damage can be caused by head injuries even when there has been no loss of consciousness. The result can be not only long-term physical disability, but emotional, intellectual, and social trauma. Just in the last few years, National Football League studies and Congressional hearings have documented these symptoms among professional football players who have suffered concussions, and some of them have committed suicide.

If you wonder how a bump on the head can lead to major brain injury, think of it this way: In a moving car struck by another moving car, in a split second your brain goes from the car speed to zero.

We learned in science class how complex the human brain is, directing all of our thoughts and actions from this tiny control center in our head. So it’s not surprising that so many things can go wrong when the brain is subjected to severe trauma, especially one inflicted at high speed.

Generally, head injuries are classified as either open or closed:

• Open head injuries are those incurred when an object fractures and goes into the skull. In a traffic accident, for example, this can happen when a driver or passenger goes through the windshield.

• A closed head injury is one where an accident or object causes a strong blow to the head, but the skull is not broken. For example, this could happen to a driver who hits the steering wheel or another hard component of a car.

If you suffer either kind of head injury, there is a very real possibility of long-term brain damage. That’s why immediate diagnosis and treatment are so important.

One serious consequence of head injury is brain contusion, or bruising, which happens when the brain’s soft tissue is squashed against the hard skull. Blood may collect on the surface of the brain, called subdural hematoma. Subarachnoid hemorrhage is bleeding between the brain and the tissues that cover the brain. Large amounts of blood can cause areas of the brain to stop working, and can result in permanent brain damage.

These kinds of serious brain injuries can cause coma; chronic headaches; loss of sensation, hearing, vision, taste, or smell; paralysis; seizures; or speech and language problems.

Perhaps the most common kind of traumatic brain injury is a concussion, a closed-head injury that occurs when the brain is shaken back and forth in the skull by the impact of, for example, a motor vehicle accident. A concussion can be mild or severe, resulting in short-term loss of consciousness or a longer-term coma.