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NASCAR races are measured in hundreds of miles, but it's a sport in which inches and milliseconds often mean the difference between life and death.

At the Talladega Speedway on Sunday, Dale Earnhardt Jr. suffered his second concussion in six weeks - the first was in August during a practice run - as his car piled up along with 24 others in an epic crash at 200 m.p.h.

On the track after the accident, he appeared woozy. Later he said he suffered a lingering headache.

"The fact that I felt the way I did was what concerned me after the accident in Talladega because it wasn't that hard of a hit," Earnhardt said during a news conference Thursday. "I knew having those concussions back to back was not a good thing."

He announced Thursday that he would be taking a two-week-long break from racing - and ending his chances at a championship.

"In a machismo world, that is auto racing where drivers are scared to death to lose their ride," said ESPN's Marty Smith today. "He did [take] that initiative."

It was a stunning decision in a sport in which drivers rarely remove themselves from the race. NASCAR is the latest sport dealing with the impact of concussions.

In the last five years, according to ESPN, there have been fewer than a dozen concussions reported in NASCAR's 3 series. The symptoms of a concussion can include headaches, drowsiness, blurred vision, numbness and reduced reaction times.

Dr. Steven Olvey, a University of Miami neurologist who has been studying NASCAR concussions for 10 years, estimates that every driver in the circuit has likely suffered at least one concussion in his career. Most, he says, have suffered many.

Given the neck-snapping speed of the cars hurtling around the track at 200 m.p.h., the loss of reaction time, Olvey says, is the most dangerous symptom. Even weeks after a concussion, reaction time can be half of an athlete's baseline reflexes.

"If your reflexes are affected," Olvey said today, "you can not only endanger yourself, you can endanger pit crew members, other drivers - even spectators. [The] car can go airborne, go into a crowd, that kind of thing."

The Associated Press contributed to this story.