NEW YORK (CNN/Money) - The National Highway Traffic Safety Administration passed two new regulations this week designed to make car seats safer.

On Tuesday, the agency announced a new standard for head restraints aimed at reducing the number of whiplash injuries. The following day, NHTSA unveiled a rule requiring lap and shoulder safety belts for all rear center seats.

More than 270,000 whiplash injuries occur annually in motor vehicle crashes, the agency said.

The new standard will require that head restraints be higher and positioned closer to the head. It will also mandate that all head restraints be adjustable and that they lock in place once adjusted.

"By standardizing the best practices in head restraint performance, we can reduce the most common form of injury in rear-end collisions," said NHTSA Administrator Jeffrey W. Runge, M.D. "Many of these injuries are more than just a pain in the neck. They become chronic, painful, debilitating and costly."

All passenger vehicles, including pick-ups and sport/utility vehicles, that are manufactured starting in September 2008 will have to comply with the head restraint standard. NHTSA estimates the cost per vehicle of meeting the standard at about $4.51 for front seats and $1.13 for rear seats equipped with head restraints.

In recent tests by the Insurance Institute for Highway Safety, most of 73 seat/head restraint combinations received a rating of "Poor" for protecting occupants in slow to moderate-speed rear-end collisions.

Car manufacturers will also have to comply with the new rear center safety belt rule by 2008. Currently, 23 percent of new cars and about half of new light trucks -- a designation that includes SUVs -- have only a lap belt in the rear center seating position.

The new seatbelt rule will save and estimated 10 to 23 lives each year and will prevent 245 to 495 injuries a year, according to the agency.