Metal slides, off-balance merry-go-rounds and metal chain swings have all but disappeared from U.S. schools and parks. A University of Georgia expert said new, safer play areas have vital parts you might not think of: the surfaces under them.

"Kids love to play on equipment," said Don Bower, an Extension Service human development specialist with the UGA College of Family and Consumer Sciences.

"If they play on it, climb on it or slide down it, somebody is likely to fall off," Bower said. "What they land on can make a big difference in whether a bone gets broken."

Nationwide, more than 200,000 children go to hospital emergency rooms with playground injuries. The Consumer Product Safety Commission reports that most of those injuries happen when the child hits the ground.

"Generally, the harder the surface, the more likely children will be injured in a fall," Bower said.

Testing by various agencies has assigned a head injury criteria (HIC) number to most surfaces that might go under playground equipment. Testers drop a head form from a likely height onto different materials. The extent of the possible injury sets the HIC.

"For example," Bower said, "if concrete has an HIC of 1,000, pea gravel may have an HIC of less than a quarter of that."

Playground designers choose from two main types of materials, unitary or loose fill, to cushion the falls, spills and even safe landings on playgrounds.

Unitary materials are those spongy tiles that join to make the floor or ground surface softer. Bower said these are most often used in relatively small areas like enclosed playgrounds at fast-food restaurants.

"Unitary materials are some of the most expensive at first," he said. "But they need almost no maintenance."

Designers put loose-fill materials -- sand, wood chips, pea gravel or other small pieces -- where playing children are likely to land: at the end of a slide, under swings or under climbing equipment.

Bower said loose fill costs less, but needs more care. "All those materials can be kicked, carried or moved out of their area," he said. "If it's not there, it can't cushion falls. So even if you see it in place during your first visit to a new play area, check often to make sure it stays put or is refreshed."

Parents need to look in their own backyard, too.

"Even soil or grass can compact and get hard enough to cause injury in a fall," Bower said. "Building a play area of landscape timbers enclosing a foot-deep pit of pea gravel can be less expensive than a trip to the emergency room."

Bower suggests checking out the play area at the fast food restaurant your kids love. Large corporations want to keep your kids happy and safe in their play equipment.

"Injury there means potential lawsuits," he said. "So their equipment is usually designed with the latest safety features."

Corners are rounded, fall or landing areas are well-cushioned and all bolts are hidden under covers or designed out of sight.

"Very little in these playgrounds can cause injury when used as designed," Bower said. "And even if you can't afford one of those play areas, you can measure the play equipment in your backyard against it."