They look safe. Unlike trampolines, you can’t fall off of a big bouncy inflatable bubble house. They’re a hit with kids and parents; showing up at private homes for birthday parties and family get-togethers. They’ve also become a staple at amusement parks and fairs.

According to a new study, they may not be quite as safe as they appear.

Kids often pack themselves into bounce houses, jumping high and hard. That can send smaller children flying through the air causing them to land outside the house or to collide with another child.

Looking at the data, the numbers suggest 30 kids a day are treated in emergency rooms for broken bones, sprains, cuts and concussions from bounce house accidents. As the popularity of bounce houses has increased, the number of injuries has also climbed. In 1995 there were fewer than 1,000 injuries reported. In 2010 the number is closer to 11,000. That's a 15-fold increase, and a doubling just since 2008.

"I was surprised by the number, especially by the rapid increase in the number of injuries," said lead author Dr. Gary Smith, director of the Center for Injury Research and Policy at Nationwide Children's Hospital in Columbus, Ohio.

Smith and his colleagues analyzed data on ER treatments for nonfatal injuries linked with bounce houses. The records are kept by the U.S. Consumer Product Safety Commission (CPSC.)

About 3% of children were hospitalized, mainly with broken bones. More than a third of the injuries occurred in children under the age of 5. The CPSC already recommends that children under the age of 6 not be allowed to use full size trampolines. Smith believes that barring children 6 and under from entering bounce houses is a good idea that should be considered. If the CPSC is unwilling to bar them for young children, he thinks parents should consider making that a house rule. No small children in the bounce house. Bounce houses come in a variety of sizes – but smaller or larger isn’t necessarily a factor in accidents. "There is no evidence that the size or location of an inflatable bouncer affects the injury risk," Smith said.

Other recommendations, often listed in manufacturers' instruction pamphlets, include not overloading bounce houses with too many kids and not allowing young children to bounce with much older, heavier kids or adults, said Laura Woodburn, a spokeswoman for the National Association of Amusement Ride Safety Officials.

The study only looked at nonfatal accidents, but there have been recorded instances of fatalities. Separate data from the product safety commission show four bounce house deaths from 2003 to 2007, all involving children striking their heads on a hard surface.

A group that issues voluntary standards says that bounce houses should always be supervised by a trained operator, and that children should be prohibited from doing flips and purposely bouncing into other children.

Bounce house injuries can be similar to those linked with trampolines, and the American Academy of Pediatrics has recommended against using trampolines at home. The author notes that policymakers should offer the same special consideration to bounce houses.

The study was published in the journal Pediatrics.