SAN DIEGO -- They're fun and popular with kids -- air-filled Jumpees, slides and obstacle courses that are all in one location.

"It is so much fun and there is a bunch of places to go," said Jumpee fan Lily Martinez.

Inflatable party zones are all the rage with the under-6 crowd.

However, in Washington state, a recent birthday party turned deadly at the Hoppin House when two adults fell on 3-year-old Jacob Pierce.

The coroner's office said the boy died when his skull was crushed.

Rady Children's Hospital Dr. Ghazala Sharieff said, "Think about playing football without a helmet. So, if you have a large adult or teenager falling on top of a child, not only do you worry about head injuries but suffocation injuries as well."

Dr. Sharieff said it's also easy to suffer fractures.

Richard Clark, owner of inflatable party zone Pump It Up, said, "Safety is paramount and it's a primary concern."

Clark said in Pump It Up's 3 years of business there has been only one injury -- when an adult fell and twisted her ankle.

"That was one of our concerns when we decided to have the party here (at Pump It Up)," said parent Raul Duran.

At a party for Duran's 6-year-old daughter, a safety video reassured him of Pump It Up's commitment to safety.

Every child watches the video before they get to play.

Clark said, "That's the one thing that will close your door… one kid gets hurt."

The Consumer Product Commission said injuries from inflatable rides have increased significantly from 1,300 in 1997 to 4,900 in 2004 – a nearly 400 percent increase in eight years.

The Web site RideAccidents.com tracks inflatable ride injuries. For example, in June 2005, 12 children were injured. In July 2005, 6 were hurt and the list continues.

One young girl told 10News, "We get to jump on it. When we fall, there's a cushion."

The air-filled cushions could give children a false sense of security so doctors at Rady Children's Hospital stress adult supervision.

Between 2002 and 2005, the CPSC reported 4 deaths from inflatable rides not including the death of Jacob Pierce.

Pierce's mother said the accident could have been prevented with better supervision.