SOUTH PORTLAND — Human hamster balls were the main attraction for the group that came from New Hampshire on Monday.

The school kids expected to step into transparent plastic spheres and roll around the surface of the shallow pool at Summit Adventures at the Maine Mall.

“It’s my favorite thing,” said Brianna Hanson, a 12-year-old from Exeter, N.H., who was part of the group on school vacation.

But Summit Adventures — which offers activities like laser tag, rock climbing and indoor snow tubing in what once was Filene’s — shut down its showcase attraction in January, after the State Fire Marshal’s Office indicated it would not approve hamster balls for use in 2012.

The fire marshal’s office, which inspects amusement park rides in Maine, decided that the balls would not be allowed because of a federal warning and the absence of a standard from the body that develops standards for rides.

Brianna said human hamster balls are so much fun because she can walk on water, bounce around, make others fall down or just relax. Her brother, 9-year-old Eric, likes being inside the plastic ball as it gets inflated.

Customers have had so many questions about the missing balls that Summit Adventures’ staff has been briefed about how to explain the situation, said Jeff Hunnewell, who owns the entertainment center.

A flier on a counter also explains, and gives customers a contact at the fire marshal’s office.

In March, the U.S. Consumer Product Safety Commission warned about risks of suffocation, drowning and impact injuries in hamster balls. The commission noted that the balls — sometimes called water-walking balls — are airtight and can be opened only from the outside, that people can be hurt if the balls collide or fall out of the pool, and that the balls can leak or be punctured.

The government cited one incident in which a child was unresponsive after being in a ball for short time, and another in which someone suffered a fracture when a ball fell onto hard ground.

The commission encouraged state officials not to permit the balls, saying it was unaware of any safe way to use them.

But Hunnewell said his business took precautions. The pool had a padded rail, the floor around it was padded with foam, the water was no deeper than 14 inches, and participants were under constant supervision during a strictly enforced five-minute time limit.

He said he even hired a company to test whether the oxygen levels in the balls were safe.

“We’re just trying to persuade the state to take a look at it. Is it safe to do it the way we’ve been doing it?” Hunnewell said Monday.

Hunnewell said 21,000 people used the hamster balls last year, with only two incidents — both knee-to-nose situations — that required ice packs. For laser tag, he said, about 20 incidents required ice packs.

Assistant State Fire Marshal Rich McCarthy said the federal commission’s report was concerning enough that his office decided that hamster balls can’t be used until there is a standard for them.

He said ASTM International, formerly known as the American Society for Testing and Materials, is working on one.

“The problem is, (for) the hamster balls, there is no standard,” McCarthy said. “They’re new. So there is no standard.”