Two Florida students were found dead inside a large, deflated helium balloon that they apparently pulled down to earth and crawled inside, Tampa police said.

The bodies of Jason Ackerman and Sara Rydman, both 21, were found Saturday partially inside a deflated helium balloon at the entrance of a condominium complex near Tampa.

The 2.4-metre balloon was being used to advertise the complex.

"It was more a fun thing they thought they were doing," said Linda Rydman, mother of Sara Rydman, adding that the two were apparently breathing helium from the balloon to make their voices go higher.

But the U.S. Compressed Gas Association warned that inhaling helium is dangerous because it can quickly lead to brain damage and death from lack of oxygen.

The county medical examiner said Sunday the official cause of death will not be released for six weeks, until toxicology results come back.