At least two of the victims had serious respiratory complaints or complaints of burning eyes, said Sacramento City Fire Department Assistant Chief Niko King.
"It started out with just a few people with complaints of burning respiratory tracts and eyes," King said.
He and hospital officials did not immediately have conditions on the victims. Three of those taken to hospitals were park employees.
Lifeguards quickly moved visitors away from the wave pool after the 2 p.m. incident.
King said it appears a mechanical failure in the pool's equipment caused the excessive chlorine release.
"The water dilutes it pretty well. But obviously, because it's so strong, it turns to gas. That's what they're affected by," he said.
Chlorine is used to sanitize pool water, but in high concentrations, particularly once it's airborne, the gas can turn toxic.
The California Division of Occupational Safety and Health, known as Cal/OSHA, dispatched investigators from its amusement ride and tramway unit, agency spokeswoman Erika Monterroza said.
The Sacramento district office of Cal/OSHA will open a separate investigation Tuesday because water park workers were injured and exposed to a safety risk, she said. The second investigation is because the division also has jurisdiction over workplace safety and injuries.
"We were in the wave pool and a big cloud of chemical was released. All of a sudden it was just really hard to breathe. It hurt the back of your throat. It was hurting people's eyes. We just had to get out of the water," visitor Alyssa Dronenburg told KCRA-TV in Sacramento.
KCRA said Raging Waters issued a statement saying it would cooperate with the Cal/OSHA investigation and will keep the wave pool closed until the investigation is complete.
State records showed one minor safety violation at the Sacramento water park since 2008, unrelated to the chlorine system.
"You're at a water park, there's going to be chlorine, but they should manage the chlorine, you know? Have it under the levels that are supposed to come out at one time," Dora Chavez of Manteca told KXTV in Sacramento. She said the smell made her feel sick.
David Ong, a spokesman for UC Davis Medical Center in Sacramento, said nine people were taken to his hospital, including three women and six children. Their conditions were not released.
Mark Schiller told KXTV his wife and 5-year-old son were among those taken to UC Davis Medical Center.
"My son couldn't get his breath at first and they administered oxygen," he said.
No one answered the phone at Raging Waters' Sacramento office, and repeated messages left with the park's parent company, Newport Beach-based Palace Entertainment, was not immediately returned. A message left with Cal Expo, the state-owned site where the park is located, also was not immediately returned.
Raging Waters operates three parks in California: in Sacramento and San Jose in Northern California, and San Dimas, east of Los Angeles.
Associated Press writers Adam Weintraub, Juliet Williams and Judy Lin contributed to this story.