Emergency services were called to Wild Duck Holiday Park in Belton near Great Yarmouth in Norfolk at 14:35 BST.
Police said the entertainment complex was evacuated but the pool was declared safe at about 17:30 BST.
Norfolk Fire Service said vehicles including an environmental protection unit dealt with a chlorine leak.
An East of England Ambulance Service spokeswoman said swimmers had been treated for breathing and asthma problems. Eight were taken to hospital in a stable condition.
Tom Cassidy, a spokesman for the park's owners Haven Holidays, said about 40 people were treated with 33 of them taken to hospitals in Gorleston, Norwich and Ipswich as a "precaution".
He said: "There was a chemical leak in the indoor swimming pool in the entertainment complex and it was evacuated while police and fire and rescue were on their way.
"The casualties have been described to me as walking wounded."
He said other guests had been allowed back into their caravans, although the swimming pools and entertainment complex remained sealed off while health and safety officials carried out their investigations.
Police said none of the injuries were thought to be serious.
The Wild Duck Holiday Park has indoor and outdoor pools, a restaurant, play area and sports court.
About 40 people were treated by paramedics for breathing problems following a chemical leak at a holiday park swimming pool.