THOUSAND OAKS, Calif.—Authorities say 18 people including children have been sickened—seven critically—by exposure to chemicals at a Southern California swimming pool.

The Ventura County Fire Department says authorities were called to Daland Swim School in Thousand Oaks Oct 3. evening after several people reported they were having trouble breathing.

Seven people were listed in critical condition and 11 were said to have been moderately affected.

There’s no word on the type of pool chemicals involved.

A call to the swim school wasn’t immediately returned.