Alton Towers shut its waterpark today after visitors suffered "irritation of the eyes and throat".
Staff at the theme park took the decision to close the section at around 1pm.
Medical crews, fire brigade staff and police officers attended the site.
At the scene, 39 people were treated. They were discharged with no further medical treatment needed.
There were six children among those examined at the site in Staffordshire.
An Alton Towers spokeswoman told the Standard: “At around 1 o’clock today we took the decision to close the Waterpark after a small number of people reported irritation of the eyes and throat.
"The Fire Brigade, WMAS and Staffordshire Police attended the resort, as is standard for any site where chemicals are used.
"39 people, including 6 children, were assessed by medical staff but all have been discharged with no treatment required.
"We are helping the Fire Brigade with their ongoing investigation."
The waterpark is due to reopen on Wednesday. Tuesday had been planned to be a closed day due to it being the low season.
"The Waterpark remains closed whilst we complete our investigations," the spokeswoman added.
West Midlands Ambulance Service confirmed its attendance at the incident.
