Alton Towers has closed its water park after almost 40 guests, including a number of children, said their eyes and throat had become irritated in the Staffordshire amusement park.
The attraction, owned by Merlin, said it took the step at about 1pm because "a small number of people" had reported feeling unwell.
Emergency service official raced to the scene, while an air ambulance was dispatched to offer support.
However after 39 people were assessed, including 6 children, all were found to require no further treatment.
An Alton Towers Resort spokesperson added: “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.”
Staffordshire Fire and Rescue Service said it was called to the site at about 1.50pm after reports of both staff and guests "with irritation to the eyes and throat".
A fire service spokesman said crews were "carrying out an assessment".
Staffordshire Police confirmed their officers attended after being alerted at 1.45pm by West Midlands Ambulance Service (WMAS).
The water park is expected to remain closed until Wednesday. The main theme park, which can see two million pass through its gates in a year, is due to open at the end of the month following the winter break.
