School kids in Kitakyushu City in southwestern Japan got sick after a pool class on June 23. 25 out of 46 students complained of rashes and numbness on their hands and abdominal pain, and were checked at the hospital.

The city’s board of education released a photo on June 27, saying that it was possible that the pupils got pressure marks on their hands from touching plastic tiles laid out on the side of the pool.

The city also confirmed there were no detrimental substances found in the pool or surrounding trees and plants.