Police say a rider was carrying the drain cleaner, and it accidentally came in contact with passengers on the bus. Someone may have tripped over the container, or the cleaner may have eaten through an improper container.
Police originally classified the incident as an "attack", but instead called it an accident.
Three people suffered burns, including two children, ages 8 and 5. All injuries are considered minor.
The man who brought the liquid on board is in custody, but police say it is unlikely he will be charged with anything.