Fire crews were called to a chemical incident in Leyland after three people were taken to hospital.
It is understood the three patients worked for a water company which had been collecting water samples from a stream in Wigan Road on Wednesday.
The three casualties attended hospitals in Preston, Wigan and Rochdale suffering various symptoms due to exposure of unknown chemical in a water source.
Fire-fighters from Bamber Bridge and Chorley attended the area at around 11.45pm and were then joined by a hazardous materials team.
Crews spent around five hours at the scene assessing the chemical in the water. It was later found to be low risk.