A chlorine explosion at a water treatment plant injured four people Friday and heavily damaged the facility, authorities said.

No water-quality problems were expected because other plants in the system were still operating, said Judy Dahl, business manager for the Little Thompson Water District.

A truck was unloading chemicals at the Carter Lake Filtration Plant, about 40 miles north of Denver, when the explosion happened, but it wasn't clear what set it off, Dahl said. Chlorine is used to disinfect water but is toxic as a gas and poses a risk of fire and explosion. In World War I, it was used as a chemical weapon.

Two of the four injured workers were treated at a hospital, Larimer County sheriff's spokeswoman Eloise Campanella said, but none of the injuries was considered serious.

Authorities blocked roads into and out of the area and advised nearby residents to close windows and turn off air conditioners, Campanella said.

Debra Graves of the Berthoud Fire District said it was not known how much chlorine leaked.