About 30 residents of the northern B.C. town of Mackenzie have been forced to leave their homes because of a chlorine dioxide leak at a mothballed pulp mill.

A spokesperson for the provincial emergency program said Sunday the evacuation was precautionary after the toxic chemical was found to be dripping from a faulty valve on a 380,000-litre tank.

Carla Seguin said a team was able to tighten the valve to make the leak smaller.

She said officials would decide Sunday evening whether residents could return to their homes and whether nearby forestry plants that were also evacuated could reopen.

Liquid chlorine dioxide is an ingredient in making pulp, but as a gas it can damage the lungs.

The Worthington Properties pulp mill was shut down in May but the B.C. Ministry of Environment intervened in January after workers who hadn't been paid in weeks threatened to shut down its boiler and leave, which officials feared could trigger a chlorine dioxide spill.