A sewer testing exercise in Clarkstown this week is causing smoke to come pouring out of vent stacks in homes along Tucker Avenue.

The smoke testing is "to find water sources that aren't supposed to be coming through the sewer system," says Michael Saber, of the Rockland County Sewer District. "We call it inflow and infiltration. It comes either from the rain or the groundwater. It takes away capacity from our system."

The county says the smoke has a slight odor, but is not harmful to people or pets, and does not create a fire hazard.