PITTSBURGH — The Pittsburgh Water and Sewer Authority is telling about 100,000 customers to boil their water before drinking it after low levels of chlorine were found in tests conducted by the state Department of Environmental Protection.

The DEP has been testing the city’s water as part of an ongoing investigation into its water treatment system.

The issue has prompted Pittsburgh Public Schools to close 22 schools and two early childhood centers today.

Officials say low levels of chlorine could enable giardia to grow in the water. The parasite causes severe diarrhea.