More than 2.2 million people are impacted by the notice. Beyond drinking water, any water used for bathing or brushing teeth should be boiled for at least three minutes as well. Filtered waters should be also be boiled, the city says. Residents are also being advised to avoid ice and dispensed water from their refrigerators.

The situation has prompted officials to cancel school classes in four districts on Monday. Microbes that may have entered the water pose greater risks to children, seniors and those with weakened immune systems, the city said.

Officials say they are optimistic there are no bacteria in the water, since the power outage and subsequent drop in water pressure took place for a short period of time (less than one minute). However, because the pressure dipped below a certain point, the city was legally obligated to warn the public.

“The reason we’re doing the boil water notice is a regulatory requirement. Our system maintained pressure. We never lost pressure fully, so there was never an opportunity for anything to enter our system. They just fell below the regulatory requirements,” Houston Water Director Yvonne Williams Forrest told CBS affiliate KHOU.

Regulators have collected water samples for testing, a process that takes roughly 18 hours. Results are expected late Monday night or early Tuesday. The city said it will notify customers when the water is confirmed to be safe.

Some residents are questioning why the city took as long as it did to issue the boil water notice. The water pressure at one purification plant fell below minimums at 10:30 a.m. on Sunday due to a power outage of unknown origins, but the alert from the city didn’t come for more than eight hours.

This is the third time Houston residents have been advised to boil their water in the past two years. It previously occurred last February after the Winter Storm that caused a widespread power crisis and in February 2020 after a water main break.