Houston, Texas issued a boil water notice on Sunday night for over 2.2 million people after a power outage occurred at three water treatment facilities on Sunday morning. Residents of Houston should boil all water used for drinking, bathing, or even just brushing teeth, for at least three minutes, according to a tweet sent by the city of Houston just before 7:30 p.m. local time, 8:30 p.m. ET.

Residents of Houston are encouraged not to use chilled water that runs through pipes in home refrigerators. The city also notes residents should stay away from ice cubes created in automatic ice machines.

Regulators are collecting water samples, which take about 18 hours to process, meaning the boil water notice will likely be lifted early Tuesday if everything appears to be safe. As the Houston Chronicle notes, Houston has seen two other boil notices in the past two years—during Winter Storm Uri in Feb. 2021 and after a water main break in Feb. 2020.

The power outage occurred around 10:30 a.m. local time, according to the Texas Tribune, causing water pressure to drop below levels considered safe by health regulators. Low water pressure can result in a reversal of the system, allowing potential contaminants to enter the drinking supply, though city water officials don’t believe that has happened.

“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 local TV news channel KHOU.

Even though Houston water officials don’t seem too concerned, the public school district has cancelled all classes for Monday. School offices will also be closed.

“Due to the Boil Water Notice issued by the City of Houston late this evening, all Houston ISD schools, offices, and facilities will be closed Monday, Nov. 28, 2022. #HISD will closely monitor the situation and provide additional updates regarding operations tomorrow,” the school district said in a tweet.

Texas Governor Greg Abbott issued a directive Sunday night Texas Division of Emergency Management (TDEM) and the Texas Commission on Environmental Quality (TCEQ) to use necessary resources to make sure Houston’s water was safe to use.

“The State of Texas is immediately responding and deploying support to Houston as they work to get a safe supply of water back online,” Governor Abbott said in a statement published to his website.

“We have been in contact with Mayor Turner to offer the full support of the state, and we’re currently working to fulfill the city’s request for help with rapid turnaround of water sample results. I thank TDEM and TCEQ for swiftly responding to help address this issue. We urge those that the boil water notice affects to continue heeding the guidance of local officials and take adequate precautions when boiling and using water. Together, we will ensure our fellow Texans are supported while the city’s water supply returns.”