(NewsNation) — Houston’s 2.2 million residents are being told they must boil their tap water before drinking or using it to brush their teeth, cook or bathe until further notice.

At least four school districts canceled classes Monday due to the boil order, and other areas schools that held classes said they would provide drinking water, the Texas Tribune reports.

People in the country’s fourth-largest city need to boil water for at least 2-3 minutes and then let it cool before drinking or using it to be safe. Officials have said the order will likely not be lifted until Tuesday morning.

A power outage at a water plant caused the pressure to drop, which triggered a regulatory requirement to issue a boil water notice. The pressure is what prevents contaminants from getting into water.

Texas Gov. Greg Abbott said in a statement Sunday that power was lost at three different water treatment plants, and that an investigation is underway. Samples are being collected to determine the safety of the water and when the order can be lifted.

The notice was issued six hours after the pressure drop, but officials say anyone who drank water prior to the boil water order should still be safe and that it is just a precaution.

Local residents turned to social media to express outrage that the notification took so long, The Guardian reported, although officials said the delay was due to workers needing to check the cause before alerting the public.

Residents in the affected area should also avoid using water or ice from automated dispensers.

Mary Hall contributed.