All Houston residents were told Sunday to start boiling their water, after pressure from the city’s main water system dropped.

The water pressure has been restored, according to CNN, but the order is likely to stay in place at least through Monday. Houston’s Public Works Department said it would not be lifted until proper testing takes place to ensure the system is working well and is safe.

The Texas Commission on Environmental Quality requires 20 PSI for the water to be deemed safe. But during a power outage at the East Water Purification Plant, that pressure dropped and officials said the system might not be fixed until Monday night or Tuesday morning, according to KPRC-2, an NBC News affiliate in Houston.

When pressure drops, the worry is that harmful bacteria or microbes will infiltrate the system.

CNN quoted Erin Jones, the spokesman for public works, noting that “once the water sample plan is approved, the water samples have to be collected by the City of Houston Public Works who will then submit the water samples to (the commission) and the samples have to sit in its lab for 18 hours to see if anything grows on them.”

The article noted that the state lab has to give the OK to lift the boil-water order. The city “cannot rescind the boil order independently,” Jones told CNN.

The city’s order covers water used to cook, drink, wash hands and face, and brush teeth, the city said. Those requiring boiling water for three minutes to kill bacteria and microbes that could be harmful. The order also warns against using water from refrigerator lines or automatic ice machines.

The Houston Independent School District canceled classes for Monday as a result of the boil-water order. And CBS affiliate KHOU-11 said other districts were following suit, though it noted that Houston Community College would offer services remotely.

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Most area colleges and universities said they’d be open, but food services and potable water would not be available, so they warned students to make their own arrangements.

And dozens of wholesale customers who get water purified by the same plant were told to follow their jurisdiction’s guidance on whether to follow the same boil-water order, per KPRC-2.

The news station said it is “investigating the City of Houston water department,” noting an earlier report that “more customers reported problems with low water pressure than in any other major metropolitan water district surveyed by JD Power last year.”

The notice said those most at risk of harmful bacteria are young children, older adults and those with a weakened immune system.