(CBSDFW/AP) – Millions of people in Texas remained in the dark early Tuesday amid subfreezing temperatures.

Nearly 4 million homes and businesses were without power early Tuesday in Texas, where temperatures dipped into the single digits overnight.

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As of Monday night, 34 patients were treated for hypothermia and 13 were taken to the hospital in Tarrant County.

“What we’re seeing is a large number of hypothermia calls not because people are outside, but because they’re home but have no electricity and have no heat,” said Matt Zavadsky with MedStar.

Oncor said it is working to get the lights back on, especially with another round of winter weather on the way, but the demand on the system is taking a toll.

“It’s two separate power emergencies,” said Oncor spokesperson Kerri Dunn. “On one hand we have the winter storm we have those weather impacts, So for that we’ve been prepositioning our crews and our resources.”

Dunn said the other problem is supply and demand.

“Our focus for that, although very challenging for our customers and for our personnel that are working it, is ultimately to help preserve the integrity of the electric grid.”

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Oncor has tried to move around the outages, but the grid is under such strain that they can’t. For now, they are asking North Texans who have power to try to keep their temperature at 68 degrees to lessen the load. As for why some people have experienced long outages and others none, Oncor said that has to do with how the system is set up.

The rolling outages are designed so that hospitals and critical infrastructure won’t be affected. Thus, if you’re on a power line that also supplies power to a hospital, you likely won’t see your lights go out.

Dallas Mayor Eric Johnson tweeted about his displeasure with the situation Monday night, saying, “I have been as frustrated as everyone else today. I know this has been extraordinarily difficult. These power outages are unacceptable, and ERCOT needs to answer for them.”

Meantime, across the Metroplex, emergency warming shelters have opened up.

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In Houston, a woman and a girl died from suspected carbon monoxide poisoning at a home without electricity from a car running in an attached garage, police said.