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Natural gas prices in the futures market hit $5.18 per 1,000 cubic feet Friday, the highest level in three and a half years.

Prices have risen 29 percent in two weeks, and 50 percent year-over-year.

Natural gas is burned for heat and electricity.

The U.S. Department of Energy said supplies have fallen 20 percent below a year ago — and that was before this latest cold spell, the AP reported.

Cold weather has spread across the U.S. from Texas to Maine twice this winter, straining natural gas supplies.

In some urban areas of the Northeast, natural gas was selling “for 10 times [the] benchmarks because producers couldn’t get their gas into packed pipelines, according to Citibank energy analyst Anthony Yuen,” the AP said.

An unscheduled shutdown of a nuclear power station in Maryland sent the spot natural gas price to a record $120 per 1,000 cubic feet in some East Coast markets.

“To put that in perspective, that’s equivalent to oil at more than $700 per barrel,” the AP said.