Democratic lawmakers are pushing Bitcoin mining companies in the US to divulge how much electricity they use in an effort to assess how that will impact nearby residents and the environment. Eight members of Congress sent letters yesterday to six companies that mine Bitcoin in the US asking them about the energy and environmental impact of their operations.

“Given the extraordinarily high energy usage and carbon emissions associated with Bitcoin mining, mining operations raise concerns about their impacts on the global environment, local ecosystems, and consumer electricity costs,” the letters say. The letters were signed by Senators Elizabeth Warren (D-MA), Sheldon Whitehouse (D-RI), Jeff Merkley (D-OR), Margaret Hassan (D-NH), and Ed Markey (D-MA). Representatives Katie Porter (D-CA), Rashida Tlaib (D-MI), and Jared Huffman (D-CA) also signed.

The congresspeople asked the companies to answer questions about how much electricity they use, where it comes from, and how they plan to grow in the US. This comes on the heels of an oversight hearing on cryptocurrency mining’s impact on energy held by the House Energy & Commerce Committee last week. During the hearing, experts and lawmakers debated whether cryptocurrencies might play a role in promoting renewable energy or totally derail US climate goals.

Mining certain cryptocurrencies, most notably Bitcoin, gobbles up huge amounts of energy. If Bitcoin were a country, it would rank 27th in the world for its electricity consumption — using more electricity than the country of Ukraine in a single year. If that electricity is generated from fossil fuels, it comes with a lot of pollution that adds to the climate crisis.

The reason Bitcoin uses so much electricity is that it relies on an energy-intensive process called “proof of work” to keep its ledger secure. It requires miners to use specialized computing equipment to solve ever-more-complex puzzles in order to verify transactions. The miners are rewarded with new coins in return.

The proof of work system, which is intentionally energy inefficient in order to dissuade people from tampering with the ledger, is what worries some policymakers and many environmental experts. Different processes use just a fraction of the energy required for proof of work.

In the US, miners’ hunger for energy has already revived aging natural gas and coal plants. For instance, one of the companies that received a letter yesterday, Stronghold Digital Mining, bought two Pennsylvania power plants that now burn coal waste primarily to mine Bitcoin. The company says on its website that, by burning coal waste, it’s helping to clean up “areas that have been ravaged by its presence.” But once burned, the waste pollutes the air and heats up the planet. The EPA has recently cracked down on power plants that dispose of coal waste on site, including two that have been used to power cryptocurrency mining, E&E News reported last week.

Coal waste aside, industry advocates argue that Bitcoin and other cryptocurrencies that use proof of work can clean up their act with renewable energy. And since battery technology currently isn’t advanced enough to store renewable energy whenever there’s more solar and wind power than the grid can handle, they argue that mining operations can make use of the excess energy so that it doesn’t go to waste.

That argument, however, is still largely theoretical. The answers to the questions in the letter lawmakers sent might reveal whether the companies are actually running on clean or dirty energy. There’s some skepticism that miners would be willing to modify the hours during which they run their equipment in order to best make use of renewable energy. The machines typically run 24/7 to maximize profits — not just during daytime when solar power is available.

If mining companies aren’t using excess renewable electricity but instead competing with other customers to fuel their operations, that could drive up utility bills for households and other businesses. That’s already been the case in Plattsburgh, New York, Congress members point out as one example in their letter. Residential utility bills ballooned by up to $300 in 2018 after Bitcoin miners set up shop nearby.

Stronghold and the other companies were asked to respond to the lawmakers’ letter by February 10th. Riot Blockchain Inc., Bitdeer, Bitfury Group, Bit Digital, and Marathon Digital Holdings Inc. also received letters.

“We look forward to having a productive dialogue about the many benefits that we and the rest of our industry have for the United States,” Charlie Schumacher, director of corporate communications for Marathon Digital Holdings, said in an email to The Verge.

“We are proud to do business in America, and we look forward to thoroughly reviewing and earnestly responding to the requested information in the letter we received from the Senate,” Riot Blockchain communications director Trystine Payfer said in an email. A spokesperson for Stronghold Digital Mining also said in an email that they are “excited to engage with Senator Warren and her colleagues.”

The other companies did not immediately respond to a request for comment.

The global center for cryptocurrency mining has shifted from China to the US over the past couple years as China cracked down with policies banning the practice. Now, more than a third of all mining happens in the US, and that’s expected to grow.

Correction 4:43PM Jan. 28: A previous version of this article incorrectly said that Stronghold Digital Mining bought two Pennsylvania power plants that burn coal waste exclusively to mine Bitcoin. This story has been corrected to say that the plants primarily power bitcoin mining; they also provide power to the grid during times of high demand. The story has also been updated to include a statement from Stronghold.

Correction 11:10AM Jan. 31: A previous version of this article incorrectly said that the EPA has recently cracked down on power plants seeking to burn coal waste. The power plants were seeking permission to continue keeping coal waste on site.