Some have been built where there wasn't enough power currently available from the grid but would later be available, and were allowed to operate using temporary power sources until then which can cause serious pollution issues for nearby communities.
A notorious example was/is xAI's Memphis data center. They started out using a loophole that let them power it with dozens of gas turbines without permits that didn't have to meet air pollution standards. (I don't remember for sure, but I think they were using them under a rule that is meant for emergencies like natural disasters).
The surrounding communities, mostly poor and minority with not much political clout, already had air that was significantly worse than wealthier and whiter parts of the area, and the data center pushed that to levels health officials considered to be alarming, like a 79% rise in peak NO2 nearby.
This was actually bad enough that even the Trump administration had to act and the EPA now had a rule making it harder for data centers to exploit that loophole.
But they are still using gas turbines, 15 instead of dozens, and they now are permitted and have more pollution controls, but independent data suggests it remains a major source of the smog in the surrounding neighborhoods. They are expected to be fully on the grid later this year.
