Dangerous and potentially lethal bouts of heat — driven by the unabated burning of fossil fuels — could fast spread to parts of the United States unaccustomed to such blazing hot conditions.

That’s according to a new report by the Union of Concerned Scientists, published Tuesday in the journal Environmental Research Communications. According to the findings, if humanity doesn’t dramatically rein in greenhouse gas emissions by midcentury, more than 6 million people from California to Louisiana to Kansas could regularly experience what experts call “off the charts” heat.