Sustainability researchers from the Rochester Institute of Technology point out that there is significant variation in the types and amounts of critical materials present in different reservoirs of coal waste. This means that not all waste will be profitable to purify. As the researchers have written, “The value of rare earths in a single ton of coal ash can vary from US$99 at a coal plant in Ohio to $534 at a West Virginia plant. With extraction costs expected to range between $380 and $1,200 per ton, not every coal plant’s ash will be a profitable place to find rare earths.” There are also concerns that the chemicals used to harvest critical minerals could be damaging.