An opinion piece on the CleanTechnica website has attacked proponents of direct air carbon capture technology for their uncritical embrace of it. Making some back-of-the-envelope calculations, the piece estimates that it would take around 4.4 MWh of electricity, which would cost US$53.00 (€46.80) at current average US electricity prices, to push enough air through sorbent columns to extract one tonne of CO₂ . This does not include capital costs for the fans or sorbent columns, and neither does it include heating of the sorbent, nor compression, transport and storage of the CO₂. Part of the problem with direct air carbon capture is, somewhat paradoxically, the low concentrations of CO₂ in air (at around 410 ppm, or 0.04% by volume), and this is one reason why capture from power plants and other heavy industry is vastly more economical, the flue gases being heavily concentrated. Another criticism of direct air carbon capture is that there are natural alternatives, such as reforestation. The CleanTechnica piece concludes, however, that the solution to elevated levels of carbon dioxide in the air is to stop burning fossil fuels and wait the hundreds of years until it declines naturally, though it concedes that there may be some opportunity to reuse some captured carbon in products such as cement.