Studies on external costs focus instead on the potential ecological and noise effects of wind power. Mining and transportation result in emissions similar to those that have been described for coal and uranium mining, most notably PM. Steel production as such emits some PM such as black carbon over and above the emissions resulting from electricity generation. Further hydropower capacity in OECD countries however is limited, with the best resources already tapped and the emergence of significant environmental concerns. Like wind or nuclear, electricity generation from hydropower does not involve any direct atmospheric emissions.