Significant environmental impacts may result from the addition to the environment of various substances from coal combustion, nuclear plant or fuel cycle operations, and from photovoltaic manufacturing and applications. The emissions of coal plants have been studied extensively, though there is still uncertainty about the ultimate environmental fate of these emissions. Coal has a long history of environmental and health impacts that stem from decades in which there was a lack of scientific understanding concerning the potential impacts as well as a lack of regulation. As concerns were recognized, scientific research helped to clarify causes of various environmental and health issues, and methods for mitigating or abating many impacts. Regulations were passed and best practices developed to limit future impacts, and mitigate or abate past abuses. This research continues. It should be noted that not all the environmental and health concerns attributed to coal are valid. There have been many cases where concerns about an issue in one situation are raised in another area where they do not apply.