Environmental Externalities The environmental impacts of fossil fuels often result in real costs to society, in terms of human health(i.e., loss of work days, health care costs infrastructure decay(i.e., from acid rain), declines in forests and fisheries, and perhaps ultimately, the costs associated with climate change. Dollar costs of environmental externalities are difficult to evaluate and depend on assumptions that can be subject to wide interpreta tion and discretion. Although environmental impacts and associated dollar costs are often included in economic comparisons between renewable and con- ventional energy, investors rarely include such environmental costs in the bottom line used to make decisions.