The National Environmental Policy Act requires federal agencies to evaluate each of the alternatives thoroughly to support comparisons about their implementation and their impacts on resources in natural and physical environments. Alternatives also are evaluated and compared in terms of their social and economic impacts. Under NEPA, “effects” and “impacts” may be used interchangeably and mean the same thing.
"Environment" under NEPA is defined as the natural and physical environment and the relationship of people with that environment. This means the environment considered in a NEPA process includes land, water, air, structures, living organisms, environmental values at the site or sites, and the social, cultural and economic aspects of the action.
The list of potentially impacted resources evaluated under NEPA is not identical for each study. Rather, resources are defined early in a specific NEPA study that considers conditions specific to the study area. Resources also are determined based on input from an interdisciplinary project team, stakeholders and the public.