Doing an Environmental Assessment (EA) involves determining the significance or importance of likely environmental impacts. The regulations direct that this be done by considering two variables: "context" and "intensity."
- "Context" is the geographic, biophysical, and social context in which the effects will occur. The regulations mention society as a whole, the region, and affected interests as examples of context. Considering contexts does not mean giving greater attention to, say, effects on society as a whole than to effects on a local area. On the contrary, the importance of a small-scale impact must be considered in the context of the local area, not dismissed because it does not have impacts on larger areas.
- "Intensity" refers to the severity of the impact, in whatever context(s) it occurs. The regulations require that a number of variables be addressed in measuring intensity. Impacts that may be both beneficial and adverse:
- Effects on public health and safety;
- "Unique characteristics of the geographic area such as proximity to historic or cultural resources, park lands, prime farmlands, wetlands, wild and scenic rivers, or ecologically critical areas";
- The potential for controversy on environmental grounds;
- Uncertainty about effects or unique risks;
- The potential for establishing a precedent or representing a decision in principle that defines the parameters of a further action;
- Cumulative impacts;
- Potential adverse effects on "districts, sites, highways, structures, or objects listed in or eligible for listing in the National Register of Historic Places," and the potential for "loss or destruction of significant scientific, cultural, or historical resources";
- Potential adverse effects on an endangered or threatened species or its habitat, or on a critical habitat; and
- Potential for violation of a Federal, state, or local law or requirement "imposed for the protection of the environment." (40 CFR 1508.27)
NEPA requires that a broad range of effects be considered:
- Direct effects such as actually changing an ecosystem, filling a wetland, knocking down a building, or digging up an archeological site;
- Indirect effects such as causing economic change in a community that changes the environment over the long run (through development, increased taxes, etc.) or causing long-term erosion in a watershed; and
- Cumulative effects – the "straws that break the camel's back." An individual action may not have much effect, but it may be part of a pattern of actions whose effects are significant. For example, widening a bridge may not itself have much effect, but it may be the last piece of highway improvement that allows rampant development of a pristine valley.