An Environmental Site Assessment is a report prepared for a real estate holding which identifies potential or existing environmental contamination liabilities. The analysis, often called an ESA, typically addresses both the underlying land as well as physical improvements to the property; however, techniques applied in a Phase I ESA never include actual collection of physical samples or chemical analyses of any kind. Scrutiny of the land includes examination of potential soil contamination, groundwater quality, surface water quality and sometimes issues related to hazardous substance uptake by biota. The examination of a site may include: definition of any chemical residues within structures; identification of possible asbestos containing building materials; inventory of hazardous substances stored or used on site; assessment of mold and mildew; and evaluation of other indoor air quality parameters. Contaminated sites are often referred to as "brownfield sites." In severe cases, brownfield sites may be added to the National Priorities List where they will be subject to the U.S. Environmental Protection Agency's Superfund program.

Actual sampling of soil, air, groundwater and/or building materials is typically not conducted during a Phase I ESA. The Phase I ESA is generally considered the first step in the process of environmental due diligence. This type of study is alternatively called a Level I Environmental Site Assessment. Standards for performing a Phase I site assessment have been promulgated by the US EPA and are based in part on ASTM in Standard E1527-05. If a site is considered contaminated, a Phase II Environmental Site Assessment may be conducted, ASTM test E1903, a more detailed investigation involving chemical analysis for hazardous substances and/or petroleum hydrocarbons

http://en.wikipedia.org/wiki/Phase_I_Environmental_Site_Assessment