A Phase I ESA (or Phase 1) is a report that identifies potential hazards from toxic or hazardous materials on a property. A phase I substantially site assessment is usually required when a company or other entity purchases a property. An assessment of the property is done, which includes a review of soil conditions and any improvements to it.

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This may include the identification of asbestos traces on buildings, the determination of chemical residues on structures, and the inventory of hazardous substances that have been stored on or used at the site.

The 1998 Law emphasized the importance of a Phase I Environmental Site Assessment. Real estate entities were required to conduct a Phase 1 ESA to comply with the strict standards of ASTM E-1527: Standard practice for environmental site assessments – Phase I Environmental Site Assessment Process.

In order to prevent another harmful incident from occurring, amendments to laws and statutes introduced environmental due diligence to properties to be sold or purchased. Due diligence is required before any construction or other development takes place in order to make sure there aren't hazardous substances or contamination on the property. 

These procedures were also studied by experts to ensure that the most rigorous standards of inspection are applied to any property. These include Phase I and II ESA, site remediation and risk assessment, as well as property condition assessments.