Often a multi-disciplinary approach is taken in compiling all the components of a Phase I study, since skills in chemistry,  atmospheric physics, geology, microbiology  and even botany are frequently required.  Many of the preparers are environmental scientists who have been trained to integrate these diverse disciplines.  Many states have professional registrations which are applicable to the preparers of Phase I ESAs; for example, the state of  California had a registration entitled "California Registered Environmental Assessor Class I or Class II" until July 2012, when it removed this REA certification program due to budget cuts.
Under ASTM E 1527-13 parameters were set forth as to who is qualified to perform Phase I ESAs. An Environmental Professional is someone with:
# a current Professional Engineers or Professional Geologists license or registration from a state or U.S. territory with 3 years equivalent full-time experience; or
# a Baccalaureate or higher degree from an accredited institution of higher education in a discipline of engineering or science and 5 years equivalent full-time experience; or
# have the equivalent of 10 years full-time experience.
A person not meeting one or more of those qualifications may assist in the conduct of a Phase I ESA if the individual is under the supervision or responsible charge of a person meeting the definition of an Environmental Professional when concluding such activities.
Most site assessments are conducted by private companies independent of the owner or potential purchaser of the land.