Hi Scott:
I have performed hundreds of Phase I ESA's and the primary reason most don't address this is for the exact reason you stated. According to the ASTM E1527-13 Standard it does recognize Radon as an Environmental Risk according to ASTME1527-13;
13.1.5.12 . Appendix X1; X5.2 However, Radon is excluded from CERCLA. Phase I's are focused around this Act as it was designed primarily to protect innocent land owners and seek compensation from those responsible.
"CERCLA authorizes the federal government to respond to releases of hazardous substances,7 to
seek reimbursement from potentially responsible parties (“PRPs”)8 or to order PRPs to abate releases
or threatened releases of hazardous substances that may present an “imminent and substantial
endangerment” to the public health or welfare or the environment."
So the inclusion of Radon testing and mitigation is strictly left up to the discretion of the user. Each state has the authority to include it or a bank that is lending can require based on a what is called a Business Environmental Risk (BER).
The additional challenge is Radon is site specific and until it is shown that the diffusion into a structure could have been caused by someone else, most Environmental Consultants shy away from included this as part of a Phase I. In reality, Radon levels can be increased by someone else. For example, I have seen how development of neighboring properties have dramatically increased the radon levels in neighboring homes. When development of vacant land occurs the Karst topography is being altered which in turn alters radon migration pathways.
Finally, I would recommend that this to be included in states that have a Radon Testing and Mitigation Requirement in place for Day Cares and Nursing Homes. They are classified as commercial properties which fall under the customary practice of requiring a Phase I ESA during the purchase process. Radon testing should be included as part of the due diligence process for these types of transactions as the new property owner will have to address it at some point under State requirements.
So, long and short of it is that ideally it should be performed, following the lead of HUD's new requirements. To make this happen however, is going to take time, money and advocacy by the Radon Community to get this in place.