Henri, you can find the HUD MAP Guide here:

https://www.hud.gov/sites/dfiles/OCHCO/documents/4430GHSGG.pdf

The Radon section is 9.6.3, which begins on pg 340. The Radon Professional credential language is found in Section 9.6.3.2.B, the standards are listed in 9.6.3.2.D, and OM&M is in 9.6.3.2.I. I hear that OM&M has often been overlooked but is currently being enforced when mitigation systems are installed.

The clearance testing section of ANSI/AARST MAMF-2017 Rev1-21 is found in Section 7.3.2:
https://standards.aarst.org/MAMF-2017/26/index.html

thanks so much. I wasn't sure where to find the latest HUD revision.

It also occurs to me that:
1) a client's resistance with respect to the need, in clearance testing, to retest units that previously tested low is probably common, and therefore
a) more education to practitioners about these requirements and
b) advance advice to clients about what they can expect regarding post-mitigation clearance testing
may be called for;
and
2) an explanation to the client of the rationale for the clearance testing requirements may also be helpful. I seem to recall that there were those mitigators who have described cases in large buildings of elevated radon levels that arose after the initial round of mitigation. The issues of
a) reasons for the appropriateness of full retesting,
b) rough likelihoods of what clients can expect, and
c) how further clearance testing is to be carried out if one or more units are found to be elevated after the first or subsequent round of mitigation
are all things that both professionals and their clients should have clarity about.

Henri, if you could cite specific aspects where the current standards appear to be unclear and if you have specific recommendations for how that lack of clarity could be remedied, these are always the kinds of things that the standards committees appreciate learning about and want to address. As I'm sure you know, procedures and forms are available here: https://standards.aarst.org/public-review/

Thanks. I believe the confusion stemmed from the fact that this project started long ago, before HUD changed it's stance. (There were numerous long delays for COVID, Brownfield issues, etc.)