Bill and Colleagues,

Here are several papers that begin to address your question:

Findlay, Robertson and Scott 1990 Testing of Indoor Radon Reduction Techniques in Central Ohio Houses: Phase 2 (Winter 1988-1989), Research Triangle Park, NC: EPA AEERL

Findlay, Robertson and Scott 1991 Follow-up Durability Measurements and Mitigation Performance Improvement Tests in 38 Eastern Pennsylvania Houses having Indoor Radon Reduction Systems, Research Triangle Park, NC: EPA AEERL

Scott and Robertson, 1989 Follow-Up Alpha-Track Monitoring in 40 Eastern Pennsylvania Houses with Indoor Radon Reduction Systems (Winter 1987-88), Research Triangle Park, NC: EPA AEERL

There may be some more AARST Symposium papers that I could search if they do not pop-up from others.

Thank you for your work on the Science Advisory Committee considering the threshold question,

Bill A

Thank you Bill. These are helpful, but the study I was thinking about was more of a national study. I know year long follow-up tests were performed in Iowa and other states.

Bill,
NJ did a Follow-up study on Mitigated homes. Then they looked at ones that failed and that was the basis of a training course and update for the manuals. I think Camroden and Bill Brodhead were involved. I may have a copy home that I could access during the week.

This is the study that NJ did -
https://www.semanticscholar.org/paper/INITIAL-RESULTS-FROM-FOLLOW-UP-OF-NEW-JERSEY-HOMES-Brodhead-Clarkin/a7166a87a5924fe2a26db2edb26afb9302454165?p2df

Thank you for sharing link. Do you know how to get a copy of results or the full pdf paper?

Bill Brodhead posted a link to the 1993 study on his website:

https://wpb-radon.com/Radon_research_papers/1993%20Denver,%20CO/1993_38_Initial%20Results%20From%20Follow-Up%20of%20New%20Jersey%20Homes%20Mitigated%20for%20Radon.pdf

There were formal grant reports on this project. I scanned a bunch of these before I retired. I will see if I can find this them this week now that I have access to my old laptop and flash drives. I will also check with Anita Kopera at NJDEP.

Thank you!

Found it. Bill Broadhead to the rescue. I also added another paper looking a post mitigation by Dan Steck.