Identify and obtain the governing standard for residential radon mitigation system design (ASTM E2121 / ANSI-AARST)

Steps

1. Identify ASTM E2121 (current edition E2121-21), 'Standard Practice for Installing Radon Mitigation Systems in Existing Low-Rise Residential Buildings,' as the classic reference standard for existing-home mitigation system design.
2. Purchase the current PDF edition from the ASTM International bookstore (store.astm.org) or the ANSI webstore (webstore.ansi.org).
3. For the actively maintained ANSI/AARST family that supersedes/complements ASTM numbering for many building types, browse standards.aarst.org, which lists a distinct ANSI/AARST standard per building type (existing low-rise, new construction, schools, multifamily).
4. Use AARST's free online-only viewing of the current standards where offered, understanding that downloadable/printable copies must still be purchased.
5. Confirm which specific standard version your state program or NRPP/NRSB certification actually requires, since some now reference the ANSI/AARST edition by name rather than the older ASTM number.