The ambient interior concentration of asbestos after the completion of actions described in the most current guidance document, other than the type of action described in sections
2643
(f) of this title and subsection (c) of this section, shall not exceed the ambient exterior concentration, discounting any contribution from any local stationary source. Either a scanning electron microscope or a transmission electron microscope shall be used to determine the ambient interior concentration. In the absence of reliable measurements, the ambient exterior concentration shall be deemed to be—