The Environmental Protection Agency (EPA) is proposing to approve a revision to the San Diego County Air Pollution Control District (SDCAPCD) portion of the California State Implementation Plan (SIP). This revision concerns emissions of oxides of nitrogen (NO<INF>X</INF>) from boilers, process heaters, and steam generators. We are proposing to approve a local rule to regulate these emission sources under the Clean Air Act (CAA or the ``Act''). We are taking comments on this proposal and plan to follow with a final action.