multiple successive days of training. (B) The impact of multiple types of heavy weapons being fired in close succession. (C) The feasibility of cumulative annual or lifetime exposure limits. (D) The minimum safe distance for observers and instructors. (3) Updated training guidance Not later than 180 days after the date of the completion of the review under paragraph (1), each Secretary of a military department shall update any relevant training guidance to account for the conclusions of the review. (4) Updated review (A) In general Not less frequently than once every two years after the initial review conducted under paragraph (1), the Assistant Secretary of Defense for Health Affairs, in coordination with each Assistant Secretary for Manpower and Reserve Affairs of a military department, shall conduct an updated review under such paragraph, including consideration of the matters set forth under paragraph (2), and update training guidance under paragraph (3). (B) Consideration of new research and evidence Each updated review conducted under subparagraph (A) shall take into account new research and evidence that has emerged since the previous review. (5) Briefing required The Assistant Secretary of Defense for Health Affairs, in coordination with each Assistant Secretary for Manpower and Reserve Affairs of a military department, shall brief the Committees on Armed Services of the Senate and the House of Representatives on a summary of the results of the initial review under paragraph (1), each updated review conducted under paragraph (4), and any updates to training guidance and procedures resulting from any such review or updated review. (d) Research on improved personal protective equipment (1) In general The Under Secretary of Defense for Research and Engineering shall establish a research and development program to improve the efficacy of personal protective equipment to reduce blast pressure or protect against brain injury from exposure to blast pressure waves. (2) Report required Not later than one year