This study performs the numerical simulation of hydrogen dispersion in a partially open space. The space under investigation measures 2.9 m x 1.22 m x 0.74 m and a leak flow rate of 2 standard cubic feet per minute is assumed. The effects of various roof vent positions and their areas on the ability to recognize the dispersion and accumulation of hydrogen, and its natural ventilation, are shown and discussed. Based on the results, this paper proposes an innovative approach to the sensing-based adaptive risk mitigation control of hydrogen dispersion and accumulation in a partially open space. By adaptively opening roof vents near the leak source, and closing other neighboring roof vents, concentrated hydrogen is exhausted rapidly and efficiently. It is also shown that cases exist where sufficient area of the proposed adaptive roof vent can be determined by the finite-time sensing ofhydrogen concentration near the roof. The effects of the delay time of a sensor, and a method to cancel these effects, are also discussed. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.