Origins of toxic gas clouds may be diverse, including accidental releases due to industry or to hazardous materials transportation, or biological or chemical attacks. A protection to such a phenomenon consists in taking advantage of the protection offered by buildings against airborne pollutants. In this event, people can shelter in a building and wait until the toxic plume has gone. European directive concerning major accidents hazards control, known as SEVESO II, requires from Member State’s local authorities, to ensure that appropriate measures are taken to guarantee people’s protection when located in areas close to Seveso facilities. In France and in Catalonia, two different political approaches are set to assess the effectiveness of shelter in place in the event of toxic gas releases. This paper exposes and discusses both approaches, including: 1- a brief presentation of the political, legal and technical strategy developed in both countries to face up the imminent risk due to industrial premises, storing and manipulating hazardous materials; 2- a description of the methodology developed in both countries to assess shelter in place effectiveness in buildings; 3- a study case illustrating both approaches in order to assess their consistency.