Abstract: After the terrorism attacks in the US on September 11，2011 international anti-terrorism experts have indicated that it is not possible to completely avoid the risk of a CBRN terrorism; it's occurrence is not a scenario of "if", but a scenario of "when". However, there are several measures can be taken to reduce the likelihood and mitigate consequences of such an attack; one of these is the proper protection and control of a HVAC system. Although the simple suggestion given by many websites is to shut off the entire HVAC system when facing a chemical release whether it is inside or outside the building, it may not be the most effective response option. Such immediate action can reduce the penetration rate of any chemical agent into the building and the spread rate of the chemical contaminant within the building. However, it may not be necessary, the most effective strategy depends on the building's HVAC system and design. Based on documents prepared by the US and British governments after 911 attacks, this article identifies the strategy and tactics of response when facing a deliberate chemical release on high speed train subway stations.

Key words: CBRN terrorism, subway station, chemical attack, HVAC