Should cities invest in sheltering-in-place measures against chlorine truck attacks by terrorists?

After an intentional release of chlorine in an office district, public responses such as sheltering-in-place could save many lives if rapid enough. However, previous work does not estimate how fast and effective such responses would be for several possible investments in attack detection, public alert, and building ventilation, nor whether such measures would be cost effective. We estimate public response times with investment options in place, and resulting changes in fatalities as well as system costs, including false alarm costs, and cost effectiveness in terms of cost per net death avoided. The measures do have life-saving potential, especially if all response times are at or near the lower limits of the ranges assumed in this article. However, due to uncertainties, it is not clear that responses would be rapid enough to save many people. In some cases total fatalities would increase, since sheltering after chlorine vapor has already entered buildings can increase occupants' chlorine exposure. None of the options considered have median cost per statistical life saved meeting a cost-effectiveness threshold of $6.5 million across all of the chlorine exposure dose-response and ingress-delay models considered here, even if there were one attack per year in the area covered by the system. Given these and other issues discussed in this article, at this point investments to improve sheltering-in-place capability appear not to be robust strategies for reducing fatalities from chlorine attack in an office district.