The objective of this work is to optimize the arrangement of Phase Change Material (PCM) packets in a cooling vest by targeting torso segments that can trigger comfort when cooled. An integrated fabric-PCM and bio-heat simulation model is used in the optimization to predict human segmental core and skin temperatures to determine overall human thermal comfort during moderate activity in hot environment over a specified working period. The variables of the optimization were the number of PCM packets on the upper and lower front and back torso segments. The constraints to be met were: a core temperature not to exceed 38 degrees C, a comfort level not to be less than -1.4 on a comfort scale varying from very uncomfortable at -4 to very comfortable at +4 and a PCM melted mass fraction between 0.85 and 1.0. Three environmental conditions were considered at 28 degrees C, 35 degrees C and 45 degrees C for optimization and the results showed that 8, 18 and 20 PCM packets were needed to provide cooling for the outdoor worker at acceptable comfort level, respectively. At the very high ambient temperature of 45 degrees C and when increasing the PCM packets number to 32 packets, it was shown that the working duration can be extended from 45 min recommended by labor standards to two hours similar to duration permitted at 28 degrees C and 35 degrees C. (C)2017 Elsevier B.V. All rights reserved