Changes in the environmental heat stress need to be properly evaluated to manage the risk of heat-related illnesses, particularly in the context of climate change. The wet bulb globe temperature (WBGT) is a useful index for evaluating heat stress and anticipating conditions related to heat-related illness in the present climate, but projecting the WBGT with a sufficiently high temporal and spatial resolution remains challenging for future climate conditions. In this study, we developed a methodological framework for estimating the site-specific hourly resolution WBGT based on the output of general circulation models using only simple calculations. The method was applied to six sites in Japan and its performance was evaluated. The proposed method could reproduce the site-specific hourly resolution WBGT with a high accuracy. Based on the developed framework, we constructed future (2090s) projections under two different greenhouse gas emission pathways. These projections showed a consistent rise in the WBGT and thus the capacity to perform physically demanding activities is expected to decrease. To demonstrate the usefulness of the projected WBGT in planning adaptation measures, we identified the optimal working schedules which would minimize outdoor workers' exposure to heat at a specific site. The results show that a substantial shift in the working time is required in the future if outdoor workers are to compensate the effect of increased heat exposure only by changing their working hours. This methodological framework and the projections will provide local practitioners with useful information to manage the increased risk of heat stress under climate change.