Fig. 5. Difference in liveability between older and younger adults for shaded or sun-exposed conditions.

The difference in liveability—or maximum safe metabolic rate (${\mathit{M}}_{\mathbf{max}}$)—between young and older adults in sun-exposed (a) and shaded/indoor (b) conditions. The dotted area indicates air temperature and relative humidity combinations that, with aging, shift the situation from liveable to only survivable (i.e., people cannot increase their activity without continuously storing heat inside the body, which will lead to a continuous in core temperature, but heat stroke death after a 3-hour exposure would not occur). For example, in the shade, a young adult can perform 1 METs more work that an older adult within the green-shaded area. Note: The $\mathrm{\Delta }$MET values can also be seen by directly comparing Fig. 4a, b (for 5a) or Fig. 4c, d (for 5b). Source data are provided as a Source Data file.