Fig. 1.

Biophysically modelled humidity-dependent threshold temperatures at which electric fan-use and modest skin-wetting becomes detrimental; above the line fans exert a heating effect, below the line fans exert a cooling effect. Individual data points represent the highest 1-h ambient temperature and associated relative humidity recorded between 2000 and 2019 (inclusive) for each of the 105 metropolises analyzed. Data points are colour-coded to identify percentage of summer days in last 20 years on which environmental limits were exceeded where green: 0%; yellow: >0% but ≤0.5%; red: >0.5%. (For interpretation of the references to colour in this figure legend, the reader is directed to the web version of this article.)