I'll defend the islanders and their weak constitutions.
At high enough humidity you can't cool down from sweat, so a 88°F/31°C wetbulb temperature can be lethal. Add in sunlight, combining radiation with convection, which gives the extra 9°F/5°C needed to kill you. Humidity and direct sunlight can make 79° lethal, especially when combined with physical exertion.
It's not just the gout and aristocratic inbreeding that's killing them I swear!