What is the effect, high humidity in the air, for humans?
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It prevents our body from cooling. We eliminate excess heat through sweat thanks to a process known as evaporative cooling. Basically, when water evaporates off a surface it brings a little heat with it, resulting in that surface getting cooler. But if air humidity is high, evaporation cannot occur, because the air is already saturated. That means evaporative cooling cannot take place and our body begins to overheat.
Which brings us to wetbulb temperature, ie. the temperature measured by a thermometer wrapped in a wet tissue. If the air is dry and evaporation occurs, a wetbulb thermometer will measure an air temperature several degrees lower than the actual temperature you would measure with a regular, “drybulb” thermometer. If moisture is very high, evaporation won’t be as efficient, and the wetbulb temperature will be closer to the drybulb temperature.
A wetbulb temperature as “low” as 35°C (96F) results in a fatal heatstroke. Such value has only ever been reached a couple of times in the Persian gulf and in Pakistan, thankfully in mostly uninhabited areas and for short amounts of time. But global warming is making wetbulbs rise, and there is legitimate concern that sooner or later the 35°C wetbulb threshold will be reached in a major settlement, causing thousands or maybe millions of people to just drop dead.
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