[mikewarot]

Humidity is a percentage of the amount of water that could be in the air, which is when the vapor pressure of water is equal to the partial pressure.[1]

So if you're in a room 20 °C, the partial pressure of water is 0.0231 Atmospheres * 40% --> 0.00924 water (0.924%)

Steam is 100% water, and generally can only happen at 100 °C or higher.

The highest humidity/temperature I've experienced is about 100% at 99F, which works out to around 7% water in the air. It was a miserable day, and I was a young/healthy kid at the time. [1] https://en.wikipedia.org/wiki/Vapour_pressure_of_water