What are ‘wet-bulb’ temperatures — and why can they be so deadly?

Humans lose about 80 per cent of heat through sweating, but when both the humidity and air temperatures are high, it's harder to shed that heat. 

What are ‘wet-bulb’ temperatures — and why can they be so deadly?
Humans lose about 80 per cent of heat through sweating, but when both the humidity and air temperatures are high, it's harder to shed that heat.