In the hottest year on record, with scorching conditions claiming lives from India to Mexico, and Greece sweltering in its earliest-ever heat wave, experts are sounding the alarm over heat stress.

The condition kills more people than hurricanes, floods or any other climate-related extreme, but what is heat stress, exactly, and how is it measured?

Heat stress occurs when the body's natural cooling systems are overwhelmed, causing symptoms ranging from dizziness and headaches to organ failure and death.