Final answer:
Firefighters are safest working upwind when wind speeds exceed 10 mph because it minimizes the risk of the fire advancing toward them and reduces exposure to smoke.
Step-by-step explanation:
When a firefighter is combating wildfires and wind speeds are 10 mph (15 kph) or faster, the safest position is to work upwind. Working upwind means that the firefighter is in the direction from which the wind is coming. This position is preferable because it reduces the risk of being overtaken by the fire as the wind could potentially push the fire toward them if they were downwind. Additionally, it minimizes exposure to smoke and airborne particulates, which can be hazardous to health.