Student or Learner
Shall we put a comma before the last “and” (and toxic gas)? I would have done so. They are not single-word items listed in the paragraph:
Hazards on the peak, transformed into an eerie moonscape by grey ash, are already numerous, ranging from ash flung up by helicopter blades to loose stones that make footing dangerous and toxic gas.
It's optional. Like you, I would use one.