Mt. Whitney Portal Store & Hostel Forums General Mt. Whitney Portal Store Message Board drinking too much water on the trail

I think if all things are equal you would need more fluid intake at high humidity(muggy conditions). The sweat runs off your body without cooling.
Altitude may have somewhat of an opposite effect but the same result, lower pressure will allow the sweat to evaporate easier with less cooling of the body which would make you dehydrate quicker without even noticing even with little or no exertion.In both cases your body would produce more sweat to attempt to maintain the 98.6 core temperature so you should drink replacement fluids.

Though I've never had problems with hyponatremia or known anyone who has the issue is discussed frequently on the yahoo group GrandCanyon Hikers. I think the difference in temparature between the Whitney climb and Grand Canyon climbs may well account for the difference. There was an interesting thread about a year ago about an individual who nearly died from it.

Salt as a treatment for passing out in the heat was developed by the Desert Research Institute during the construction of Hoover Dam. Workers were apparently passing out regularly in the 110 degree heat of the Colorado River Canyon.