provided a highly useful overview to this rare but serious condition that occurs when plasma sodium levels fall dangerously below normal.
Hyponatremia, also known as water intoxication, is diagnosed in patients with less than 135 millimoles of sodium per liter of plasma (mEq/L).
If you are an excessive, salty sweater, your sodium loss will be greater than that of a moderate sweater.
In this way, an excessively hot and humid day can contribute to cases of hyponatremia (through inadequate replenishment of the large amounts of sodium lost in sweat), as well as can an unexpectedly cold day (on which it's more likely a marathoner will have overdrank for the amount of fluid the body will use).
These athletes are out there so long that they are more likely to lose too much sodium through sweat than they are to overdrink.