Hot weather, exercise, old age, and the kidneys.

Thirst is not a very accurate measure of water need; optimal fluid replacement may lag 2 to 3 days behind actual needs, particularly for persons unaccustomed to a hot climate. If an older person begins an exercise program during warm weather or travels to a warmer climate, the intensity and duration of exercise should be restrained initially, then gradually increased over 10 to 14 days to allow for acclimatization.