Thirst has no single exact percentage

Thirst can occur after a relatively small change in fluid balance, but its threshold depends on osmolality, age, temperature, activity, and health. Percentages such as 1%, 5%, or 10% of body mass appear in some dehydration classifications, but they are not a precise universal sequence: fainting does not inevitably occur at exactly 5%, and death risk depends on severity, rate of loss, heat, illness, and treatment. Major fluid loss can be an emergency; seek medical assessment rather than waiting for a threshold.