A method is presented for predicting heat stroke of a subject. The method includes an earbud covered with a waterproof moisture permeable membrane allowing for moisture penetration, the earbud including an infrared (IR) temperature sensor for measuring core body temperature of the subject, wherein the IR temperature sensor is covered with a waterproof IR transmittable film to inhibit water drops from contacting a detector of the IR temperature sensor, a first humidity sensor positioned within a sweat flow path within the earbud, a second humidity sensor positioned outside the earbud, and a sodium ion (Na+) concentration sensor for measuring hydration levels of the subject.