There is no specific clinical limit to how many hours a child’s high fever of 39.5°C cannot exceed. Once the baby has a fever, it is recommended to deal with it in a timely manner to avoid the occurrence of febrile convulsions when the body temperature continues to increase. Fever is divided into low fever, moderate fever, high fever, ultra-high fever, high fever is 39.1 ~ 41 ℃, more than 41 ℃ is ultra-high fever. So 39.5 ℃ belongs to high fever, persistent high fever may lead to febrile convulsions, manifested as a sudden loss of consciousness, eyes staring, squinting or upturned, the head and limbs in tonic or clonic convulsions, apnea, facial bruising, most of the convulsions after the occurrence of a quick return to sanity, if the persistence of repeated occurrences, it indicates that the seriousness of the condition requires timely medical attention. If the fever lasts for a long time, you should also consider whether it is a bacterial or viral infection or a specific disease such as Kawasaki disease. Therefore, when a baby has a high fever that does not go away, it is advisable to consult a doctor promptly to find the cause of the illness and treat it aggressively.