The gray is conventionally used to express the severity of what are known as "tissue effects" from doses received in acute exposure to high levels of ionizing radiation. These are effects that are certain to happen, as opposed to the uncertain effects of low levels of radiation that have a probability of causing damage. A whole-body acute exposure to 5 grays or more of high-energy radiation usually leads to death within 14 days. LD 1 is 2.5 Gy, LD 50 is 5 Gy and LD 99 is 8 Gy. The LD 50 dose represents 375 joules for a 75 kg adult.