Final answer:
The largest exposure of radiation before effects become apparent is below 100 mSv (10 rems), termed as a low dose. Symptoms of ARS, which can eventually lead to death, are most likely to appear after higher exposures, like around 500 rems, which can result in a 50% chance of death within 30 days.
Step-by-step explanation:
Acute radiation syndrome (ARS) symptoms appear when the body receives a high dose of radiation over a short period. The largest exposure before effects become apparent is known as a low dose, which is less than 100 mSv (10 rems). ARS symptoms can range from changes in blood chemistry to nausea, vomiting, and hair loss, and can ultimately lead to death if the exposure is extremely high.
The most sensitive tissues to such exposure are bone marrow, intestinal tissue, hair follicles, and reproductive organs due to their rapidly dividing cells. Importantly, radiation has a cumulative effect on the body, thus repetitive low-dose exposures over time still pose a serious health risk. A single, short-term dose that can cause lethal effects is approximately 500 rems, where there is about a 50% chance of death within 30 days (LD50/30).