What is the threshold dose for acute radiation syndrome (ARS)?

The threshold dose for acute radiation syndrome (ARS) is around 1000 mGy (or 1 Gy). ARS typically occurs after exposure to high doses of ionizing radiation over a short period of time. At doses above approximately 1000 mGy, clinical symptoms of ARS can manifest, including nausea, vomiting, fatigue, and, in severe cases, can lead to death if the dose is lethal.

Lower doses, such as those listed in the other choices, generally do not lead to ARS but may cause other radiation effects or increase cancer risk over time. Hence, while 100 mGy and 500 mGy are significant exposures, they fall below the threshold associated with acute radiation syndrome.