What unit is used to express the maximum permissible weekly dose of ionizing radiation for humans?

The correct choice for expressing the maximum permissible weekly dose of ionizing radiation for humans is the REM, or roentgen equivalent man. The REM is an important unit in radiological health as it takes into account the biological effects of radiation on human tissues. It quantifies the dose of ionizing radiation by considering not only the amount of energy deposited per unit mass but also the type of radiation and its potential for causing biological damage.

While Gray is a unit that measures the absorbed dose of radiation, it does not factor in the biological effectiveness of different types of radiation, which is crucial when determining safety limits for humans. The Sievert, like REM, is also a unit that accounts for biological effects but is more commonly used in modern contexts compared to REM. Roentgen measures the exposure to X-rays and gamma rays but does not directly translate to a biological dose, making it less applicable for setting permissible dose limits for humans. Therefore, the REM remains a key unit for assessing human exposure and establishing safety standards in radiation protection.