The rem is a unit used to measure:

The rem is a unit that specifically measures dose equivalence in the context of radiation exposure. It quantifies the biological effect of radiation on human tissue, taking into account the type of radiation and its potential to cause harm. This is particularly important in the field of radiation safety and protection, as it allows for the assessment of health risks associated with different types of radiation doses.

Using the rem helps ensure that safety standards are appropriately tailored, as it takes into consideration factors like the quality of radiation (e.g., alpha particles, beta particles, gamma rays) which can cause varying levels of biological damage. By measuring dose equivalence, health and safety protocols can be effectively established to protect workers exposed to radiation, thereby enhancing their safety.

The other options deal with different concepts: contamination refers to the presence of radioactive material on surfaces or in the environment; decay rate pertains to how quickly radioactive substances lose their radioactivity; and chemical activity involves the reactivity of chemical substances, which is distinct from the measurements of radiation exposure or effects. These distinctions clarify why the rem is specifically linked to dose equivalence.