Derived Air Concentration (DAC) is defined as the concentration of radioactive material in air that would result in one ALI of breathed for 2000 hours.

• A. concentration of radioactive material in air that would result in one ALI breathed for 2000 hours
• B. The maximum allowable concentration in air
• C. The average concentration in room
• D. The dose rate in air

Answer: (A)

The main idea is translating an intake limit into an air concentration that a worker could be exposed to over a typical work year. Derived Air Concentration is the amount of radioactive material in each cubic meter of air that would deliver the Annual Limit on Intake if a worker breathed that air for about 2000 hours—the standard full‑time work year.

In other words, DAC answers: if air contained this much activity, inhaling it at a normal breathing rate for 2000 hours would reach the ALI. This makes DAC a practical air monitoring threshold, expressed in units like Bq per cubic meter, to keep workplace air within safe limits.

Why this definition fits best: it ties the regulatory intake limit (ALI) directly to a measurable, environmental quantity (air concentration) over the typical working period, enabling monitoring and controls. Other concepts—such as a general maximum permissible concentration, an average room concentration, or a dose rate in air—do not specify the exact link between ALI and inhaled exposure over a defined working time, which is what DAC provides.