What device is actively used to measure radon levels continuously?

The correct answer is the continuous radon monitor. This device is specifically designed to continuously measure radon levels in the environment, providing real-time data on radon concentrations. Continuous radon monitors utilize advanced technology to detect radon gas and provide immediate feedback, which is particularly useful for ongoing assessments and mitigation efforts.

In contrast, the other options are typically used for specific testing scenarios rather than continuous measurement. A charcoal adsorber is a passive device that collects radon over a period of time but does not provide real-time data. Similarly, a passive radon detector also captures radon levels over a predetermined timeframe and requires laboratory analysis for results. A short-term test chamber is typically employed for a brief measurement period but does not continuously monitor radon levels like the continuous radon monitor does. This makes the continuous radon monitor the preferred choice for those needing ongoing radon level data.