How a continuous monitor differs from passive devices
Unlike passive devices such as charcoal canisters or alpha-track detectors, which simply absorb or record radon exposure for lab analysis afterward, a continuous monitor uses an internal sensor, commonly a pulsed ion chamber or similar detection method, along with an internal pump or diffusion chamber to actively sample air throughout the test. It calculates and stores hourly radon concentration readings directly in its memory.
This active, hourly logging means results don't need to be mailed to a lab; the device itself performs the calculation and can display or transmit results as soon as the test period ends. It also means the device can record contextual data alongside radon readings, such as temperature, humidity, and barometric pressure, which help explain fluctuations.
Tamper detection and why it matters
Continuous monitors typically include movement sensors and can detect if the device was picked up, tilted, or relocated during the test, along with logging any interruption in operation. Some models also detect significant changes in light exposure, which can indicate a device was opened or exposed by removing a cover, all of which would be recorded as a flag in the final report.
This matters most in real estate transactions, where there's a financial incentive for someone to want a lower reading. Tamper flags give both buyers and sellers, along with their agents, confidence that the reported result reflects an undisturbed test rather than one that was manipulated.
- Movement or tilt detection during the test period
- Logged interruptions in power or operation
- Hourly data points rather than a single averaged figure
- Temperature, humidity, and pressure logging for context
Reading an hourly data graph
An hourly graph from a continuous monitor typically shows radon levels fluctuating somewhat throughout the test, often rising overnight when a home is closed up and less air exchange occurs, and dipping slightly during the day with more household activity. This pattern is normal and expected rather than a sign of a problem.
What a technician looks for beyond the overall average is consistency of the pattern; a graph with a sudden, unexplained flat line, an abrupt drop to near zero, or an isolated spike inconsistent with the rest of the data can indicate a sensor issue, a brief tamper event, or an unusual household occurrence worth noting in the final report rather than simply averaging over it.
When continuous monitoring is the preferred method
Continuous monitoring is generally the preferred method for real estate transactions given its speed, same-day-available results after retrieval, and built-in tamper documentation, which reduces disputes between parties. It's also useful for homeowners who want to understand how their home's radon levels vary hour to hour, for example, to see whether levels spike specifically overnight or during particular HVAC cycles.
It's less commonly used for the multi-month long-term testing format, where simpler passive alpha-track detectors are more practical to leave in place for extended periods without needing battery changes or data retrieval partway through.
What to expect during a professionally conducted continuous test
A technician places the monitor following the same closed-house and positioning protocol used for any short-term test, then returns after the test period, commonly 48 hours, to retrieve the device and download the data. The report typically includes the calculated average result alongside the hourly graph and any tamper or condition flags recorded during the test.
Our practice is to review the full hourly dataset, not just the final average, before finalizing a report, since context from the graph, such as a spike coinciding with a documented interruption, can be relevant to correctly interpreting an unusual result rather than reporting a number in isolation.
