Charcoal radon test kit vs digital radon monitor: differences that matter

An activated charcoal detector uses activated carbon, which binds gases strongly, to absorb radon from the room air. It sits open in the room for the exposure time, are resealed, and go back to a laboratory for analysis. The charcoal does not tell you anything on its own; the lab works out the level.

A digital monitor, also called an electronic radon monitor, works the other way round. Air reaches a detection chamber by diffusion or a small pump, and a detector there registers alpha particles emitted by radon and its decay products. The monitor's electronics turn those counts into a radon level and keep updating it, so the result is on the device, not at a lab.

Passive and active devices

The two groups of radon test. Passive devices, such as charcoal canisters, need no power and are sent to a laboratory. Active devices, such as continuous monitors, need power and continuously measure and record radon.

This is the difference most people feel first, and it is less simple than it looks. A charcoal kit needs its 2 to 7 days in the house, then the post, then the lab; results come within a few days or weeks, and you can ask for expedited service. A monitor shows a first reading much sooner: the models C-NRPP approved give first readings from 90 minutes to 24 hours.

But a first reading is not an answer. A short reading can sit well above or well below your real average, so run a monitor for at least 3 months before deciding, and follow up with a long-term radon test if you have less than 91 days of data. A charcoal kit is itself a short-term test: if its result is at or above 4 pCi/L, EPA recommends a follow-up test before deciding.

Both passive and active devices can measure accurately when used to protocol. What differs is where accuracy comes from. A kit's result is only as good as the laboratory that reads it, which is why you should use a device from a qualified laboratory, and why your state may list do-it-yourself devices from qualified labs. A monitor's accuracy belongs to the model: C-NRPP tests consumer monitors against a reference radon chamber, but only models whose makers enter, and Health Canada has recalled several monitors for reading radon inaccurately.

Both can be spoiled by how they are used, and improper testing gives inaccurate results. For a charcoal kit that means closed-house conditions, the right room and a prompt return to the lab. For a monitor it means the right room, enough time and reading the long-term average rather than the latest hour.

For more on choosing a spot for either one, see where to place a radon test.

We do not print prices in our copy, because they change; current prices, where we have them, appear only from the retailer's own data with the time they were checked. The shape of the cost is different, though, and worth knowing before you buy:

• A charcoal kit is a one-time cost per test. Passive devices are generally inexpensive. Check whether the price includes the laboratory's analysis and return postage; if not, add them. A confirmed result means at least two kits.
• A monitor is a larger cost up front, then little or none per test. There is no lab fee or postage. A battery model needs batteries; a plug-in needs an outlet. It can be used again in another room or after a fix.
• Free kits change the sums. State radon programs and local health departments sometimes give kits away. See free radon test kits by state.

Neither test comes out ahead; each fits some questions better. Each list has five points.

Read this figure as textHide the text version

Test	How long it runs	What it is for
Short-term test	Stays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.	A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term test	Stays in the home for more than 90 days.	It is more likely than a short-term test to tell you the home's year-round average radon level.
Continuous monitor	Logs readings for the whole test, run as a short-term or a long-term test under the same durations in EPA guidance.	Logs a graph over time, so you can see trends and spot anomalies rather than a single number.

Show the whole figure

Whichever you use, read the result the same way. EPA recommends fixing a home when the radon level is 4 pCi/L or higher. EPA recommends considering a fix when the level is between 2 and 4 pCi/L. Every kit we cover is on radon test kits, and every monitor, with its independent evidence, is on radon detectors and monitors.