Radon test kits: when one is enough, and when it is not

Which one do you need?

Six things that make a kit result unusable

None of these is exotic. Every one is common enough that it is worth reading before the kit is opened rather than after the number arrives.

1. Closed-building conditions were not held. A short-term test assumes windows and external doors stay shut apart from normal entry and exit, generally for 12 hours before and throughout. A house aired out mid-test reads lower than it lives, and the number describes the ventilation rather than the house.
2. The device sat on the wrong floor. EPA’s guidance is the lowest lived-in level. A kit on the ground floor of a house whose family room is in the basement is measuring a room nobody occupies, and will usually under-report what the household actually breathes.
3. It was placed against an exterior wall, in a draught, or in a wet room. Kitchens, bathrooms and laundries have their own air movement and humidity. Placement close to a window, an external door, a vent or a fan puts the detector in the exception rather than in the room.
4. It was moved, or the dates were not recorded. A kit with no start and end time cannot be defended later, and a device that moved mid-run produces one number from two locations. In a transaction this is the argument that actually happens.
5. It was posted late. Charcoal-based kits keep absorbing after the exposure window and are analysed against an assumed timeline. A kit that sits in a car for a week is measuring something other than the stated period.
6. One short test was treated as the final answer. Radon moves with season, weather and how the house is run. A single short measurement is one sample under one set of conditions, which is enough to decide whether to look further and rarely enough to settle a number on its own.

Each of these has its own entry in our reference library, written for someone who has already run a test and is trying to work out whether to trust it — start with things that go wrong during a radon test, which covers placement, movement and conditions case by case.

You have a number. Now what?

A result is a starting point, not a verdict. What radon levels mean explains the units and the bands and, more usefully, what a single figure can and cannot support. Testing versus mitigation covers where measurement ends and acting begins — they are separate jobs, often done by separately credentialed people. If you are ready to talk to somebody, how to find and verify a radon professional explains why the credential matters more than the distance. And if the reading surfaced during a sale, what Illinois radon law requires and what it does not separates the statutory duties from the recommendations they get confused with.