Geiger Counters and Radon: What a Handheld Counter Shows and Misses

What radon is and how it is measured

Radon is a naturally occurring radioactive gas. It is a concentration in the air, and official programmes measure it as an average over a set period of days or months rather than as a quick spot reading. In the UK, results are reported in becquerels per cubic metre of air (Bq/m³); the EPA defines one becquerel as one radioactive decay per second. In the United States, the EPA uses picocuries per litre (pCi/L), a very small unit on the older curie scale. The EPA also notes that radon coming from the soil is a far more common household issue than radiation from materials inside the home, such as a granite worktop.

The time frame is the core of the method: a radon test stays in place for its measurement period, and the result is a single figure describing that air over that time. For wider context, UKHSA puts the average radiation dose in the UK at about 2.6 mSv per year, of which about 1.3 mSv comes from radon. Those are population averages quoted by UKHSA, not a limit and not a verdict on any particular house, so they cannot replace a test of your own home.

Why a Geiger counter cannot give you a radon figure

A Geiger-Muller tube is, in the words of HyperPhysics at Georgia State University, a wire at the centre of a cylinder held at high voltage. When a particle ionises a few gas molecules inside, it sets off an avalanche of electrons and a pulse of current, which the device turns into a click. A pocket counter adds up those pulses as counts per minute (CPM) and converts them into an indicative dose rate in µSv/h. It responds to beta, gamma and X-ray events that reach the tube close to the device. It does not draw in air, it does not average a volume of gas over days, and it never reports Bq/m³ or pCi/L.

There is a second, more basic reason. Radon decays by emitting alpha particles, and those alpha decays do not reach the inside of a glass tube. A glass Geiger-Muller tube does not detect alpha at all. So even in a room where a radon measurement pack would come back high, the counter has no way to turn that gas into the concentration a radon test reports. A figure on its screen in a cellar is a local count near the tube, not a radon result, and it should never be read as one.

What a Geiger counter may still show

A counter can show a change in readings near some objects and materials. The EPA explains that uranium glass (yellow-green, and it glows green under a UV lamp), some pottery made before 1973 with a uranium glaze, and old self-luminous watch or clock dials that may contain radium can make a Geiger counter react when it is held close. Granite can give off a little beta and gamma radiation that varies from stone to stone and drops quickly with distance. Background levels also vary from place to place and rise with altitude. In one independent test, a J321-type glass tube read about 20 CPM at background. CPM figures cannot be compared between different counters, and on beta sources the µSv/h figure runs high because the tube has no beta shield, so read CPM there.

What the counter says nothing about is the air in the room. Its readings are indicative, and it is not a calibrated measuring instrument, not personal protective equipment and not a radon test. If you see an unusually high or persistent reading, move away from the source, do not handle the object and contact UKHSA or your local authority (in the US, your state radiation control programme or the EPA). In an emergency, call 999 in the UK or 911 in the US.

• Keep old objects intact, and never take apart a radium dial.
• Do not serve food or drink in old uranium-glazed pottery or uranium glass.
• Wash your hands after handling such objects.
• If one breaks, ask your local authority how to dispose of it.

How to test a home for radon

In the UK, UKHSA runs UK Radon at ukradon.org. Radon there is measured with a radon measurement pack, reported in Bq/m³, and UKHSA publishes maps of radon Affected Areas that show where testing matters most. The pack is the tool built for the question you are asking: it stays in place for the period its instructions give, then reports an average concentration for that air. A sensible order is to look up your area on the Affected Areas maps first, then order a pack through UK Radon. According to UK Radon, if a home's level turns out to be high, fairly simple building work can reduce it.

In the United States, the EPA's radon testing guidance points to radon test kits, found at hardware shops, online or through the state radon programme, or to a radon measurement professional, with results given in pCi/L. In both countries, the official programme, not a handheld counter, is where the decision about your home should start, including where and how to place a test and what to do with the result once you have it.

Granite worktops: what the EPA says

According to the EPA, granite can contain traces of naturally occurring radioactive elements that give off some beta and gamma radiation, and the amount varies from one slab to another. That radiation drops quickly with distance from the stone. The EPA adds that measuring a worktop precisely requires calibrated instruments and a trained user. A pocket counter held against a kitchen worktop may show counts above your usual background, or it may not, and neither result is a precise measurement of the stone.

The EPA's larger point is about priorities: radon coming from the soil is the far more common issue in homes. A worktop reading, high or low, tells you nothing about the radon concentration in the air of your kitchen or cellar. If radon is your concern, the answer is the same as everywhere else in this guide: use a radon measurement pack through UK Radon (a radon test kit in the US) and follow the official programme.

What to do next

Start with the official source for your country. In the UK, check the radon Affected Areas maps on ukradon.org and order a radon measurement pack through UK Radon, then place it exactly as its instructions describe. In the US, the EPA radon testing page and your state radon programme play the same role, with a radon test kit run as instructed. Keep any Geiger counter readings separate from that process: they are a different measurement, in different units, of a different thing, and they add nothing to a radon result.

A Geiger counter is still a useful tool for what it actually does: watching background counts and trends, and looking at readings near objects you already own. Geiger Counter Shop sells one pocket Geiger counter with a glass GM tube (J321 type, as listed by the supplier), a 2.4-inch colour screen, µSv/h and CPM, a dose-rate trend graph, adjustable alarms and a 1,800 mAh USB-C battery, delivered free from UK stock for UK orders. It is not a radon test and is never sold as one.