Radiation Map UK: How to Compare Your Geiger Counter Readings

The official sources: UKHSA, UK Radon and the US RadNet example

In the United Kingdom, the natural starting point is the UK Health Security Agency (UKHSA). Its page Ionising radiation: dose comparisons, on gov.uk, puts average UK exposure at about 2.6 mSv per year, a transatlantic flight at 0.08 mSv and the average annual dose from radon at 1.3 mSv. For radon, UKHSA runs ukradon.org, whose maps of radon Affected Areas show where testing is worthwhile. These pages give context rather than a live reading for your street, and none of these figures is a threshold for what your counter shows.

For an example of a live station map, look across the Atlantic. The US Environmental Protection Agency runs RadNet (epa.gov/radnet), which, according to the EPA, has 140 air monitoring stations across all 50 states. They measure gamma radiation in near real time, and the results appear on a public page, the RadNet Dashboard, which the EPA describes as showing normal environmental background. It illustrates how a station network behaves: pick one station and see how much its values move from day to day.

What a monitoring station measures versus what your pocket counter shows

A monitoring station has its own detector, height, calibration and units. A pocket counter with a glass Geiger-Muller tube counts pulses: a particle that ionises a few gas molecules in the tube sets off an electron avalanche, and the device turns each pulse into a click (HyperPhysics, Georgia State University). The count rate in CPM depends on that particular tube, and the µSv/h figure is the counter's own conversion of those counts. So a station value, or any published figure, and your screen will not match number for number.

The working rule is simple: compare trends, not numbers. If your averaged background at home stays steady for weeks and a station you follow also stays flat, the two agree in the way that matters. If only one moves, look for a local cause first, such as a new object on the shelf. CPM cannot be compared between two different counters either, even two sitting side by side on the same table.

• Detects: beta, gamma and X-rays through a glass tube (J321 type as listed by the supplier).
• Does not detect: alpha particles, EMF, Wi-Fi or 5G.
• Is not: a radon test, a food or water test, a calibrated measuring instrument or protective equipment.
• Shows: indicative dose rate in µSv/h, count rate in CPM or CPS, cumulative dose and a trend graph.

How to log your own background

A useful comparison starts with your own baseline. Choose one spot, such as a shelf or a window sill, and always measure there at the same height. Switch the counter on, give the reading time to settle, then average over several minutes rather than trusting a single glance, because short readings jump about. Note down the average dose rate, the CPM (the raw count from the tube), the date, the time, the weather and whether you were indoors. The dose-rate trend graph on the 2.4-inch colour screen shows whether the value is drifting or just flickering. After a few weeks you have a proper baseline for that place.

A notebook or a spreadsheet is enough. The counter keeps its cumulative dose, trend curves and alarm settings after power off, but your notes on conditions are what make later readings meaningful. The manufacturer also describes a PC data download over USB-C with its own software, which the shop does not supply. For long logs, plan your charging: the manufacturer states 30 days or more in long-life mode, while an independent test reported about 5 days in normal use.

Why readings vary from place to place

Background radiation is not the same everywhere. The US EPA says so directly: it comes mostly from minerals in the ground, from water and from cosmic radiation, and it varies from one location to another (EPA, Radiation Sources and Doses). Building materials made from stone or earth can carry the same natural minerals, so indoor and outdoor readings can differ. The EPA's page on granite worktops (countertops, in its wording) notes that granite can contain traces of natural radioactive elements, that the amount varies from stone to stone, and that radiation drops quickly with distance.

Altitude matters too. According to the EPA, cosmic radiation rises the higher you are in the atmosphere, so Denver receives more of it than Miami, and cosmic radiation makes up about 5% of the average annual dose in the United States. For comparison, UKHSA gives average UK exposure of about 2.6 mSv per year, while the EPA cites 6.2 mSv per person in the US (NCRP). These averages explain variation; they are not a line that makes any reading acceptable.

When your reading looks unusual

Most surprises have an ordinary cause: a different spot or an object nearby. According to the EPA, some old objects can make a Geiger counter react when it is held close: uranium glass (yellow-green, glowing green under UV light), some uranium-glazed pottery made before 1973 and luminous radium dials on watches and clocks. Keep such objects intact, never take apart a radium dial, do not use them for food or drink, wash your hands after handling, and ask your local authority if one breaks. On beta sources the µSv/h figure also runs high because the tube has no beta shield, so read the CPM.

If a reading is unusually high or stays high, move away from the source and do not handle the object. Contact UKHSA or your local authority, and in an emergency call 999. The two adjustable alarms, by sound, flashing light or vibration, can flag a threshold you set, but the readings are indicative and the number on the screen is not a verdict on an object.

What radiation maps cannot tell you

A map of air monitoring stations shows background gamma over a wide area. It says nothing about the object on your desk or the air in your cellar. Radon is the clearest example: it rises from the ground, and neither a station map nor a Geiger counter measures its concentration. In the UK, ukradon.org offers a radon measurement pack, and its Affected Areas maps show where to test; if the level is high, simple works can reduce it. In the US, the EPA likewise points to a radon test kit, sold in hardware shops, online or through state radon programmes.

A quiet map for your region says nothing about radon in one particular house, which is why UKHSA and the EPA point to a measurement kit, not a map. Use official figures as a backdrop for your region and your own log for your shelf. If you want a counter for this kind of logging, Geiger Counter Shop sells one pocket model with a glass GM tube (J321 type as listed by the supplier), a 2.4-inch colour screen, µSv/h and CPM, a trend graph, alarms and a USB-C battery, delivered free from UK stock for UK orders.