Can I trust that my Geiger counter (GQ GMC 500) is in fact measuring radiation?

I'll reply to this later as I haven't got time right now, and this is getting to be a fairly complex subject, which takes time to explain.

Essentially, no GM tubes are the same. They all have different sensitivities, so you can't rely on CPM between different tubes. Each tube has what is known as a conversion factor. This is the conversion from CPM to uSv/hr. Two different tubes, one measuring a background of 24 CPM and another (in the same setup) measuring 130 CPM background are actually measuring exactly the same dose rate. So the conversion from the 24 CPM may be 0.14uSv/hr and the other - 130 CPM will be exactly the same 0.14uSv/hr.

I'll reply in some detail later, but if you get chance, have a read up on GM tube sensitivities, conversion factors and dose rate, etc.

Also 0.200 uSv/hr is really really low. Nothing, IMHO, to worry about. I am averaging around 0.15uSv/hr at home (sometimes 0.11, sometimes 0.20, and fluctuates between). That's quite normal, and as I said, different parts of the world, even different places in the same country will have differing background levels. There are other factors too such as cosmic rays, muons, that can cause rates to increase, but they are, in fact, entirely harmless.

Like I said, it's getting complex, so I'll reply to this later.

Edit: It would be really handy to know what actual tubes are in the GMC-500. From what I understand it has two, a SBM-20 and a M4011, but this in unconfirmed. It could have two M4011's or J305's. Also as the GMC-500 has two tubes, they probably run some kind of algorythum to work out the conversion factor for it.

Alright, lets see if I can put this as something that makes sense.

[All numbers are fictitious - I haven't looked up the proper numbers, they are for the examples only.)
First of all it is important to understand the counts per minute (CPM) and counts per second (CPS) are not measures of dose or does rate. They are simply dependant on the tube used. At background level, lets assume 0.1 uSv/hr may be 20 cpm using a SBM-20, but a larger tube, such as the SI-21G, the count may be 130 cpm, at 0.1 uSv/hr. The dose rate is the same despite the cpm between both tubes being worlds apart. This is down to different sensitivities of different tubes. All types/models of GM tube are different. They will all give different counts for the same radioactive source.

Imagine it like this - It is raining outside at a fixed rate of 1mm per hour. We have two buckets to collect the rain. Both buckets are exactly the same height. One bucket is 20cm in diameter and the other bucket is 10cm in diameter. Both buckets are placed in the rain. after an hour the larger bucket has collected 4000 drops of rain, and the smaller has collected 1000 drops of rain. (20cm diameter is four times the area of 10cm diameter). Even though the smaller bucket has only collected a quarter of what the big bucket collected, the amount of rain falling from the sky is exactly the same for both buckets. The reason the smaller bucket collected half as many drops is due to size (collection area), and can only collect rain from a smaller space. We would say that the small bucket is less sensitive to collecting rain, and the larger, more sensitive to collecting rain. Now, change rain for radiation, and buckets for GM tubes, and the same thing happens.

To combat this we use a conversion factor to convert the cpm into something more useful, such as uSv/hr. This is just a simple calculation based on the tube's conversion factor. Each type/model of tube has a different conversion factor. This conversion factor is calculated from the sensitivity of a tube. Generally the sensitivity is shown in the datasheet for the tube so we can use that for calculating the conversion factor. There are a few posts in the forum talking about conversion factors so I'm not going to go into the different numbers for different tubes now. This is more the fundamentals. So to recap, I put Geiger counter with an SBM-20 tube 20cm from a source, it may be counting 200 cpm and showing 1.00 uSv/hr. Then I put another Geiger counter with a SI-21G the same distance from the source and it counts 2000 cpm, but also shows 1.00 uSv/hr. This is due to a calculation happening within the Geiger counter. So in a nutshell, cpm means nothing as a means of quantifying radiation. Sieverts, or other SI units are the numbers that matter when working out dose rate.

By now, if you are still reading this, you are probably wondering 'But... but radmon.org uses cpm and not uSv/hr.... WTF?' And that is correct. There are some reasons for this. Dan never wrote it into the code, or at least he wrote some into the code but never used it on the site. Radmon.org is not a bonefide radiation monitoring site that should be used to reference the actual radiation levels in a certain location. The stations generally all use cheap counters and low cost GM tubes. The best they can ever give is an idea, or an indication towards something. They shouldn't be used for guidance. The idea of using radmon.org for background monitoring (IMHO) is to base any detection's on a baseline. So a station is set up. It runs for ages and the graphs generally show exactly the same thing since forever. That's the baseline. Then any rise in radiation or spikes can be compared to the baseline and we see something. Just something. What was it? Nuclear power station leaking, or a user putting some am-241 near the tube to make it count some more? People experiment with their stations, and a lot of people don't really look after them either, thus leads to ambiguous readings and graphs. In order for radmon.org to display uSv/hr for each station, every user would have to know the conversion factor for their tube and input it, say in the control panel. I know for a fact that at least 25% of the users simply wouldn't. A lot of users wouldn't understand (maybe would after reading this), and a lot of users simply set up their stations, see it is recording to radmon.org, and hardly ever look again. So unless each and every user was 100% on board it couldn't be done effectively. So we keep CPM and use it as a monitoring site, based on baselines. And that is pretty much how I use my counters at home. I know my baseline has been ~0.15uSv/hr average (or the corresponding cpm for the tube) for years now. If it goes up and stays up for a bit, I'll know the radiation has increased. If I look at a station somewhere on the map and it shows a rise, I will look at the other stations around it and if none have risen also, I'll just disregard it. But if the other stations showed a rise also, then that would be something to investigate.

The 0.2 uSv/hr threshold in the Netherlands is the yearly average, so would have to sit at that level for an entire year for it to trigger an investigation. However, that rate could also be achieved in a matter of hours if there was a large release of radiation. Obviously this would be detected, and investigated. However, if the radiation rose to 0.5 uSv/hr, then after an hour it dropped back down to say, 0.11 uSv/hr, then this would simply be recorded, and not investigated. At home I get between 0.1 uSv/hr and 0.2 uSv/hr, approximately. One of my counters with a SBM-20 tube would be reading 24 cpm, another one with a SI-21G tube would be reading 130 cpm. It does go up and down so it is good to take a 5 or 15 minute average, or even longer, and that should smooth things out a bit. The 15 minute average for a counter in my office is about 0.15 uSv/hr. So pretty typical for my environment. If I go out in the countryside and take the same counter it would reduce, quite a lot, but really does depend on the environment. I have had it as low as 8 cpm / 0.046 uSv/hr in places in the past. The reason it is higher at home is due to all of the different materials around me. It is an old house with old bricks. The clay used to make the bricks will have some small amounts of NORM (naturally occurring radioactive materials), the same for the cement mortar. The roof tiles, screws and nails used in construction and even the wood (floors/joists/doors/casings) can be minutely radioactive if the tree absorbed something during growth. So radiation is everywhere, and when measuring background it is usually those things and others that have a little bit of NORM in them that we are detecting, with a few cosmic rays too. 0.2 uSv/hr is tiny. It's not going to do much at all. We are all subject to radiation all of the time, from all sorts of things.

NORM (naturally occurring radioactive materials) can actually cause cell damage (ionize cell atoms, break bonds or cause DNA damage), but is very, very minimal. A healthy immune system does it's job of fixing this damage as it occurs. This is something to consider regarding NORM/background radiation and cell damage - for every 1 cell that gets damaged by background radiation, 1000 cells in the body get damaged due to everyday items, such as perfumes/deodorants, cleaning chemicals (those you use in the kitchen/bathroom/on the car/at work), air pollution, and so on - See VOCs, Phthalates, BPA etc. Even microplastics can cause cell damage by inflammation from physical irritation, then they can leach out chemicals once inside the body (Phthalates, BPA etc.) Microplastics can bind heavy metals and other pollutants such as PCBs and pesticides (think Roundup). So really for me, radiation is right at the bottom of my scope of how not to become dead. I do enjoy it as a hobby though.

I hope this clears things up a bit for you, and other users too.

I used AI to generate a table with exposure levels and their effects and outcomes. That is in the image below.

It also gave me a few notes:

• The LD50 (lethal dose for 50% of people) is around 4–5 Sv, untreated
• Cancer risk starts to be statistically significant at 100 mSv
• Regulations aim to keep public exposure below 1 mSv/year (excluding medical)