For radon, we installed a suction fan inside our house that sucks air from under the house. That lowers the pressure under the house, and radon doesn't seep in. It made the measurements go from ~120bq to 20bq.

> Radon buildup is also a potential concern

Citation needed?

As far as I am aware unless you are building deep underground or have built your house on radon rich bedrock, or your house is entirely made of granite, this is absolutely not a concern for the average person.

If you are in doubt you can look up what is the expected radon exposure on the ground region where you live. Most people will have a higher radiation exposure from an arm x-ray than from ground leaking Radon.

It's location dependent, but more people live with radon seeping up from the ground than you would think. Some countries now have building codes that demand the foundation gets sealed to prevent it.

in the UK there are many areas that Radon can be an issue

from: https://www.ukradon.org/information/ukmaps

"Every building contains radon but the levels are usually low. The chances of a higher level depend on the type of ground. Public Health England has published a map showing where high levels are more likely."

getting a sensor at a reasonable price is tricky since Radon emits Alpha radiation - I managed to get one at around £300

It's not that much of a concern, but mostly because tests are widely available. Test your basement for radon, and only worry about it if the levels are high.

Radon is an alpha emitter which gets into your lungs, so that comparison to an arm x-ray isn't entirely fair. Otherwise I agree, it shouldn't be a concern unless you live in the first floor on granite.

Alpha particles penetrate barely any at all. An arm x-ray would also hit you with particles that hit more (and more valuable) cells in your body.

It's right they don't penetrate. However, the lungs is one place they don't need to penetrate, they get direct access to some very valuable (to us) cells in our bodies, which are especially vulnerable to cancer, and do grave damage to them. From 10 to 1000 times more damage than equivalent energy gamma or beta radiation, according to Wikipedia. Radon as a health problem isn't some crank thing.

My gut feeling tells me activated carbon is not enough but you need HEPA.

Many purifiers contain a carbon and HEPA filter.