The next topic in this series on invisible hazardous substances is radioactive materials.

We don't usually think of ourselves as being in danger from radioactive particle radiation, but we are all exposed to radon every day.

What is Radon, why is Radon dangerous?

Radon is a radioactive noble gas (chemically inactive). It is created when Uranium or Thorium decay on their way to becoming non-radioactive lead. When Radon decays, it creates other radioactive isotopes like Polonium that also have a short half life. When a Radon atom undergoes a decay event it ejects an Alpha particle from its nucleus. These Alpha particles are initially traveling at a rate of 54,000 km/hr, (44 times faster than the speed of sound) so it is not hard to imagine they could damage a cell if they hit some DNA at this speed. An Alpha particle is simply 2 protons and 2 neutrons (a helium nucleus). In air an Alpha particle will collide with so many gas molecules it loses its speed in about 5 cm. In flesh it might penetrate 50 microns (less than the thickness of a sheet of paper) before it stops, but this is still up to a half dozen cell layers.

About 0.1% of each lung cell is DNA, so there is about 0.5% probability (going through 6 cells) that an Alpha particle spawned in a lung will hit some DNA.

How dangerous is Radon?

Outside air may average about 0.4 pCi/L (0.015 Bq/L or 0.9 decay events per minute per L).

Indoor air may average about 1.3 pCi/L (0.048 Bq/L or 2.9 decay events per minute per L).

We breathe about 8 l/minute of air, so on average there could be more than 3 Alpha particles generated per minute in a pair of lungs as Radon may tend to stay in the lungs longer than other air molecules (due to its weight).

Extrapolated to 50 years – this implies there are about 76 million Alpha particles generated in a pair of lungs or about 380 thousand Alpha particles colliding with DNA. Since lungs have several trillion cells, this amounts to DNA damage to about 1 cell in every 10 million cells. Not all DNA damage will result in cancerous cells, and single strand DNA breaks can often be repaired in a cell, but the problem is of course that once created, cancer cells reproduce. Alpha particles are much more likely to create double strand DNA breaks than gamma radiation.

In Canada about 1,900 deaths per year are attributed to Radon, or about 0.6% of deaths may be due to Radon. This is roughly the same as the risk of death from car accidents.

Why are we only concerned about lung cancer from Radon? What about skin cancer?

The outer layer of our skin is also subjected to damage from Alpha particles, but there are many mitigating circumstances that reduce the effects on skin cells:

  • Since Radon is a gas, we can easily breathe it into our lungs
  • in a lung every Alpha particle will hit live tissue, no matter what direction it is emitted, whereas outside more than half of all Alpha particles are traveling away from the body and never hit it.
  • in a lung every Alpha particle in a liter of air is still traveling at high speed when it hits tissue, outside less than 50% of Alpha particles traveling towards the skin in a “cubic” liter of air could reach the body at high speed
  • any clothing, hair, nails, callous or even sweat could stop an Alpha particle before it reached a living cell (perhaps a good excuse to have long hair...)
  • we shed skin cells at a rate of between 40,000 cells and 1 million cells per day, so the entire outer layer of skin cells is removed in a few weeks. The top 18 to 23 layers of skin are made of dead cells which is enough to stop Alpha particles and further damage in dead cells is not dangerous and won't get reproduced. (new cells are generated deeper in the skin at the bottom of the epidermis) We generate enough new skin cells to about equal to the weight of our head each year.

Where is the Radon?

The average Radon “radiation” level indoors is 46 Bq/m3 (46 decay events per second per cubic meter) which is 3 times higher than outdoors. This occurs because buildings need to draw in new air to maintain appropriate levels of oxygen and CO2. When Radon gas is drawn into a building it tends to stay there because it is 9 times heavier than air and is not easily exhausted from the building.


What can we do to minimize exposure to Radon?

  • Get a low cost kit and test your house for radon
  • Radon is heavy, so generally the higher we are the less Radon will be present. Sleeping on a mat on the basement floor is not a good strategy to minimize Radon exposure.
  • One strategy is to allow more fresh air to enter the house since outside air usually has a lower concentration of Radon. It turns out my wife's insistence on keeping windows open has merit in this regard.
  • In houses where the furnace blows air from the basement upstairs, It may be worth positioning your bed so air coming out of the vent does not blow over your bed where Radon could fall on you while you sleep. It may be useful to use vent hoods on all vents to direct air coming into each room along the floor, making it less likely to rise to the level where you are breathing.
  • Radon mitigation systems are designed to extract air from the lowest areas in a house where Radon concentrations are highest. It may also be useful to pressurize your house drawing in air near the top of the house to create positive pressure which will minimize Radon being drawn into a house at ground level and below.
  • It may even be useful to use cyclonic vacuum cleaners because they may tend to propel Radon (with “centrifugal” force) into the dust trap instead of kicking it up back into the room air. There's a product idea - for a cyclonic Radon remover.

How can we measure Radon concentrations?

Even average Radon radiation levels are of concern, but they are only generating a few alpha particles per minute in a liter of air. If we have a Geiger counter set up it will generally only be sensitive to particles through its detection chamber from a hemispherical or (much) smaller arc and these particles will only be at high energy if they are generated within a few cm of the detector. The size of the detection chamber is small relative to a liter, so the % of alpha particles generated in a liter that actually get detected by a Geiger counter is probably less than 0.01%. This means a Geiger counter might see about 1 high energy alpha particle from Radon every 55 hours, assuming an optimal air flow past the detector. To achieve any kind of statistical accuracy, a Radon detector needs to monitor for weeks or months. Also the chamber walls in most Geiger counters are going to stop alpha particles from even getting into the chamber, so only special open-ended Geiger counters will even detect alpha particles.

Gas electron multiplier (GEM) technology is probably better – it can have a fairly large detection area and can be made to detect charged progeny of Radon decay.

Low cost passive devices are much more common, but need to be sent to a lab for analysis. They require no electrical power and generally trap radon or its daughter products for later analysis by a laboratory. Passive devices include charcoal canisters, charcoal liquid scintillation detectors, alpha track detectors and electret ion detectors.


Charcoal canister and charcoal liquid scintillation devices absorb radon or its products on to the charcoal. In the laboratory, the radioactive particles emitted from the charcoal are counted directly by a sodium iodide counter or converted to light in a liquid scintillation medium and counted in a scintillation detector.

The alpha track detectors have a plastic film that gets etched by the alpha particles that strike it. In the laboratory, the plastic is chemically treated to make the tracks visible, then the tracks are counted.

Electret ion detectors have a Teflon disc, which is statically charged. When an ion generated from radon decay strikes the Teflon disc, the electrical charge is reduced. In the laboratory, the charge reduction is measured and the radon level is calculated.

But all these methods require long periods to obtain results. It might be possible to concentrate radon using some cyclonic system to make it easier to measure, but concentrated radon is something you really want to avoid. I will not be attempting anything like that.

So, after all this research, it turns out I probably can't build a sensor that will safely measure radon concentrations in a reasonable time frame. I did order a radiation detector that can measure alpha particles, beta particles (high speed electrons) and gamma radiation, but I will just use it to see if it can detect anything in my environment. For example many smoke detectors have a radioactive source (americium). Americium produces mainly alpha particles which won't escape the smoke detector package. There is also some weak gamma radiation at about 59 KeV, so maybe it is detectable.


I chose this Radiascan 701 detector because:

  • It uses an ARM Cortex M3 for faster computation than competing products
  • It has a USB port allowing data to be read by an external computer
  • The USB port allows extended operation from USB power without depleting batteries
  • It is a very sensitive, high accuracy detector capable of detecting alpha, beta, gamma radiation and it has many features

Here is my 3D printed desk stand:


I am also designing a 3D printed wrist-mount for this sensor and expect to blog about test results using it in my environment.

My next blog will either be an unboxing (if the kit arrives) or a discussion of RF radiation.


All links to blogs related to this project can be found in the first blog here:

Safe and Sound - Invisible Hazardous Environmental Factors Monitoring System - blog 1