Answer
Overview
To summarise, even when radon gas accumulates at what we consider high levels, it is still a relatively low amount of ionising radiation compared to other events, partly because of the dispersed nature of a gas resulting in a lower density of molecules compared to solids like uranium. Also, radon and it’s decay products primarily decay by releasing alpha particles. These are highly ionising but cannot penetrate materials to a significant depth.
Radiation Units
One challenge for the non-specialist (myself included) with radiation is that the units can be confusing; there are different units to express rate of radioactive decay, ionisation power and dose received by biological tissue.
For example, radon gas is often quantified using pico curies per litre (pCi/L), or becquerels per cubic meter (Bq m-3). One becquerel is one radioactive disintegration per second and 1 pCi/L = 0.037 Bq m-3. These units quantify radioactive decay rates per unit volume and are commonly used for gases.
Here are some of the commonly-used radiation units:
SI Units Common Units
Radioactivity becquerel (Bq) curie (Ci)
Absorbed Dose gray (Gy) rad
Dose Equivalent sievert (Sv) rem
Exposure coulomb/kilogram (C/kg) roentgen (R)
Absorbed dose (mesures in gray) is commonly used in medicine to plan radiotherapy, where it represents a total dose, which will be delivered in multiple fractions.
The sievert is useful for determining damage to the body tissue, but needs a conversion factor depending on the type of ionising radiation and the tissue. For example, alpha particles (helium nuclei) are significantly more ionising than gamma rays (electromagnetic radiation), but are much less penetrating.
Thus, alpha particle radiation can be stopped by a few centimetres of air, or a layer of clothing, whereas gamma rays will penetrate all tissues. However, radon (or decay products that emit them) can be dangerous if they remain in body tissues, such as the lungs, releasing alpha particles repeatedly into adjacent tissue. Although they will not penetrate deeply, they will cause ionisation damage to the nearby layers of cells.
Both the gray (Gy) and the sievert (Sv) measure radiation dose and represent energy delivered per unit mass of tissue (joules per kilogram, J/kg), but the sievert is adjusted depending on radiation type (the conversion multiplier is 1 for x-ray and gamma radiation but 20 for alpha particle radiation).
Radon progeny
Radon originates from the decay of uranium over geological timescales. It’s own decay products (progeny) are shown below. They are all radioactive as well.
Radon gas is created by the radioactive decay of radium in the earth (which in turn is created by the decay of other elements, starting with uranium).
The numbers show the half-life of the radioactive decay process. You can see that decay to lead-210 happens over a timescale of minutes but then further decay has a half-life of years as lead-210 is relatively stable. These progeny also decay through alpha radiation and generally deliver more radiation dose than the radon itself.
Source: nuclear-power.com
Radiation Effects
From the World Health Organisation (WHO):
Beyond certain thresholds, radiation can impair the functioning of tissues and/or organs and can produce acute effects such as skin redness, hair loss, radiation burns, or acute radiation syndrome. These effects are more severe at higher doses and higher dose rates. For instance, the dose threshold for acute radiation syndrome is about 1 Sv (1000 mSv).
So a dose of 1 Sv will cause acute radiation sickness. However, smaller or more gradual doses can still have longer term effects if the damage to DNA causing mutations surpasses the cell’s ability to repair the damage.
If the radiation dose is low and/or it is delivered over a long period of time (low dose rate), the risk is substantially low because there is a greater likelihood of repairing the damage. There is still a risk of long-term effects such as cataract or cancer, however, that may appear years or even decades later. Effects of this type will not always occur, but their likelihood is proportional to the radiation dose.
Radon is estimated to cause between 3% to 14% of all lung cancers in a country, depending on the national average radon level and smoking prevalence.
Lung cancer risk is higher for smokers due to synergistic effects of radon and cigarette smoking.
From your example:
100 pCi/L = 3700 Bqm-3 (i.e. 3700 disintegrations per second per cubic metre of air)
According to IRCP, converting a radon level to an effective dose is done using the dose coefficient as follows:
Effective dose = radon level × time × dose coefficient
We can use a dose calculator for radon like the one at ICRPaedia. With a few assumptions detailed there, there, the dose can be estimated with 6.9 x10-6 mSv per Bq h m-3.
In your example, this works out at approximately 25 millisieverts per 1000 hours presence at that concentration.
By comparison, in a nuclear incident at Los Alamos involving physicist Louis Slotin, an arrangement of plutonium and beryllium went prompt critical and he received a whole body dose of about 21 sieverts in just a couple of seconds and died a few days later due to acute radiation sickness. This is almost 1000 times the dose you would get from 1000 hours in the the radon scenario you describe.
Your example is broadly similar to estimates of standard background radiation exposure (from cosmic rays and radon in the population):
Naturally-occurring background radiation is the main source of exposure for most people. Levels typically range from about 1.5 to 3.5 millisievert per year but can be more than 50 mSv/yr.
This is also a comparable dose to a whole-body CT scan.
Summary
Radon does not cause acute symptoms due to radiation damage at the concentrations it is commonly found in.
However, it can contribute to to DNA damage and increase the risk of lung cancer in the longer term (especially in people who smoke), due to deposition of radon and its decay progeny in the lungs.
pCi/L(picocurie per litre) is equivalent to 37 Bq/m^3 (becquerel per cubic metre). A becquerel is one disintegration per second.