We recently received a solicitation to review a government document regarding radon in multi-family buildings. It is great to see the identification of radon hazards occurring in more and more situations. However, what disturbed me was a preamble explanation as to how radon causes lung cancer. The document contained the following sentence:
“The most common pathway for human exposure to radon is inhalation indoors. When inhaled, some radon gas remains trapped in the lungs, and sensitive lung tissue can be exposed to radiation as it decays”.
On the surface this may seem like an okay statement for the general public. However, it is inaccurate and perpetuates a common misconception about the health effects of radon which is contrary to the known mechanism. One may assume this was written by an uninformed staffer, but of greater concern if this was written by a radon professional who does not fully understand the science.
So, what is wrong with the statement?
The biological effects of elevated indoor radon are not from radon but rather the short-lived decay products of radon. Most notably Polonium 218 and 214. If there is radon in the air within a home, there will also be the attendant radon decay products which behave as particulates and have strong electrostatic charges.
When one breathes in, air containing both radon and its decay products are inhaled. However, when one breathes out, the radon is exhaled, and because of the electrostatic charges of the radon decay products, they stick to the breathing passage and lungs. Before your lungs have an opportunity to clear themselves through the escalatory function (mucociliary transport), the short-lived Polonium 218 and Po214 can decay, delivering a powerful alpha particle punch to the lung cell to which they are clinging.
It may take several hours for a normal lung to clear particulates. Perhaps you may recall working in a dusty environment like a crawlspace without respiratory protection (not a good idea) and you blow your nose and see the guick on the tissue. It may be several hours before that goes away. Furthermore, you can smell or taste it for even hours more. Within that time frame, a Polonium 218 atom with a 3-minute half-life and a Polonium 214 atom with a half-life of 163 microseconds can surely decay and release an alpha particle while it is still in the lung or air passage.
The Figure below is a slide from the CERTI Entry-Level radon course that shows the relative time frames at which radon and its decay products radioactively decay. Sure, a radon atom might decay while it is in the lung before it is respired, but it is the attachment of the radon decay products, coupled with their short half-lives that make them the actual health risk.
So, the health effect of radon is not because radon is trapped in the lungs as stated in the solicitation, but rather, the decay products of radon that are inhaled. If radon, as well as other gasses were held up in the lung, we would quickly expire from accumulated carbon dioxide.
One can see this mechanism explained in many authoritative works on the health effects of radon, such as:
Biological Effects of Ionizing Radiation (BEIR VI)
“Alpha particles released by 2 radioisotopes in the radon decay chain, polonium-218 and polonium-214, deliver to target cells in the respiratory epithelium the energy that is considered to cause radon-associated lung cancer”
Technical Support Document for the 1992 Citizen’s Guide to Radon:
“Of the short-lived radon decay products (polonium-218. Lead-214, bismuth 214 and polonium-214), the polonium isotopes contribute most of the radiological dose to the lung. The risk from inhaled radon-222 is small compared to the risk from inhaled radon 222 decay products”
Exposure vs Dose
It comes down to the difference between being exposed to a hazard, versus receiving a body-harming dose. Radon in indoor air represents an exposure much like the sun represents an exposure for potential skin melanoma, but the actual risk is what is absorbed by the body — or if you are indoors or outdoors, or you wear a hat or long-sleeved shirts.
In the case of elevated indoor radon, there are a known amount of radon decay products that will be formed in the air around us. However, that does not mean that is what will enter the lungs as we breathe. A large portion of the radon decay products (50 to 60%) will attach to physical objects within a room and no longer be breathable. This is reduced further with the use of particulate air filters and air circulation as occurs in schools and large buildings. What remains after these reductions is the actual dose that can be imparted to the lungs.
So why do we routinely measure radon rather than radon decay products?
Actually, many of the miner studies linking radon induced lung cancer are based upon direct radon decay product measurements. Even more recent epidemiological studies of lung cancer incidence in residential settings show a much better correlation to radon decay product measurements than radon alone. That is because they are a better measure of dose received rather than an inferred dose from exposure.
Radon measurements are indeed easier to make. They are simple and affordable. But realize a radon measurement is a surrogate or substitute measurement for estimating health risks of the radon decay products. In many situations, quantifying the radon potential is adequate and reducing radon entry via active soil depressurization reduces radon and, in-turn, reduces the attendant radon decay products proportionately.
However, there are cases where an additional quantification of radon decay products is prudent. This certainly would be the case in larger buildings where efforts by EPA to reduce indoor particulates via better HVAC systems and filters can have a significant impact on reducing the dose from radon. Examples of this are focused reduction in asthma triggers and most recently Covid-19 viruses.
How one would apply these measurements is described within Section 9 of the ANSI/AARST MAH 2019, Protocol for Conducting Measurements of Radon and Radon Decay Products in Homes. In essence, the protocol requires:
- Concurrent radon and radon decay products measurements to allow one to determine the actual percentage of radon decay products rather than to assume a 50% relationship.
- Identify aspects of the building that may be reducing radon decay products or increasing radon decay product doses due to higher particulate air counts.
- Determine if those aspects of the building that reduce radon decay products are likely to be maintained, such as operation of an HVAC system during occupied hours, etc.
So why get upset about inaccurate statements?
Inaccurate statements inhibit innovative approaches. Many of us are members of a trade association called the American Association of Radon Scientists and Technologists. Note the emphasis on the word Scientists. One would assume that, as scientists, we would be seeking out new and innovative ways of solving a problem. However, simplified models or inaccurate statements inhibit growth and innovation.
It has been my experience when students contacted me about a very difficult mitigation, that many of them forget what the real objective is, which is to reduce radon related lung cancer to as low as reasonably achievable. Yes, Active Soil Depressurization is a great tool, but it doesn’t have to be the only tool, especially when the tools may already exist within a building for reducing radon decay products, but they are unaware of it because they are either unaware of their significance or do not know how to measure radon decay products.
In closing, I am reminded of several conversations with Dr. William Field regarding the research indicating that 2/3 of the radon induced lung cancers come from exposures less than 4.0 pCi/L and the only way to reduce exposures further is via radon decay product reductions.
For those who would like to learn more about radon decay measurement and reduction as another tool in quantifying and reducing radon risks, we encourage you to take the following course, approved by both NRPP and NRSB:
C-4-110 – Addressing Radon Decay Products – Another Tool in the Tool Box (CERTI-324)
Who says there is nothing new in radon?
Technical Consultant to CERTI
 US Department of Housing and Urban Development, Departmental Policy for Addressing Radon in the Environmental Review Process, Notice CPD-21-136.
 National Research Council, National Academy Press, Health Effects of Exposure to Radon, BEIR VI, 1999
 US EPA, Technical Support Document for the 1992 Citizen’s Guide to Radon, EPA 400-R-92-011, May 1992.