1.2 Radon History & Health Risk

Radon History & Health Risk

The known health risk of radon comes from three primary categories of radon research: 1) Underground Miners, 2) Animals, and 3) Residential [epidemiological] studies.

Miner Studies

The miner studies occurred long ago and are what helped us understand that radon [in general] causes lung cancer. Unlike radon gas measurements, working level measurements were made in the mines considered to be occupational settings. Exposure times were also recorded to determine a time and dose record for each of the miners. The term used to express the time-dose relationship is working level months (WLM), a calculation of the amount of RDPs to which a person is exposed, multiplied by the duration of exposure expressed in fractions of a month.

Animal Studies

Although animal studies have helped demonstrate that radon causes lung cancer, human epidemiological data provides the most important basis for assessing risks of radon exposure, proving that radon is a Class A carcinogen.

Epidemiological Studies AKA Residential Studies

Multiple epidemiological studies were conducted in INDOOR residential air and pooled together in what is called the North American Residential Radon Pooling Study, including the Iowa Lung Cancer Study [IRLCS] (Epidemiology, March 2005) and the European Residential Radon Pooling Study (British Medical Journal January 2005). These studies combined data from twenty previous residential studies to confirm the radon risks predicted by extensive occupational studies of underground miners who breathed radon for a period of years. By pooling the residential data, studies showed an increased lung cancer risk consistent with the predicted 12% per 2.7 pCi/L based on a linear model developed by the National Research Council.

According to EPA, the North American and European pooled studies effectively end any doubts about the risks to Americans of having radon in their homes and confirm that breathing low levels of radon can lead to lung cancer. 

The EPA estimates over 21,000 deaths per year are caused by radon-induced lung cancer in the United States. The World Health Organization (WHO) estimates that radon causes up to 15% of lung cancers worldwide. The epidemiological studies outlined above are based on the Linear No-Threshold risk model and effectively provide evidence that radon in residential dwellings contributes to radon’s health risk.  

People are at the greatest risk at home where 1:15 homes has elevated radon.

According to the EPA Citizen’s Guide to Radon, one out of every 15 homes in the U.S. is estimated to have elevated radon levels and people are at greatest exposure risk at home where they spend most of their time. Since every home is built and occupied differenty and it is impossible to understand substrate differences between any given home, testing is the only way to know if there is a problem.

The EPA and U.S. Surgeon General recommend testing all homes below the third floor for radon.All new construction homes should be tested before and after occupancy.

As well, the EPA and U.S. Surgeon General recommend testing all homes below the third floor for radon, and every new construction home should be tested after occupancy, even if it was built radon-resistant. If radon levels in a new construction home with a passive system exceed 4 pCi/L, the passive system should be activated by having a qualified mitigator install a vent fan.

Health Risk to Workers – Occupational Exposure

Special consideration must be paid to monitoring worker exposure to limit the risk of lung cancer. Although the term Working Level Month comes from occupational exposure records that pertain to the underground mining industry, it is universally used to calculate radon exposure for all radon mitigation practitioners but predominantly radon mitigation contractors.

A Working Level Month [WLM] The unit of measure of the cumulative exposure to Radon Decay Products over a period of months.

There is currently no single recommended method of monitoring radon mitigation workers for exposure to radon and RDPs. Since the risk is primarily attributed to Radon’s progeny or by-products, called Radon Decay Products, worker exposure should be limited to no more than 4.0 Working Level Months per year. A Working Level Month [WLM] is the unit of measure of the cumulative exposure to Radon Decay Products over a period of months. Working Level Month [WLM] can be calculated mathematically as illustrated below [Ref. Chapters 5 and 7]:

1 WLM = 1WL for 170 hours.

(This assumes 170 working hours per month).

Linear No-Threshold Risk Model

One such epidemiological study that definitively identified radon as a Class A carcinogen in humans is the Linear No-Threshold model (LNT) in which the dose-response model is observed. This LNT model is applied in radiation protection to estimate stochastic health effects such as radiation-induced cancer, genetic mutations and teratogenic effects on the human body due to exposure to ionizing radiation.[1]

DNA damage can occur at any level of radon exposure.

The epidemiological study degree of risk is based on an assumption of a linear no-threshold relationship between the concentration of exposure and the duration of exposure which diminishes the risk by 50% in length of time. For example, the LNT would indicate that a person exposed at 4 pCi/L for 1 year is at the same risk as someone who is exposed at 2 pCi/L for 2 years or 16 pCi/L for 3 months, etc.

Since even a single alpha particle can cause major genetic damage to a cell, it is possible that radon-related DNA damage can occur at any level of exposure. Therefore, no level of radon is safe. Figure 1.2

Source: EPA.         Figure 1.2 Cumulative Lifetime Risk Graph

Cumulative vs Acute Risk

Increased lung cancer risk has been observed at low cumulative exposures calculated by the Working Level Months of exposure (WLM), comparable to cumulative lifetime residential exposures complicated by environmental factors and the direct correlation with smoking. In other words, radon exposure is not an acute risk, rather it poses a cumulative health risk over long periods of time and there is no level of immunity.

In fact, in some studies, as summarized in the WHO Handbook on Indoor Radon, A Public Health Perspective, 2009, low exposures over longer periods of time were associated with greater risk than high exposures of shorter duration. This may be particularly important to the estimation of risk from residential radon exposure, as residential exposures tend to be lower exposures received over longer periods of time, while occupational exposures tend to be higher levels received over shorter time periods.

If the lower exposure rates do pose more risk, then current radon risk assessments may be underestimating the risk from residential exposures.

Despite limited data, a National Academy of Science Committee concluded in its report entitled “Biological Effects of Ionizing Radiation” (BEIR IV) that lung cancer risk increases exponentially with smoking. In other words, the risks from combined exposure to radon and smoking is greater than the sum of the risks from exposure to either active alone. This risk is indicated in the US EPA risk chart for smokers and non-smokers (EPA Citizen’s Guide to Radon, 2016).

Figure 1.3 DNA Effects of Ionizing Radiation

Risk Estimates

A number of uncertainties are associated with both the risk factor and the estimate of average residential radon exposure may be quantified in risk assessment. Worth mentioning are the following factors considered in the uncertainty analysis are:

  • The effect of age on radon risk
    • Children have been reported to have greater risk than adults of certain types of cancer from radiation, but there are currently no conclusive data on whether children are at greater risk than adults from radon.
  • Possible differences between homes and mines in terms of dose per unit exposure
  • The effect of exposure-rate on radon risk
  • The effect of mine exposures other than radon (silica, diesel, etc.)
  • The relationship between radon risk and smoking.

Risk Assessment is Difficult at Low Levels

Due to the difficulty of measuring radon at low levels, radon results at or close to the 4.0 pCi/L Action Level, has a 50% chance of being slightly above or slightly below the Action Level.
  • Short-term tests are less definitive about whether or not your home is above 4 pCi/L.
  • This can happen when your results are close to 4 pCi/L. For example, if the average of your two short-term test results is 4.1 pCi/L, there is about a 50% chance that your year-round average is somewhat below 4 pCi/L.
  • EPA says no level of radon is safe. Even radon levels below 4 pCi/L pose some risk
10-Fold Risk
62:1000 people risk developing lung cancer from radon exposure; versus 7:1000 never-smokers.

EPA also recommends that Americans consider fixing their home at radon levels between 2 pCi/L and 4 pCi/L. *This is not a health-based action level.

The Tobacco Correlation

According to the World Health Organization, radon is much more likely to cause lung cancer in people who smoke. Of these sources of uncertainty, the relationship between radon risk and SMOKING is often of the greatest concern because smoking is the leading cause of lung cancer. Based on the EPA’s risks calculations, lifetime exposure at the EPA action level of 4.0 pCi/L, 62:1000 or 62 out of 1000 smokers risk developing lung cancer from radon exposure versus 7:1000 or 7 out of 1000 for those who have never smoked. That equates to a ten-fold risk for lung cancer in smokers.

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