In 1984, a worker at a Pennsylvania nuclear power plant triggered the radiation detector as he was getting ready to go home. This would not be unusual for such a facility, but there was no nuclear fuel on site at the time. The alarm went off every time he left work.
One day, he triggered the alarm as he crossed the detector on arriving at the plant, leading him to suspect that he was bringing radiation from home. He eventually convinced the plant’s health physicists to check his home, although at first they were opposed to the idea. The results revealed high concentrations of radon everywhere, especially in his basement.
Radon was already known to be associated with health risks in underground miners at that time. This event revealed that a naturally occurring radioactive gas could be found in households at potentially hazardous concentrations.
The incident captured the public’s attention, and the Environmental Protection Agency (EPA) and the US Centers for Disease Control and Prevention (CDC) recommended that nearly all homes be tested.1,2 In 1988, the International Agency for Research on Cancer classified radon as a human carcinogen, and Congress passed the Indoor Radon Abatement Act in response to growing concern over health risks.3 This law funded state and federal measures to survey schools and federal buildings for radon levels, to educate citizens, and to develop programs for technical assistance. The long-term goal was to reduce indoor levels nationwide to no more than outdoor levels.
Radon is still considered an important public health hazard. From 15,000 to 21,000 people are estimated to die of lung cancer as a result of radon exposure each year in the United States, making it the second most common cause of lung cancer, behind smoking.4
Considering the relevance of this issue, this article will review the unique characteristics of radon as a risk factor for lung cancer.
WHAT IS RADON?
Radon is a noble gas that occurs naturally as a decay product of uranium 238 and thorium 232. It is colorless, tasteless, and imperceptible to our senses. Its most common isotope is radon 222 (222Rn), which has a half-life of 3.8 days and decays by emitting an alpha particle to become polonium 218. The decay chain continues through several intermediate steps until the stable isotope lead 206 is formed (Figure 1). Two of the isotopes in this chain, polonium 218 and polonium 214, also emit alpha particles.5–7
Radon is primarily formed in soil. Its most important precursor, uranium 238, is ubiquitous, found in most soils and rocks in various concentrations. Radon can also be found in surface water, metal mines (uranium, phosphorus, silver, gold), residue of coal combustion, and natural gas.
Outdoor levels are usually much lower than indoor levels, as radon dissipates very quickly. Indoor radon mostly comes from the soil under the house or building, but it can also originate from coal combustion, gas appliances, and water (especially from private wells). In municipal water systems or surface reservoirs, most of the radon dissipates into the air or decays before the water reaches homes.8,9
Radon’s only commercial application in the United States is in calibrating measuring instruments. In the past, it was used in radiography and to treat cancer but was later replaced by other radiation sources that cost less and pose less hazard of alpha radiation.10
HOW RADON CAN HARM
Alpha particles, emitted by radon 222 and its progenies polonium 218 and polonium 214, are highly effective in damaging tissues. Although they do not travel far or fast, with their two protons and two neutrons, alpha particles are heavy and therefore can cause considerable damage at short range. Although alpha particles can be stopped by a thin barrier such as a piece of paper or the skin, if the source is inhaled or ingested and lodges against mucosal linings, the alpha particles emitted can destroy cells.11
The main route of radon exposure is by inhalation. Since radon is biologically inert, it is readily exhaled after it reaches the lungs. However, radon’s progenies can also be inhaled, either as free particles or attached to airborne particles such as dust, which they tend to attract as a result of their charged state. This attached fraction is believed to be more carcinogenic because it tends to deposit on the respiratory epithelium, notably in the carinae of bronchi. The smaller the dust particle, the deeper it can travel into the lung. The radiation emissions damage the genetic material of cells lining the airways, with the potential to result in lung cancer if the repair process is incomplete.5,8,9
Other routes of exposure include ingestion and dermal exposure. Radon and its progenies can be swallowed in drinking water, passing through the stomach walls and bowels and entering the blood.12 Dermal exposure is not considered a significant route unless the dermis is exposed, since in usual circumstances the skin protects the body from alpha radiation.13
Possible biologic mechanisms by which radon exposure might increase the risk of cancer include gene mutations, chromosome aberrations, generation of reactive oxygen species, up- or down-regulation of cytokines, and production of proteins associated with cell-cycle regulation.14–16
HOW IS RADON MEASURED?
Several devices are commercially available to measure radon levels at home. The most common ones are activated charcoal detectors, electret ion chambers, alpha-track detectors, electronic integrating devices, and continuous monitors. There is no evidence that one device is better than another, but devices that measure radon gas are usually preferred over those that measure decay products because they are simpler to use and more cost-effective. These devices are divided into those used for short-term testing (2–90 days) and long-term testing (Table 1).17
Radon levels can be expressed as follows:
Working levels. One working level (WL) is any combination of radon progeny in 1 L of air that ultimately releases 1.3 × 105 MeV of alpha energy during decay. In studies of miners, the radon progeny concentrations are generally expressed in WL. The cumulative exposure of an individual to this concentration over a “working month” of 170 hours is defined as a working level month (WLM).
Picocuries per liter. In the United States, the rate of decay is commonly reported in picocuries per liter (pCi/L): 1 pCi/L translates to 0.005 WL under usual conditions. The outdoor radon level is normally around 0.4 pCi/L.
Becquerel per cubic meter (Bq/m3) is an International System unit of measure: 1 WL corresponds to 3.7 × 103 Bq/m3, and 1 pCi/L is equivalent to 37 Bq/m3.
Different areas have different radon levels
The Indoor Radon Abatement Act of 1988 helped identify areas in the United States that have the potential for elevated indoor radon levels. An estimated 6 million homes have concentrations greater than 4 pCi/L.
To assist in implementing radon-reducing strategies and allocation of resources, the EPA has created a map (Figure 2) that classifies counties according to the predicted indoor level.18
WHAT IS THE RELATIONSHIP BETWEEN RADON AND LUNG CANCER?
Determining the degree to which radon exposure contributes to lung cancer is a complex task. Radon can be found nearly everywhere, and there are diurnal, seasonal, and random year-to-year variations in the concentration of radon in indoor air.
A minority view
Not everyone agrees that radon is completely bad. For centuries, people have flocked to spas to “take the waters,” and the water at many of these spas has been found to contain radon. In the early 20th century, radiation was touted as having medicinal benefits, and people in many places in the world still go to “radon spas” (some of them in abandoned uranium mines) to help treat conditions such as arthritis and to feel invigorated and energized.
In 2006, a report by Zdrojewicz and Strzelczyk19 urged the medical community to keep an open mind about the possibility that radon exposure may be beneficial in very low doses, perhaps by stimulating repair mechanisms. This concept, called hormesis, differs from the mainstream view that cancer risk rises linearly with radiation dose, with no minimum threshold level (see below).