Radon is one of the main causes of lung cancer in non-smokers. It is estimated to cause around 21,000 deaths annually. It is a radioactive gas that is tasteless, odorless, and colorless. Naturally occurring in the environment, it is the decay product of uranium-238 and radium-226. As an extremely dense and highly radioactive gas, it can damage the respiratory epithelium through the emission of alpha particles. It can be found in the soil, rocks, and ground throughout the world. It can exist in water supplies and can be entrapped in homes. It has a tendency to build-up in large quantities in areas with poor ventilation, and high levels can eventually cause health concerns. Recently, there has been a statistically significant linear relationship found with increased radon concentrations and an increased risk for lung cancer.
Given that it is imperceptible by color, taste, or smell, and causes no obvious symptoms of irritation or exposure, measuring radon levels is the only way to know if there exists a high level of exposure.
Radon is a highly radioactive element that was initially discovered in 1899 by physicist Ernest Rutherford in England. It is not a chemically produced gas; it is usually found in rock and soil. It is an inert gas and the heaviest known gas, known to be nine times denser than air. It is made of a single atom and thus can easily penetrate many materials such as paper, leather, plastic, concrete, wood paneling, and insulation. Radon occurs naturally in several isotopic forms, but only two are found in high concentrations in the environment: radon-220 and radon-222. Radon-220 is formed in the decay process of thorium-232 and has a half-life of 55 seconds.
Radon-222 has a half-life of 3.8 days and is the most stable isotope of radon. It is the immediate decay product of radium-226 and is transient in the decay process of uranium-238. Due to the longer half-life of radon-222, it can diffuse from the environment into homes and further decay into more chemically reactive radon progeny (also known as “radon daughters”). These radon progenies are known as the main source of human exposure to alpha radiation, which results in cellular injury and DNA damage in the respiratory tract. These changes in the respiratory epithelium eventually result in radon-induced lung disease and lung cancer.
Human exposure to radon occurs primarily through inhalation and ingestion. Given that radon exists in groundwater, soil, rock, and confined spaces such as buildings and basements, human beings are susceptible to exposure from multiple sources, including the ground, gas appliances, water supply, or pressure-driven flow in the home. Since it is also present in natural gas, radon (and its progeny) can thus be released into the air when it is burned in furnaces, fireplaces, heaters, and stoves. The inhalation of radon ultimately poses the greatest health concern.
Radon exposure is a global environment and health issue. Recent studies in 2012 have noted the highest levels of radon in Armenia, where it is estimated to have contributed to 29% to 30% of lung cancer cases. Japan had the lowest levels, with an estimated 4% of lung cancers attributed to radon. In the US, it is believed that 9% to 13% of lung cancer cases are caused by radon exposure. Historically, it was thought that radon exposure increases in regions where cold climate causes people to spend longer periods indoors. However, climate change and air conditioning have altered this pattern. High levels of radon exposure in a given region depending on the availability of radon in the environment and its ability to make its way into confined spaces/water sources, the ability of radon to concentrate in high levels, and the human behaviors that contribute to increased exposure for long periods.
Radon is present at varying levels across the United States. Some states have mandated radon level disclosure upon the sale of a home. Levels vary from one state to another. However, it is notably higher in North Dakota and Iowa. The United States Environmental Health Protection Agency (EPA) has developed a map of “radon zones” to show the different areas of high radon levels across the nation. There are three different zones designated for each county in each state: zone 1 designates a predicted radon level of greater than 4 pCi/L, zone 2 includes counties with predicted levels between 2 to 4 pCi/L, and zone 3 includes counties with predicted levels less than 2 pCi/L. The map was developed to provide an overview of which areas could potentially have higher levels of radon. However, the EPA recommends that everyone in the United States test their homes for radon levels.
Persons in the mining and building industry who work in confined air spaces are at even higher risk of radon exposure. In particular, recent studies have shown that uranium miners were at the highest risk of exposure. Appropriate ventilation can reduce this risk in miners.
In the US, lung cancer is the leading cause of death for men and women. Smoking remains the main cause of lung cancer. Radon exposure is the second-leading cause of lung cancer overall and the number one cause in non-smokers. The risk of lung cancer is greatly increased in smokers who are also exposed to radon. It is estimated to be 10 to 20 times greater. Because children have smaller lung sizes and faster breathing rates, they are at greater risk due to higher radon inhalation.
The health hazards of radon come from its decay products. As radon decays, it emits alpha, beta, and gamma particles as well as x-rays. An alpha particle consists of a helium nucleus (two protons and two neutrons) and has the ability to transfer large amounts of energy into human tissue. Alpha particles are more biologically destructive than beta or gamma radiations, interacting much more readily with DNA and producing oxidative decimation through radiolysis. Ionizing radiation from alpha particles can cause chromosomal abnormalities, double-strand DNA breaks, and the production of reactive oxygen species, leading to carcinogenesis.
Ultimately, the decay products of radon (polonium-218, polonium-214, and lead-214) become charged and attach to aerosol particles. The charged aerosolized particles aggregate, forming clusters that readily attach to dust in the air, which is subsequently inhaled and deposit in the trachea, bronchi, and bronchioles. Upon entering the respiratory epithelium, the alpha particles can cause DNA damage, ultimately contributing to tumor genesis and lung cancer.
History and Physical
Risk factors for lung cancer include radon exposure, family history of lung cancer, history of cigarette smoking, and amount of cigarette smoke exposure. A good history is the essential first step in assessing radon risk and exposure. Important exposure history includes:
- Occupational history, especially if a person has ever worked in areas of confined air or worked where there is exposure to radon directly or indirectly (miners, builders, excavators, etc.)
- Year home was built/age of home
- Family history (to evaluate for risk of lung cancer)
- Smoke exposure (in the home or at work)
- Types of gas appliances
- Radon measurements in the home
- Time spent in confined spaces (such as basements)
- Ventilation systems in the home
Knowledge of the past medical history is important to make the diagnosis. A history of past lung disease is especially important since radon targets the lungs. Increased levels of radon exposure do not manifest with any specific signs and symptoms, which makes evaluation even more challenging. Since increased exposure to radon may lead to lung cancer, physical examination should focus on the respiratory system and signs of lung cancer. The physical examination may not be specific to radon exposure. However, it can aid in making the diagnosis. Some of these symptoms include:
- Shortness of breath
- Chest pain
- Weight loss
Other changes related to lung cancer include lymphadenopathy in the upper chest and lower neck, clubbing of the nails, and multiple bouts of pneumonia. The history and physical exam, along with clinical judgment, will determine the next step in the assessment. This may include further testing and referrals.
Lung cancer is the primary concern for long-term radon exposure. It is estimated that 21,000 deaths per year are due to lung cancer secondary to radon exposure. Since routine exposure to radon does not initially present with irritating symptoms or warning signs, it is important to screen patients for radon toxicity. The National Comprehensive Cancer Network recommends a low-dose computed tomography (CT) scan for anyone over the age of 50 with at least a 20 pack-year smoking history and a history of radon exposure. However, the American Academy of Family Physicians has concluded that there is insufficient evidence for or against using low-dose CT to screen for lung cancer in individuals that are high risk.
The United States Preventive Services Task Force (USPSTF) also has not established any benefit from screening asymptomatic patients. More studies are needed to determine the best method to screen for lung cancer in patients who are exposed to higher radon levels. Nonetheless, a detailed physical examination, history, chest x-rays, CT scan, sputum cytology, or a combination of these tests can aid in making the diagnosis.
The differential diagnosis includes:
- Lung cancer (small cell carcinoma-occurs most frequently, adenocarcinoma, large cell carcinoma and squamous cell carcinoma)
Radon is responsible for about 21,000 deaths per year in the United States. It usually is a “silent killer,” and symptoms do not present until years later when cancer symptoms begin to manifest. For this reason, the prognosis is usually guarded. However, it is estimated that lowering radon levels below 4 pCi/L can potentially decrease the mortality of lung cancer deaths anywhere from 2 to 4 percent.
- Lung cancer
- Frequent infections
- Pulmonary fibrosis
- Alzheimer disease (there has been an increased risk of Alzheimer's documented in the literature due to radon exposure)
- Pleural effusion
Deterrence and Patient Education
The EPA recommends that all home sellers/buyers test their homes for radon levels. Some areas with higher radon levels require testing by law. Thirty-four states have laws addressing radon and radon toxicity. Twenty-nine states require the discussion of radon levels and recent measurements during the sale of a house. Nine states require all new construction to be radon-resistant.
Ultimately, the primary remedy to radon toxicity is public awareness and prevention. When buying a home, asking questions about radon exposures and levels is important, especially in states with higher radon levels. If one already has a home and there is a concern for increased radon levels, reaching out to a contractor to measure radon levels is a simple start. The EPA’s website provides an array of resources to find more information about radon zones in various regions and about resources for homeowners.
Enhancing Healthcare Team Outcomes
Early prevention is key to improving outcomes of radon exposure. One important role of primary clinicians is the screening and prevention of diseases such as cancers. This presents an ideal opportunity for primary clinicians to educate patients on the risks of radon exposure. As with other cancers, a clinician's intervention may prevent the development of cancer by offering resources about the measurement and the reduction of radon levels, further decreasing mortality rates from radon toxicity.