Radon Toxicity

Etiology

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.[2] 

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.[3]

Epidemiology

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.[4][5]

Pathophysiology

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.[6][7]

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:

1. 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.)
2. Year home was built/age of home
3. Family history (to evaluate for risk of lung cancer)
4. Smoke exposure (in the home or at work)
5. Types of gas appliances 
6. Radon measurements in the home 
7. Time spent in confined spaces (such as basements)
8. 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:

1. Shortness of breath
2. Wheezing
3. Hemoptysis
4. Cough
5. Chest pain
6. 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.

Evaluation

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.[8][9][10]

Treatment / Management

The most effective way to decrease the adverse effects of radon is to prevent exposure. The amount of radon in the air is commonly measured in pCi/L (picocuries per liter of air). About 0.4 of pCi/L of radon is usually found in outside air, and the US Congress has a goal of indoor radon levels never exceeding outdoor levels. Since radon toxicity does not present with any warning signs or symptoms, it is important to measure and keep indoor radon levels low. 

Several methods exist to reduce radon levels. The EPA recommends hiring a trained contractor to assist in the evaluation and reduction of radon. Some techniques can reduce radon entry in the home, and others can reduce its levels after it has entered. One of the most common methods is soil suction, which draws radon from the house and vents it through a pipe to the air above the house where it is then diluted. This method helps prevent radon from entering the home. There are four different types of soil suctioning. Determining the most effective method depends on the type of house. 

For basement and slab-on-grade houses, radon levels are reduced by one of several types of soil suction: subslab suction, drain tile suction, sump hole suction, or block wall suction. In active subslab suction, suction pipes are installed through the floor slab and into the rock or soil underneath. They can also be inserted from the outside of the home below the concrete slab. A radon vent fan connected to this suction pipe can then draw the radon from below the house and release it into the air.  

At the same time, this creates a negative pressure under the slab. Passive subslab suction is similar to the active subslab suction. However, it is not as effective at reducing radon levels. It relies mostly on natural pressure differentials instead of a fan. Certain types of houses have drain tiles or perforated pipes near the base of the house. Adding suction can help reduce radon levels. Another variation is a sump hole suction. When a sump that usually drains water from a house is capped, it can be used as a location for a radon suction pipe that removes radon with the expelled water. Finally, block wall suction can be utilized in basements with hollow block suction walls. This reduces radon and depressurizes the wall and is usually used in conjunction with a subslab suction. 

Crawlspace houses usually utilize submembrane suction to reduce radon levels. This requires the placement of a plastic sheet over the earth-floor. A vent pipe and fan are then utilized to draw radon to the outdoors from under the sheet. 

Other types of radon reducing methods that can be used in all types of houses include the following:

1. Sealing: Sealing cracks to reduce the flow of radon inside a home. However, the EPA does not recommend the use of this method on its own because it has not been proven to significantly lower radon levels. 
2. House/room pressurization: Utilizes a fan to blow air into the basement or living area to create increased pressure. This will prevent radon from entering the house. This method is usually utilized if other methods have failed.
3. Heat recovery ventilator or air-to-air heat exchanger: Installed to further increase ventilation in a home. 
4. Natural ventilation: This occurs in all homes by opening windows, doors, vents, etc. This is not a permanent or long-term method. Once the ventilation is closed, radon levels increase within 12 hours. 

All homes should be tested for radon levels and, if elevated, should be reduced. Even a new home built with radon-resistant features should be tested to make sure that radon levels are below 4 pCi/L.

Differential Diagnosis

The differential diagnosis includes: 

• Lung cancer (small cell carcinoma-occurs most frequently, adenocarcinoma, large cell carcinoma and squamous cell carcinoma) 
• Pneumonia
• Bronchitis
• Leukemia[11]

Prognosis

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.

Complications

• Lung cancer
• Frequent infections
• Emphysema
• Pulmonary fibrosis
• Alzheimer disease (there has been an increased risk of Alzheimer's documented in the literature due to radon exposure)[12]
• Pleural effusion
• Metastasis
• Death