Radon (Rn) is a naturally occurring radioactive noble gas. It originates deep within the Earth as a product of the radioactive decay of uranium and thorium found in nearly all soil and rock. As it moves up through the ground, it can accumulate in enclosed spaces, such as homes and buildings, becoming a significant concern for indoor air quality. Exposure to elevated levels of this gas is recognized as the second leading cause of lung cancer, following only cigarette smoking.
Radon’s Appearance Under Normal Conditions
Under normal atmospheric temperature and pressure, radon gas is completely colorless, odorless, and tasteless, making it undetectable by human senses. As a noble gas (Group 18), its full outer electron shells result in very low chemical reactivity. This chemical inertness limits its interaction with visible light wavelengths, which is why it remains invisible. The gas is also extremely dense, being one of the heaviest known gases, which often leads to its concentration in lower levels of buildings like basements.
The Science Behind Radon’s Visibility
While radon gas is invisible under typical conditions, it exhibits a distinct color when subjected to extreme cold. Radon’s boiling point is approximately -61.7°C (-79°F), and its freezing point is -71°C (-96°F). When cooled to a liquid or solid state below this freezing point, the element’s intense radioactivity causes it to glow, a phenomenon called radioluminescence. This light is not the intrinsic color of the gas itself but rather energy released as the radiation strikes and excites surrounding material.
Solid radon initially emits a soft, yellowish glow. As the temperature is lowered further, approaching the temperature of liquid air, this luminescence shifts its spectrum. The color intensifies and changes to a noticeable reddish-orange hue, a direct result of the continuous radioactive decay process.
Why Invisibility Makes Radon a Health Concern
Radon’s invisibility, odorlessness, and tastelessness make it a serious public health hazard. Without any sensory warning, the gas can build up to dangerous concentrations inside a home over time. The primary risk is not the radon gas itself, which is mostly exhaled, but its short-lived decay products, often called “radon daughters.” These decay products include radioactive isotopes of polonium, lead, and bismuth.
These solid decay particles attach themselves to dust and other airborne aerosols, which are then easily inhaled and become lodged in the lung tissue. Once deposited, these particles emit powerful alpha radiation, which is highly energetic and capable of damaging the DNA in the cells lining the airways. The only reliable way to determine if a home has a problem is through specialized testing. The U.S. Environmental Protection Agency (EPA) recommends taking action to mitigate levels at or above 4 picocuries per liter (pCi/L). Testing remains the only protection against this invisible threat.