In the 1400's, a mysterious lung disease was killing silver miners in Bohemia (now the Czech Republic). It was identified centuries later as lung cancer but its cause - radon - remained unknown until the early 1900's.
German physicist Friedrich Ernst Dorn discovered in 1900 that radium was giving off a gas - "radium emanation." The gas became known as niton (from the Latin word "nitens" for "shining") and later, radon. The connection to lung cancer gradually emerged. It was the Nazi regime in Germany that instituted the first safety regulations in mines against radon. But the radon dangers were ignored when it came to forced labor in uranium mines and later, prisoners of communist regimes.
In more recent history, lung cancer killed many uranium miners in the U.S. for the sake of building nuclear bombs, although they had been assured by the US government that radon is safe. After four decades, the government apologized and the Congress approved a lump-sum compensation for all nuclear plant workers and their healthcare costs, except for workers hired through military contractors. Although most had already died, they are applauded as the unsung heroes of the Cold War.
Radon would make a perfect stealth weapon. It is invisible and odorless but radioactive. Radon is a "noble gas" - it does not want to fraternize (react) with other elements.
When we breathe in radon gas, it does not react but readily dissolves in blood and travels through our whole body. Once it reaches equilibrium, when its concentration matches the surrounding air, we breathe it out again through the mouth and skin. Quite harmless, except that some atoms happen to disintegrate during this journey, emitting radiation and producing radioactive metal particles that accumulate in our bones, fat tissues, and organs.
During its decay in the air, radon atoms transmute into radioactive metallic atoms which do not settle but float infinitely. When breathed in, some get trapped in the lining of our lungs, where they continue to decay and radiate.
At every step of its decay chain, the disintegrating atoms emit ionizing radiation that kills or damages living cells – alpha particles (helium nuclei with 2 protons and 2 neutrons) and beta particles (high-speed electrons), or gamma rays (more energetic than X-rays). Alpha particles are the largest and deadliest to living cells. The problem is not the killed cells but the survivors with mangled DNA. They can proliferate as cancer or mutate and damage future generations to come.
Radon kills very slowly but, statistically speaking, surely. Among non-smokers, it is the leading cause of lung cancer, the Number 1 cancer killer.
Radon is the heavyweight of all natural gases. The nucleus of Radon-222 weighs in at 222 neutrons and protons (mass 222). The puny atom of nitrogen weighs in at only 14 but two of them stick together to form a nitrogen molecule (N2). This makes radon eight times heavier than air (222/28). For comparison, iron is eight times heavier than water and quickly sinks. That's why this heavy gas tends to accumulate in mines and in people's basements.
Unlike oxygen or nitrogen molecules, radon is a single atom gas. Because atoms are smaller than molecules, radon gas atoms easily penetrate most common building materials like concrete, mortar, paints, sheetrock, wood paneling, and low-density plastic sheets, including polyethylene. Radon atoms are smaller than water molecules (H2O) – a material may be waterproof and damp proof but will not stop radon gas. Exterior waterproofing coating on foundations does not block radon.
Radon gas is produced by Mother Nature during the long decay chain of radioactive heavy metal elements uranium and thorium, which are dispersed throughout the Earth's crust.
Dig up the top 6 feet of an acre of land and you will find, on average, about 50 pounds of uranium. Uranium and its daughter products radium and radon gas are found in nearly all rocks and soils. Most contain only 1 to 3 parts per million (ppm) of uranium but some, like granites, dark shales, light-colored volcanic rocks, and sedimentary rocks with phosphate, may contain as much as 100 ppm. Thorium, which is even more common, also produces radium and radon.
The decay chains of uranium and thorium continuously produce radium, which then magically disintegrates into a gas – radon isotopes Radon-222 (most common in homes) and Radon-220 (thoron). On average, about two radon atoms are emitted from every square centimeter of soil everywhere on the Earth every second of every day.
Radon is all around us - its average concentration in ambient air is 0.45 pCi/L in the U.S.
Radon has been around since the creation of this planet. It had filled the caves of Neanderthals and it will outlast our species.
As the atoms of radioactive heavy metals break down, they change into lighter and lighter radioactive heavy metals until they end up as stable, non-radioactive lead.
Radon is the only gas in the long decay chain of uranium. Its parent (precursor) is radium and its daughter (progeny) is polonium. The half-life of Radium-226 is about 1,600 years. The half-life of Radon-222 is only 3.82 days but the radioactive decay chain then continues to polonium, bismuth, and lead. After 22 years, a half of Radon-222 atoms end up as Lead-206, a stable non-radioactive element.
Of the twenty known isotopes of radon, only two others occur naturally. Radon-220 (half-life 55 sec), also called thoron, is produced in the decay series of Thorium-232. It is emitted by building materials like concrete and may contribute 5% to 20% of the total radon level in homes. Radon-219 (half-life 4 sec), also called actinon, is produced in the decay series of Uranium-235 (actinouranium).
However, the Earth will never run out of radon gas! Uranium decays very slowly. The half-life of Uranium-238, when a half of its atoms decay into another element, is 4.5 billion years. Thorium-232 is just as common as uranium but its half-life is even longer – 14.1 billion years.
Radon levels on Earth have not changed since the last Ice Age.
Is there a plus side of radioactivity? Definitely! Radioactive decay makes life on Earth possible by heating its core. Japanese scientists have measured the antineutrinos produced by the decay of uranium and thorium deep inside the Earth. The decay generates about 19 million megawatts of heat, about half of all the heat generated inside the planet. The other half comes from gravitational and chemical sources. (New York Times, 7/28/05)