Radon is produced by Mother Nature during the long radioactive decay chain of uranium, which is present 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 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 is another radioactive heavy metal that produces radon, actually an isotope called thoron. Thorium-232 is just as common as uranium but its half life is even longer - 14.1 billion years.
Radium is one of the elements in the decay chain of uranium and in turn, it magically produces gas - radon. 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.
Radon is the heavyweight of all known gases. The nucleus of Radon-222 weighs in at 222 neutrons and protons. The puny atoms of nitrogen weigh only 14, but two of them stick together to form a molecule. This makes radon eight times heavier than air - similarly, iron is eight times heavier than water and it does not float much. That's why this heavy gas accumulates in mines and in people's basements.
Unlike oxygen or nitrogen (O2 or N2), radon is a single atom gas. Because the atoms are smaller than molecules, radon gas easily penetrates many common building materials like concrete, mortar, paints, sheetrock, wood paneling, and low-density plastic sheets, including polyethylene. And radon atoms are about five times 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 is a "noble gas" - it does not want to fraternize (react) with other elements. When we breathe in radon gas, it readily dissolves in blood and travels through our whole body. Once its concentration matches the surrounding air, we breathe it out again. Quite harmless, except that some atoms happen to disintegrate during this journey and the produced radioactive metal particles accumulate in our bones and organs.
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 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, except for those hired through military contractors, and all healthcare costs. Although their money has not yet been allocated and most have already died, they are applauded as the unsung heroes of the Cold War.
Radon is the only gas in the long decay chain of uranium. It comes from heavy metals and begets heavy metals. Its parent (precursor) is radium and its daughter (progeny) polonium. The half life of Radon-222 is only 3.82 days, but the radioactive decay chain then continues. At every step, it emits ionizing radiation (either alpha, beta, or gamma), which kills or damages living cells. A half of the radon atoms finally end up as a stable lead after 22 years.
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 indoors. Radon-219 (half-life 4 sec), also called actinon, is produced in the decay series of Uranium-235 (actinouranium).
Radon would make a perfect stealth weapon. It is radioactive but invisible and odorless. When it decays, it shoots out alpha particles,the deadliest radiation 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.
During its decay, radon atoms transmute into radioactive metallic atoms so light that they float in air infinitely. When breathed in, some get trapped in the lining of our lungs, where they continue to decay and radiate. Radon kills very slowly but, statistically speaking, surely.