It is well known that smoking is the number one killer of the lungs, and about 90% of lung cancer cases are caused by smoking. So what is the second killer of the lungs? Is it second-hand smoke? No, it is not. Second-hand smoke only ranks third in the list of lung killers, and the second one is radioactive inert gas radon. There are about 21,000 lung cancer deaths caused by radon in the United States every year, while there are only about 3,000 lung cancer deaths caused by second-hand smoke (compared to 160,000 lung cancer deaths caused by smoking). According to the World Health Organization, about 3% to 14% of lung cancer cases are related to radon, mainly caused by exposure to low concentration of radon in life. Radon is colorless, odorless and tasteless, and its presence cannot be detected without professional instruments. The medical profession has long noticed that miners’ susceptibility to lung cancer is related to radon inhalation, but the danger of radon in living environment has been recognized later, and only in the 1980s did this problem attract attention. In 1985, an employee of a nuclear power plant in the U.S. was found to be contaminated with radioactive substances during a routine checkup, which was puzzling because the power plant had not yet started generating electricity and the cleaning could not remove the radioactive substances from his body. It was eventually discovered that it was due to the extremely high concentration of radon gas in the basement of his house, up to 100,000 becquerel cubic meters (equivalent to 2.7 nano-curie liters. The international unit of radioactive activity is Becquerel Bq, while Curie Ci is customarily used in the U.S. (1 pCil = 37 Bqm3). He lives in such a high concentration of radon gas, the risk of causing lung cancer is equivalent to smoking 135 packs of cigarettes per day. In general, the radon content in people’s homes is not that high, and it is two or three orders of magnitude lower. Then how does radon enter the living room? There are several isotopes of radon, the main one is radon 222, which is the decay product of radium 226, and radium 226 is the decay product of uranium 238. Uranium and radium are commonly found in soil and rocks and keep on producing radon, especially soil, which is loose and the radon produced is more easily released. So radon can be said to be everywhere Because radon is mainly released from soil, it is more threatening to people living on the ground floor of bungalows or buildings. Therefore, the United States requires that if you live below the third floor, you have to measure the radon level. If the radon content is high, you should modify the room and take some measures to reduce the radon content, such as sealing the gaps on the floor and walls, eliminating the gas in the foundation soil and ventilating the room well. As mentioned above, there is no safe dose of radon, how can it be considered high? This is what is artificially set. The recommendation of the US EPA is to take immediate measures if the radon content exceeds 4 pCi/l. If it is between 2 and 4 pCi/l, measures can be considered, but it is difficult to reduce the radon concentration to below 2 pCi/l. Those who live above the third floor, soil is not a threat to them anymore, and building materials become the main risk. If the construction and decoration of houses use a lot of uranium-rich stones such as granite, it is better to measure the radon content. Some stone dealers say that the half-life of radon is only 3.8 days and it will be fine after a few days. This is a misleading nonsense. Radon is ultimately produced by uranium, and the half-life of uranium is 4.5 billion years, and only half of it has been decayed since the birth of the earth. Another danger of decorating with stone is that, due to the use of close to the key organs of the human body, the radioactive isotopes in the stone may damage the body outside the body, so it is possible that the radioactive intensity should be measured. According to national standards, radioactive stone can not be used indoors, but in the specific implementation is not necessarily in place. There are also some statements on the Internet, what dark-colored stone radiation is strong, light-colored stone radiation is weak, that is no basis. Many people are afraid of the harmless electromagnetic radiation of electrical appliances, but they do not understand radon radiation or think it does not matter. In fact, radon radiation is the only harmful radiation in daily life that is really worth noticing and there are ways to avoid it.