So is background radiation harmful to human body?
(Image source:/radiation-level-exposure-nuclear power plant -x-ray-smoking-2015-1)
First, we need to figure out what background radiation is. Many people may think of the physical concept "cosmic background radiation" or "cosmic microwave background radiation" when they see these four words. In fact, background radiation, also known as background radiation, is a concept in the field of radiation safety and protection.
We know that radiation is often divided into ionizing radiation and non-ionizing radiation according to whether atoms and molecules can ionize. The former includes alpha particles, beta particles, neutrons and other particles, as well as gamma rays, x rays and short-wave ultraviolet rays in electromagnetic waves. The latter mainly includes ultraviolet rays with longer wavelength in visible light, infrared light, radio waves and electromagnetic waves. Compared with non-ionizing radiation, ionizing radiation is more powerful, so it is more destructive to living things. When we talk about radiation protection, we usually mean ionizing radiation protection.
Imagine that there is an X-ray machine in the room. When we discuss how to protect the radiation caused by it, we must first determine how strong its radiation is. But the X-ray machine is not the only radiation source in the room. The stones on the floor may contain trace natural radioactive elements, and there is also trace radioactive element radon in the air. Therefore, we must first understand how strong all radiation except the X-ray machine is, and then we can analyze the health impact of the radiation from the X-ray machine and the corresponding protective wording. In this way, the sum of all radiation except a specific radiation source is background radiation.
Sometimes we don't choose a specific radiation source, so background radiation refers to the sum of all ionizing radiation, which is often used to reflect all radiation that people can touch except occupational radiation (such as operating nuclear reactors, testing nuclear weapons, conducting radiation breeding, and radiotherapy for patients).
So what is the composition of background radiation? Simply put, background radiation can be divided into natural background radiation and artificial background radiation. The existence of natural background radiation has nothing to do with human activities, and usually includes the following parts [1]:
Cosmic rays, energetic particles from outer space. Cosmic rays will interact with the atmosphere and be weakened, so people living in high altitude areas will be exposed to more cosmic rays, and people will also be exposed to some extra cosmic rays when flying [2].
Background radiation in the surface environment. These radiations come from various radioactive elements and are widely distributed in the atmosphere, soil, rocks and water bodies. They are also the main source of background radiation.
In addition, our daily diet also contains trace radioactive elements, which also produce radiation, and even organisms themselves have a certain amount of radioactive elements. These radiations are also considered as a part of natural background radiation.
As the name implies, artificial background radiation refers to background radiation caused by human activities. The main sources of artificial background radiation are various medical activities, such as X-ray imaging, CT and other medical imaging diagnostic means and radiotherapy and other treatment measures. In economically developed countries, medical technology is relatively advanced, and residents often have more opportunities to contact these diagnostic and therapeutic methods involving radiation, so the radiation generated by medical activities accounts for a higher proportion of background radiation. For example, the background radiation produced by medical activities in the world usually does not exceed 20% of the total background radiation [3], while the proportion in the United States is as high as 50%[4].
Main sources of background radiation (image source: http://www.bbc.co.uk/schools/gcsebite size/science/add _ aqa/atoms _ radiation/nuclearradiationrev1.shtml).
How strong is the background radiation? When we evaluate the influence of radiation on organisms, we usually consider the radiation dose, that is, the total amount of radiation absorbed by the human body in a certain period of time. The commonly used unit is Siver (Sv), but this unit is too large to be used conveniently. In fact, millisieverts (mSv, 1Sv= 1000 mSv) are often used. Rem and mrem are commonly used to describe radiation dose, and the conversion relationship is1rem =1000 mrem = 0.01SV =10 msv. The background radiation levels around the world are slightly different in different regions. On average, the radiation dose per person per year due to background radiation is about 3 millisieverts.
Although we have a deeper understanding of the harm caused by radiation, the experimental data mainly come from acute and high-dose radiation, and long-term and low-dose radiation is often encountered in daily life. Unfortunately, however, the impact of low-dose radiation on health is often difficult to verify through experiments and can only be estimated through theoretical models. At present, the "linear non-threshold" theoretical model is the main basis for evaluating the health impact of low-dose radiation. According to this theory, there is no safe dose of radiation, even weak radiation will cause damage to organisms, and there is a simple linear relationship between radiation damage to health and radiation dose. Therefore, if the harm of high-dose radiation is confirmed according to the experimental data, we can calculate the impact of low-dose radiation on health.
Whether this theory really conforms to the actual situation has always been controversial. Some researchers believe that the damage caused by low-dose radiation is not so serious because organisms have developed a mechanism to deal with radiation damage in the long process of evolution, and some even think that low-dose radiation is beneficial to health. In recent years, although there is new experimental evidence to support the linear thresholdless theory, the related debate is likely to continue. However, although controversial, from the perspective of radiation protection, the linear thresholding theory is still a good theoretical model.
But in practice, it is impossible to completely eliminate radiation, and many times we still need to use radiation to serve production and life. Therefore, the principle that is usually followed in practical operation is the lowest reasonably achievable (ALARA for short). Therefore, when we discuss whether radiation is harmful, we usually refer to whether the damage caused by this radiation is significant or acceptable.
From this point of view, whether background radiation is harmful to health can be said to be a meaningless question. Because background radiation is always around us, it is difficult to eliminate it. We can't really observe the influence of background radiation on health through experiments, because we can't find out that there is no background radiation as a control, and it is almost impossible for us to formulate operable protective measures against background radiation. Therefore, when we discuss the harm and protection of radiation, we always refer to the influence of additional radiation dose on human body on the basis of background radiation. The radiation dose safety threshold given by government agencies in various countries also refers to the additional radiation dose besides the background radiation.
From the comparison of radiation dose, the background radiation is really negligible. As I said just now, the annual radiation dose of each person due to background radiation is about 3 mSv, while the radiation dose caused by a whole-body CT is as high as 10 mSv, and the radiation dose of a mammogram is also 0.4 mSv ... According to the regulations of the International Atomic Energy Agency, on the basis of background radiation, it is acceptable that the annual radiation dose absorbed for occupational reasons does not exceed 20 millisieverts. Judging from the comparison of these figures, it is not too wrong to say that background radiation is harmless to human body.
There are indeed some places in the world that have caused extremely strong natural background radiation due to special geological reasons. The annual radiation dose absorbed by residents from background radiation is not only significantly higher than the world average, but sometimes even exceeds the upper limit of occupational exposure radiation dose set by the International Atomic Energy Agency by 20 millisieverts, such as Ramsar, China and Yangjiang, Guangdong. However, the current research has failed to find that such high background radiation has obvious negative effects on the health of local residents. One possible reason is that local residents have adapted to this high background radiation [6, 7]. Therefore, even for these special areas, the statement that "background radiation is harmless" can still be said to be true.
Finally, briefly talk about the problem of radon. Radon is a naturally occurring radioactive element, and the radiation caused by radon is the main source of background radiation, accounting for about half of the background radiation. Because radon is a kind of gas, it can not only produce radiation in the body after being inhaled by the human body, but also the decay products will remain in the human body to continue to produce radiation, so it is more troublesome than solid radioactive substances. Because indoor ventilation is worse than outdoor ventilation, radon gas is easy to accumulate, causing more serious harm.
Current research shows that radon is the main cause of lung cancer besides smoking, so government agencies in various countries generally set an upper limit of indoor radon concentration and try to control this radioactive pollutant. For example, the US Environmental Protection Agency suggested that if the indoor radon concentration exceeds 2 pCi (pCi liter (pCi is the unit of radioactive intensity), measures should be taken to reduce the radon concentration [8]. But even so, we cannot completely eliminate the radiation caused by radon. The United States Environmental Protection Agency pointed out that it is extremely difficult to reduce indoor radon concentration to below 2 pCi per liter. Exposure to this concentration of radon will still lead to lung cancer in 4 out of every 1000 non-smokers [8], and radon will still exist in the outdoor air. Therefore, for the indoor environment with high radon content, what we can do is to reduce radon radiation to the background level as much as possible.
refer to
[ 1]? http://www . NRC . gov/about-NRC/radiation/around-us/sources/NAT-BG-sources . html
[2]? http://www.ans.org/pi/resources/dosechart/
[3]http://www . world-nuclear . org/uploaded files/org/Features/Radiation/4 _ Background _ Radiation( 1)。 Extension of portable document format file (abbreviation of portable document format)
[4]http://www . NRC . gov/reading-RM/basic-ref/glossary/exposure . html
[5]? http://www . NRC . gov/about-NRC/radiation/around-us/sources/man-made-sources . html
6m. Ghiassi-nejad, S. M. J. Mortazavi, J. R. Cameron, A. Niroomand-rad, P. A. Karam, "Extremely High Background Radiation Area in Ramsar, Iran: Preliminary Biological Research", Health Physics, 2002, pp. 82 and 87.
Tao Zufan, Zha Yongru, Qiu Qiuzhuli, Sun Quanfu, Zou Jianming, Liu Yusheng, Hiroshi Katou Xiong, Sugawara, Wei, "Cancer Mortality during 1979 and 1995 in Yangjiang, China", Journal of Radiation Research, 2000, 4/kloc.
[8]https://www . EPA . gov/sites/production/files/20 16-02/documents/20 12 _ a _ citizens _ guide _ to _ radon . pdf
Text/yuyu (Editor-in-Chief of Science Park)