Is background radiation harmful to health?

Not long ago, an article titled "Common Knowledge on Ionizing Radiation" was pushed by the Science Park WeChat official account (author: Prisoner Breaking Free from Shackles), which mentioned that background radiation will not cause harm to the human body, causing a stir Some controversy. Some people think that the statement in the article is inappropriate and that background radiation is actually harmful to the human body.

So is background radiation harmful to the human body?

(Image source: /radiation-levels-exposure-nuclear-plants-xray-smoking-2015-11)

First of all, we need to understand what background radiation is. When many people see these four words, the first thing they may think of is the concept of "cosmic background radiation" or "cosmic microwave background radiation" in physics. In fact, background radiation, also called background radiation, is a concept in the field of radiation safety and protection.

We know that according to whether it can ionize atoms and molecules, radiation is often divided into ionizing radiation and non-ionizing radiation. The former includes particles such as alpha particles, beta particles, neutrons, and gamma rays in electromagnetic waves. , Compared with non-ionizing radiation, ionizing radiation has stronger energy and is therefore more damaging to living organisms. When we refer to radiation protection, we usually refer specifically to protection from ionizing radiation.

Imagine there is an X-ray machine in the room. When we discuss how to protect against the radiation it causes, we must first determine how strong its radiation is. But the X-ray machine is not the only radiation source in the room. The stone on the floor may contain trace amounts of natural radioactive elements, and there is also trace amounts of the radioactive element radon in the air. Therefore, we first need to understand how strong all the radiation except the X-ray machine is, and then we can specifically analyze the impact of the radiation from the X-ray machine on health and the corresponding protective wording on this basis. The sum of all radiation outside a given radiation source is background radiation.

There are also times when we do not choose a specific radiation source, so background radiation refers to the sum of all ionizing radiation, especially when it is often used to reflect people's occupational exposure (such as operating nuclear reactors, testing All radiation that can be exposed except nuclear weapons, radiation breeding, radiotherapy of patients, etc.).

So what are the usual components 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 are high-energy particles from outer space. Cosmic rays interact with the atmosphere and are weakened, so people living at high altitudes will be exposed to more radiation from cosmic rays, and people will also be exposed to some additional 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 amounts of radioactive elements, which also produce radiation, and even living organisms themselves contain a certain amount of radioactive elements. These radiations are also considered part of the natural background radiation.

As the name suggests, 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 methods, as well as treatment measures such as radiotherapy. Economically developed countries have relatively advanced medical technology, and residents often have more opportunities to be exposed to these diagnostic and treatment methods involving radiation. Therefore, radiation from medical activities accounts for a higher proportion of background radiation.

For example, background radiation from medical activities around the world usually does not exceed 20% of the total background radiation [3], but in the United States this proportion is as high as 50% [4].

The main sources of background radiation ( Image source http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/atoms_radiation/nuclearradiationrev1.shtml)

How strong is the background radiation? When we evaluate the impact of radiation on living organisms, we usually consider the dose of radiation, that is, the total amount of radiation absorbed by the human body within a certain period of time. The commonly used unit is Sievert (Sv), but this unit is too large and difficult to use. Conveniently, millisievert (mSv, 1Sv= 1000 mSv) is often used in practice. In addition to sieverts and millisieverts, rem (rem) and millirem (mrem) are also often used to describe radiation dose, and the conversion relationship is 1 rem = 1000 mrem = 0.01 Sv = 10 mSv.? The background radiation levels around the world vary slightly in different regions. On average, the radiation dose received by each person due to background radiation per year is about 3 mSv.

Although we have a relatively in-depth understanding of the harm caused by radiation, experimental data mainly comes from acute, large-dose radiation, and what we more often encounter and are more concerned about in our daily life is long-term, low-dose radiation. . Unfortunately, however, the health effects of low-dose radiation are often difficult to verify experimentally and can usually only be estimated using theoretical models. The current assessment of the impact of low-dose radiation on health is mainly based on the theoretical model of "linear no-threshold". According to this theory, there is no safe dose of radiation. No matter how weak the radiation is, it will cause damage to the living body. There is a simple linear relationship between the damage to health caused by radiation and the radiation dose. Therefore, if the effect of high-dose radiation is confirmed based on experimental data, Hazards, we can accordingly deduce the health effects of low-dose radiation.

Whether this theory is really consistent with reality has always been controversial. Some researchers believe that because organisms have developed mechanisms to cope with radiation damage during the long process of evolution, the damage caused by low-dose radiation It's not that serious, and some people even think that low doses of radiation are good for health. Although there has been new experimental evidence supporting the linear thresholdless theory in recent years, the relevant debate will probably continue. However, despite the controversy, from the perspective of radiation protection, the linear thresholdless theory is still a good theoretical model.

However, in actual operation, it is impossible to completely eliminate radiation, and many times we still need to use radiation to serve production and life. Therefore, the principle usually followed in actual operations is "As Low As Reasonably Achievable (ALARA)". So when we discuss whether radiation is harmful, we usually refer to the radiation caused by this kind of radiation. Whether the damage is significant or acceptable.

From this perspective, whether background radiation is harmful to health can be said to be a meaningless question. Because background radiation exists around us at all times, it is difficult to eliminate. It is impossible for us to truly observe the impact of background radiation on health through experiments, because we cannot find a situation without any background radiation as a control, and it is almost impossible to formulate operational protective measures against background radiation. Therefore, when we discuss the hazards and protection of radiation, we always refer to the background radiation as the benchmark and the impact of additional radiation dose on the human body. The radiation dose safety thresholds given by government agencies in various countries also refer to the background radiation. Additional radiation dose.

From a comparison of radiation doses, background radiation is indeed negligible. As mentioned earlier, the radiation dose received by each person due to background radiation is about 3 millisieverts per year, and the radiation dose caused by a whole-body CT is as high as 10 millisieverts, and the radiation dose caused by a mammography is also 0.4 millisieverts. The International Atomic Energy Agency stipulates that the radiation dose absorbed due to occupational reasons is acceptable if it does not exceed the background radiation of no more than 20 millisieverts per year. Judging from the comparison of these figures, it is not too inappropriate to say that background radiation is not harmful to the human body.

There are indeed some places in the world where special geological reasons have caused extremely strong natural background radiation. The radiation dose absorbed by residents from background radiation every year is not only significantly higher than the world average, but sometimes even exceeds the international level. The IAEA sets an occupational exposure radiation dose limit of 20 mSv, which is the case in Ramsar, Iran, and Yangjiang, Guangdong, my country. However, current research has not found that such high background radiation has a significant negative impact on the health of local residents. One possible reason is that local residents have already 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, let’s briefly talk about the problem of radon. Radon is a naturally occurring radioactive element. The radiation caused by it is the main source of background radiation, accounting for about half of the background radiation.

Since radon is a gas, when inhaled by the human body, not only can it produce radiation in the body, but its decay products will also stay in the human body and continue to produce radiation, so it is more troublesome than solid radioactive substances. Due to poorer ventilation indoors than outdoors, radon gas can easily accumulate, causing more serious harm.

Current research has shown that radon is the most important cause of lung cancer besides smoking. Therefore, government agencies in various countries generally set upper limits for indoor radon gas concentrations to try to control this radioactive pollutant. For example, the US Environmental Protection Agency recommends that if the indoor radon concentration exceeds 2 pCi per liter (pCi is a unit that expresses radioactive intensity), measures should be taken to reduce the radon concentration [8]. But even so, we cannot completely eliminate the radiation caused by radon gas. The U.S. Environmental Protection Agency pointed out that it is extremely difficult to reduce indoor radon concentration below 2 pCi per liter. Exposure to radon gas at this concentration can still cause 4 out of every 1,000 non-smokers to develop lung cancer [8], and Radon gas will still be present in the outdoor air. Therefore, for indoor environments with high radon content, all we can do is try to reduce radon radiation to background levels.

References

[1]?http://www.nrc.gov/about-nrc/radiation/around-us/sources/nat-bg-sources.html< /p>

[2]?http://www.ans.org/pi/resources/dosechart/

[3] http://www.world-nuclear.org/uploadedFiles/ org/Features/Radiation/4_Background_Radiation(1).pdf

[4] http://www.nrc.gov/reading-rm/basic-ref/glossary/exposure.html

< p>[5]?http://www.nrc.gov/about-nrc/radiation/around-us/sources/man-made-sources.html

[6] M. Ghiassi-nejad , S. M. J. Mortazavi, J. R. Cameron, A. Niroomand-rad, P. A. Karam, "Very high background radiation areas of Ramsar, Iran: preliminary biological studies", Health Physics, 2002, 82, 87

[7] Zufan Tao, Yongru Zha, Suminoiri Akiba, Quanfu Sun, Jianming Zou, Jia Li, Yusheng Liu, Hiroo Kato, Tsutomu Sugahara, Luxin Wei, “Cancer Mortality in the High Background Radiation Areas of Yangjiang, China during the Period between 1979 and 1995 ”, Journal of Radiation Research, 2000, 41 Suppl. 31

[8]https://www.epa.gov/sites/production/files/2016-02/documents/2012_a_citizens_guide_to_radon.pdf

Text/Wei Xinyu (Editor-in-Chief of Science Park)