On March 11, 2011, a 9.0-magnitude earthquake occurred off the eastern coast of Japan, causing a tsunami that immediately threatened the Fukushima Daiichi Nuclear Power Plant in the Fukushima Prefecture. Less than an hour after the quake, massive waves overwhelmed the walls of the plant, leading to core meltdowns and explosions. The Fukushima nuclear disaster was the largest incident of its kind since the Chernobyl nuclear disaster in 1986. More than half a million people were evacuated from the surrounding area over the course of the disaster.
|
|
As a result of accidents like Fukushima, which attract a great deal of media attention, radioactivity is often publicly perceived as being dangerous and unnatural. However, as we have discussed already, radioactivity is part of the natural environment and has been for millions of years. Humans are exposed to various forms of radiation in our daily lives, mostly from natural sources, even when we may not be aware of it. In this section we will describe the difference between hazardous and non-hazardous radiation, as well as some of the health risks associated with hazardous forms of radiation.
Ionizing vs. Non-ionizing Radiation
All radiation can be separated into categories of “ionizing” and “non-ionizing” based on the amount of energy it carries. Radiation that has enough energy to remove electrons from atoms or molecules is called ionizing radiation. Since ionization energies vary between different atoms and molecules, there is not a clearly defined boundary beyond which radiation becomes ionizing. X-rays, gamma rays, and the high frequency range of ultraviolet light are all forms of ionizing radiation. Furthermore, alpha particles, beta particles, and neutrons resulting from radioactive decay are also ionizing.
All radiation can be separated into categories of “ionizing” and “non-ionizing” based on the amount of energy it carries. Radiation that has enough energy to remove electrons from atoms or molecules is called ionizing radiation. Since ionization energies vary between different atoms and molecules, there is not a clearly defined boundary beyond which radiation becomes ionizing. X-rays, gamma rays, and the high frequency range of ultraviolet light are all forms of ionizing radiation. Furthermore, alpha particles, beta particles, and neutrons resulting from radioactive decay are also ionizing.
Non-ionizing radiation, including radio waves, microwaves, infrared, and visible light, is not energetic enough to separate electrons from atoms. Sunlight that reaches Earth’s surface is mostly non-ionizing because the atmosphere filters out ionizing radiation in the far-ultraviolet frequency range. Even if it is incapable of causing damage to cellular structure, non-ionizing radiation can thermally excite atoms or molecules, potentially leading to burns.
Sources of Radiation
We are exposed to low-level sources of radiation on a daily basis (Figure 10). About half of this radiation originates from natural background sources, many of which have existed as long as the Earth. The other half is caused by man-made sources, mostly due to medical applications such as X-rays. Notice that only a tiny fraction of radiation exposure (<0.1%) is due to industrial sources, which includes nuclear power plants.
We are exposed to low-level sources of radiation on a daily basis (Figure 10). About half of this radiation originates from natural background sources, many of which have existed as long as the Earth. The other half is caused by man-made sources, mostly due to medical applications such as X-rays. Notice that only a tiny fraction of radiation exposure (<0.1%) is due to industrial sources, which includes nuclear power plants.
Natural sources of low-level radiation include cosmic rays, radioactive minerals, and even radioactive nuclei in food. Radon is the largest source of natural background radiation. Radon-222, an intermediate product in the uranium-238 decay series, takes the form of a heavy, inert gas that tends to accumulate in basements and crawlspaces of homes. Radon is the second most frequent cause of lung cancer, after smoking. Homes can be tested for high concentrations of radon with commercially available detection kits.
Cosmic rays consist of high-energy radiation, mostly free particles such as protons that originate from outside the Solar System. Since the atmosphere provides a great deal of shielding from exposure to cosmic rays, the intensity of radiation depends on a person’s altitude above sea level. For example, a resident of Denver, Colorado receives twice the annual dose of cosmic radiation compared to a person at sea level. Thus air travel also increases a person’s exposure to cosmic rays. For this reason, workers in the U.S. with an increased risk of exposure to radiation (like pilots) are subject to strict regulations regarding their annual permitted dosage due to work activities.
|
Health Effects of Radiation
Cells are composed of molecules that are carefully structured and organized to support biological processes. Ionizing radiation, including high-energy photons and decay particles, causes severe damage to cells by breaking and altering molecular bonds. The radiation strips away electrons, creating ions and “free radicals,” which are neutral atoms or molecules that contain unpaired electrons. Ions and free radicals tend to be chemically active, and can recombine with other atoms to form either useless or potentially harmful new molecules within the cell.
Cells are composed of molecules that are carefully structured and organized to support biological processes. Ionizing radiation, including high-energy photons and decay particles, causes severe damage to cells by breaking and altering molecular bonds. The radiation strips away electrons, creating ions and “free radicals,” which are neutral atoms or molecules that contain unpaired electrons. Ions and free radicals tend to be chemically active, and can recombine with other atoms to form either useless or potentially harmful new molecules within the cell.
|
Although sufficient damage to cellular structure can cause the death of the cell, cells are capable of healing from molecular damage provided the intensity of radiation is low enough. Thus a cell can survive an otherwise lethal dose of radiation if it is absorbed over a long enough time period to permit healing. Furthermore, many of the body’s cells (with the exception of nerve cells) are replaced over time.
Cells are normally able to repair small defects to DNA molecules, but ionizing radiation can cause irreparable harm to DNA. Structural damage to a cell’s DNA molecule can have very serious and long-lasting consequences due to cellular reproduction. When cells with damaged DNA reproduce, it leads to a mutation in which defective genetic information is passed on to offspring cells. Harmful mutations can result in genetic disorders or cancer.
Sunburns, for instance, are a form of radiation burn caused by overexposure to ultraviolet radiation from the sun. Without proper protection, this radiation can be intense enough to kill living cells, which are removed from the body in the form of peeling skin and replaced with newer cells. In severe sunburns, ultraviolet photons cause damage to the DNA in skin cells, leading to an elevated risk of skin cancer.
|
|
Gamma rays tend to be the most destructive to living tissue because they penetrate the furthest into matter. Alpha particles are blocked by skin but can cause internal damage to the body if an alpha particle source is inhaled or ingested. Not all particles emitted as products of radioactivity are harmful, however. Neutrinos, a product of beta decay, interact so weakly with matter that we are not even affected by them. Nevertheless, it is estimated that about 1011 neutrinos from the sun pass through a single square centimeter of the Earth every second.