The Biology of Radioactivity

With the recent problems at the Fukushima nuclear power plant in Japan, caused by the Tohoku earthquake and consequent tsunami on March 11, attention has been drawn to how radiation can be dangerous to our health. But radiation isn’t all bad; it’s often used to improve our quality of life, from medical diagnostic tools to smoke detectors and much more, such as supplying power to techniques used in science laboratories.

Even if we wanted to, it’s impossible to completely avoid radiation, as we’re constantly exposed to it all around us. Since we evolved with constant exposure to background levels of radiation, our bodies know how to repair damage from it. But exposure to high levels can put a person’s health at risk. To decrease such risks, officials set standards for “safe” levels of radiation exposure based on studies of how different levels affect the human body.

What is radioactivity? To know how it affects us biologically and how we apply its potential, it’s important to first understand what radioactivity is in general. Radioactive materials are made up of elements that are unstable. The reason they’re unstable is that their core, their atomic nucleus, doesn’t have the normal, balanced number of components (normally it’s made up of a set number of protons and neutrons, but in radioactive materials the number of neutrons is different). So strongly does the element want to achieve stability that it changes its nucleus, its very identity, and becomes another element. At the same time, this change releases a lot of energy, which is in the form of radiation (mostly very energetic alpha, beta, and gamma particles). This whole process is known as radioactive decay.

Applications of radiation: Because some of these high-energy particles are able to penetrate many different types of solid materials and because we have tools that can detect even very low levels of radiation, radiation is used in many biological science and medical applications. Different tissues in the human body allow radiation to pass through them to different degrees (mainly based on their density), which makes possible a variety of radiation-based imaging techniques to visualize our internal organs, including x-rays and computed tomography (CT) scans, or the recently introduced backscatter x-ray machines at airports to just visualize below our clothes.

Other imaging techniques involve introducing radioactive material (called “tracers”) into a patient and monitoring the material in the body. These techniques include single photon emission computed tomography (SPECT) and positron emission tomography (PET).

Similarly, radioactive materials have been used in laboratories for decades to uncover key biological processes that take place on a molecular level; because our bodies treat a given radioactive element just as they would its nonradioactive counterpart, scientists use radioactive materials to study normal biological processes, taking advantage of our ability to detect these radioactive materials at very low levels.

And, of course, radiation can damage biological entities, and people have ironically taken advantage of this trait to improve our quality of life through a number of health and food applications. Localized doses of radiation are used in medical therapies to treat a wide range of cancers, including breast, brain, lung, rectal, prostate, cervical, and neck cancers. But probably less well-known is that radiation is also commonly used to sterilize certain food items (by killing potentially harmful microbes) and to sterilize male insects (which are then released to slow down reproduction within pest populations).

Looking beyond biology, radiation is used in a number of consumer products, including some glowing watch and telephone dials, exit signs, smoke detectors, and tobacco products. And, of course, radioactivity is used to generate power in nuclear power plants by heating up water to create steam (accounting for 20 percent of the energy generated in the U.S. in 2009).

Natural sources of radiation: Given the list above, it may seem like exposure to radiation from medical procedures and other man-made applications is very high, but for most people these sources actually contribute to less than half of their total annual radiation exposure. The amount varies, but total annual radiation exposure is generally around 2.5 to 3.6 millisieverts, or mSv, where a sievert is a measure of the amount of joules (a unit of energy) that a kilogram of tissue absorbs (one Sv equals one joule per kilogram).

The rest of our radiation exposure is from naturally occurring radioactive materials in the dirt, gases, and air around us. By far the largest source of natural radiation exposure is radon and thoron gases (making up about 1.2 to 1.3 mSv annually). Dirt and rocks contain radioactive potassium, thorium, and uranium, the last of which can decay into radioactive radon gas. The danger is that people cannot see, smell, or taste radon, and it builds up in underground dwellings, such as caves and basements where people can breathe it in. It may be a factor in 5 to 15 percent of all lung cancer cases. We’re also always exposed to cosmic radiation, which increases with altitude; exposure to cosmic radiation is why flying in a plane for a total of two days amounts to 0.3 mSv of radiation. And there’s also UV radiation from the sun which is thought to cause melanoma (which can be prevented by regularly using sunblock and, well, by staying out of tanning beds).

In summary, radiation is constantly all around us. We ourselves contain radioactive materials (and receive about 0.3 mSv of radiation every year from them). Although it is impossible to get away from all radiation, it’s thought that these normal background levels aren’t responsible for any adverse health effects. But exposure to radiation above these normal levels can do great biological harm.

How radiation affects the body: The highly energetic particles that radiation is made up of can crash into, and consequently damage, molecules in living creatures. If particularly important molecules, such as DNA, are injured, the life of the cell can be at stake. But our bodies have evolved with normal background levels of radiation and other sources of DNA damage; we have cellular mechanisms that constantly work to ensure that our DNA doesn’t get mutations and can repair any DNA that is damaged. This is why, in general, it is much easier for our bodies to deal with a dose of radiation that is spread out over a long period of time than one single, high dose of radiation. (This is part of the theory behind cancer radiotherapy.)

Because a single exposure to high levels of radiation is usually much harder for our bodies to deal with, it’s especially important to closely monitor high radiation levels from radioactive accidents to make sure workers are rotated and evacuated as necessary. Based on a normal exposure of about 2.5 to 3.6 mSv a year, normally a person receives about 0.007 to 0.01 mSv a day, although nuclear workers internationally have an exposure limit of about 20 mSv annually, averaged over multiple years, and a 50 mSv limit for a single year. Below acute exposures of about 50 mSv (and sometimes up to 250 mSv), people generally have no noticeable signs of damage from the exposure. But sometimes exposures as low as 50 mSv can result in changes in blood chemistry, including dilation of blood vessels. Between the reactors at the Fukushima nuclear plant, radiation levels reached 400 mSv/hr, and workers were evacuated when levels potentially reached 600 to 1,000 mSv/hr (on March 16).

Exposures of 500 mSv to 2000 mSv (2 Sv) can cause radiation poisoning, with symptoms including nausea, fatigue, destruction of white blood cells, vomiting, diarrhea, and hemorrhaging within hours of exposure, and hair loss within a few weeks. These high radiation levels damage cells faster than the body can repair them. Even more exposure can damage cells in bone marrow, the gastro-intestinal tract, and neural and muscular tissues (2 to 3 Sv). Doses as low as 3 Sv can be fatal within 30 to 60 days of exposure, whereas exposure to 10 Sv or more is almost always fatal within a few hours. Different tissues have varying degrees of sensitivity to radiation, but, in general, cells that are quickly multiplying and cells that re-grow slowly are the ones most damaged by radiation.

Lower levels of radiation exposure can cause other health problems, such as cancer, if the exposure is either for an extended period of time or accumulates within the body (such as through ingestion). The lowest radiation dose linked to an increase risk in cancer is 100 mSv distributed over one year, for multiple years, while several different types of cancers (including leukemia, bladder, colon, liver, stomach, lung, and more) have been associated with levels of 500 mSv or more, based on studies of survivors of the atomic bombs in Japan. Although our cells have mechanisms to repair damaged DNA, if it is damaged enough over time the cell may repair it incorrectly, potentially leading to cancer through an accumulation of errors.

In regard to radioactive materials leaking from the Fukushima nuclear power plant, the ones of most concern are radioactive iodine, cesium, and strontium, all nuclear fission by-products which may increase the risk of cancer. Low levels of radiation were detected 20 to 30 miles away from the Fukushima nuclear plant (about 4mSv over an entire day on March 16 and 17), and much lower levels have been detected in the Tokyo water supply and food products from the Fukushima prefecture.

When consumed, iodine is normally absorbed by the thyroid, and consequently when food contaminated with radioactive iodine is eaten, this is also where the radioactive material accumulates. This localized radiation exposure can cause thyroid cancer, but luckily radioactive iodine has a short half-life (after eight days half of its radioactive energy is dissipated, and after 40 days it’s only 3 percent as energetic) and potassium iodide can be taken to prevent absorption of radioactive iodine. Radioactive cesium acts like potassium and, if ingested, spreads throughout the body and can increase cancer risk. Radioactive strontium behaves like calcium; because both accumulate in bones when ingested, radioactive strontium can cause bone-related cancers, including leukemia. That said, it can be difficult to fully track the effects of leaked material, as cancer due to radiation exposure can take decades to manifest.

Although using radiation-based technology has allowed us to develop a wide variety of medical, agricultural, scientific, and energy-generating tools, the biological risks associated with the use of radioactive materials can be quite high. Clearly, thorough precautions and risk assessment guidelines must be followed to safely harness the potential of radioactive materials, and new generations of nuclear reactors and medical equipment have been designed with these concerns firmly in mind.

For more details on radioactivity and how it affects us biologically, see Joseph Magill and Jean Galy’s book Radioactivity, Radionuclides, Radiation, G. C. Lowenthal and P. L. Airey’s book Practical Applications of Radioactivity and Nuclear Radiation, Michael F. L’Annunziata’s book Radioactivity: Introduction and History, Robert G. Kunz’s book Environmental Calculations: A Multimedia Approach, the EPA’s website on Health Effects of Radiation, a very helpful visual Radiation Dose Chart by xkcd, A Layman’s Intro to Radiation website by Reed College, Risk Science Blog’s article on “The Fukushima nuclear reactor disaster…,” Discover magazine’s blog “Not Exactly Rocket Science” with many links covering the Fukushima nuclear reactor incident, or Wikipedia’s article on the Fukushima nuclear accidents.

Biology Bytes author Teisha Rowland is a science writer, blogger at All Things Stem Cell, and graduate student in molecular, cellular, and developmental biology at UCSB, where she studies stem cells. Send any ideas for future columns to her at science@independent.com.