Radio and television signals, radar, heat, infrared, ultraviolet, sunlight, starlight, cosmic rays, gamma rays, and x rays all belong to the electromagnetic spectrum and differ only in their relative energy, frequency, and wavelength. These waves all travel at the speed of light, and unlike sound they can all travel through empty space. The frequencies above visible light have enough energy to penetrate and cause damage to living tissue, damage that can be as minor as a sunburn caused by ultraviolet light or as extreme as the incineration of Hiroshima, Japan, during World War II. Lower frequencies do not penetrate, but can cause eye and skin damage, primarily due to the heat they transmit.
The energy of electromagnetic radiation is a direct function of its frequency. The high- energy, high-frequency waves, which can penetrate solids to various depths, cause damage by separating molecules into electrically charged pieces, a process known as ionization. Atomic particles, cosmic rays, gamma rays, x rays, and some ultraviolet are called ionizing radiation. The pieces theygenerate are called free radicals. They act like acid, but they last only fractions of a second before they revert to harmless forms. Adjusting the energy of therapeutic radiation can select a depth at which it will do the most damage. Ionizing radiation also does damage to chromosomes by breaking strandsof DNA. DNA is so good at repairing itself that both strands of the double helix must be broken to produce genetic damage.
Because radiation is energy, it can be measured. There are a number of unitsused to quantify radiation energy. Some refer to effects on air, others to effects on living tissue. The roentgen, named after Wilhelm Conrad Roentgen, who discovered x rays in 1895, measures ionizing energy in air. A rad expressesthe energy transferred to tissue. The rem measures tissue response. A roentgen generates about a rad of effect and produces about a rem of response. Thegray and the sievert are international units equivalent to 100 rads and rems,respectively. A curie, named after French physicists who experimented with radiation, is a measure of actual radioactivity given off by a radioactive element, not a measure of its effect. The average annual human exposure to natural background radiation is roughly 3 milliSieverts (mSv).
It is reasonable to presume that any amount of ionizing radiation will produce some damage. However, there is radiation everywhere, from the sun (cosmic rays) and from traces of radioactive elements in the air (radon) and the ground (uranium, radium, carbon-14, potassium-40 and many others). Earth's atmosphere protects us from most of the sun's radiation. Living at 5,000 feet altitude in Denver, Colorado, doubles exposure to radiation, and flight in a commercial airliner increases it 150-fold by lifting us above 80% of that atmosphere. Because no amount of radiation is perfectly safe and because radiation isever present, arbitrary limits have been established to provide some measureof safety for those exposed to unusual amounts. Less than 1% of them reach the current annual permissible maximum of 50 mSv.
It is therapeutic, accidental, and deliberate radiation that does the obviousdamage. There has not been much in the way of deliberate radiation damage since Nagasaki, but accidental radiation exposure happens periodically. Between1945 and 1987, there were 285 nuclear reactor accidents, injuring over 1,550people and killing 64. The most striking example, and the only one to endanger the public, was the meltdown of the graphite core nuclear reactor at Chernobyl in 1986, which spread a cloud of radioactive particles across the entirecontinent of Europe. Information about radiation effects is still being gathered from that disaster. There have also been a few accidents with medical and industrial radioactivity.
Nevertheless, it is believed that radiation is responsible for less than 1% of all human disease and for about 3% of all cancers. This figure does not include lung cancer from environmental radon, because that information is unknown. The figure could be significant, but it is greatly confounded by the similar effects of tobacco.
Radiation can damage every tissue in the body. The particular manifestation will depend upon the amount of radiation, the time over which it is absorbed,and the susceptibility of the tissue. The fastest growing tissues are the most vulnerable, because radiation as much as triples its effects during the growth phase. Bone marrow cells that make blood are the fastest growing cells inthe body. A fetus in the womb is equally sensitive. The germinal cells in the testes and ovaries are only slightly less sensitive. Both can be rendered useless with very small doses of radiation. More resistant are the lining cells of the body--skin and intestines. Most resistant are the brain cells, because they grow the slowest.
The relative sensitivity of various tissues gives a good idea of the wide range that presents itself. The numbers represent the minimum damaging doses; agray and a sievert represent roughly the same amount of radiation:
- Fetus--2 grays (Gy).
- Bone marrow--2 Gy.
- Ovary--2-3 Gy.
- Testes--5-15 Gy.
- Lens of the eye--5 Gy.
- Child cartilage--10 Gy.
- Adult cartilage--60 Gy.
- Child bone--20 Gy.
- Adult bone--60 Gy.
- Kidney--23 Gy.
- Child muscle--20-30 Gy.
- Adult muscle--100+ Gy.
- Intestines--45-55 Gy.
- Brain--50 Gy.
Notice that the least of these doses is a thousand times greater than the background exposure and nearly 50 times greater than the maximum permissible annual dosage.
The length of exposure makes a big difference in what happens. Over time theaccumulating damage, if not enough to kill cells outright, distorts their growth and causes scarring and/or cancers. In addition to leukemias, cancers ofthe thyroid, brain, bone, breast, skin, stomach, and lung all arise after radiation. Damage depends, too, on the ability of the tissue to repair itself. Some tissues and some types of damage produce much greater consequences than others.
Immediately after sudden irradiation, the fate of the patient depends mostlyon the total dose absorbed. This information comes mostly from survivors of the atomic bomb blasts over Japan in 1945.
- Massive doses incinerate immediately and are not distinguishable from the heat of the source.
- A sudden whole body dose over 50 Sv produces such profound neurological, heart,and circulatory damage that patients die within the first two days.
- Doses in the 10-20 Sv range affect the intestines, stripping their lining andleading to death within three months from vomiting, diarrhea, starvation, and infection.
- Victims receiving 6-10 Sv all at once usually escape anintestinal death, facing instead bone marrow failure and death within two months from loss of blood coagulation factors and the protection against infection provided by white blood cells.
- Between 2-6 Sv gives a fighting chance for survival if victims are supported with blood transfusions and antibiotics.
- One or two Sv produces a brief, non-lethal sickness with vomiting, loss of appetite, and generalized discomfort.
It is clearly important to have some idea of the dose received as early as possible, so that attention can be directed to those victims in the 2-10 Sv range that might survive with treatment. Blood transfusions, protection from infection in damaged organs, and possibly the use of newer stimulants to blood formation can save many victims in this category.
Local radiation exposures usually damage the skin and require careful wound care, removal of dead tissue, and skin grafting if the area is large. Again infection control is imperative.
There is considerable interest these days in benevolent chemicals called "free radical scavengers." How well they work is yet to be determined, but population studies strongly suggest that certain diets are better than others, andthat those diets are full of free radical scavengers, otherwise known as antioxidants. The recommended ingredients are beta-carotene, vitamins E and C, and selenium, all available as commercial preparations. Beta-carotene is yellow-orange and is present in yellow and orange fruits and vegetables. Vitamin Ccan be found naturally in citrus fruits. Traditional Chinese medicine (TCM) and acupuncture, botanical medicine, and homeopathy all have contributions tomake to recovery from the damage of radiation injuries. The level of recoverywill depend on the exposure. Consulting practitioners trained in these modalities will result in the greatest benefit.