Ecosystems and Human Health - Chapter 13 potx

Ionizing radiation, that portion of the spectrum that can cause serious cell damage, includes all wavelengths of 1000 Å or less see Table 12.. After all, humankind evolved in a world of

chapter thirteen Radiation hazards

Introduction

The electromagnetic spectrum encompasses all forms of radiant energy

Table 12 lists the various components and their wavelengths in decreas-ing order

Ionizing radiation, that portion of the spectrum that can cause serious cell damage, includes all wavelengths of 1000 Å or less (see Table 12) Ion-izing radiation, by stripping electrons from molecules as it passes through tissues, produces ionized species of everything from H2O to macromolecules like DNA These ionized species are unstable and reactive, and can produce dramatic disruptions in cell function, including mutation

There is still controversy regarding the degree of risk Radiophobia, an illogical fear of radiation hazards, has led to considerable controversy over the extent of the environmental risks of ionizing radiation Consider the following conflicting statements from Cobb (1989)

Table 12 Components of the Electromagnetic Spectrum

Infrared (includes thermal portion) 0.5 mm–10000 Å

Cosmic rays (protons 85%, alpha particles 12%, electrons, gamma rays, etc.

• Dr K.Z Morgan, a pioneer in health physics: “It is incontestable that radiation risks are greater than published.”

• C Rasmussen, nuclear engineer at MIT: “There is a lot of evidence that low doses of radiation not only don’t cause harm but may in fact do some good After all, humankind evolved in a world of natural low-level radiation.” (About 82% of our total radiation exposure come from natural sources.) This is another example

of hormesis

• R Guimond, EPA: “We can’t avoid living in a sea of radiation.”

In some cultures, deliberate exposure to natural-source radiation is done

in the belief that it has curative powers In Japan, exposure to radon is courted in “radon spas” where natural hotsprings occur

Sources and types of radiation

Sources Natural sources of radiation

Natural sources of radiation include cosmic rays from space, solar rays that intensify during solar storms (sunspots), radiation emanating from rocks and groundwater, and radiation coming from within our own bodies, mainly from decay of radioactive potassium in muscle Of considerable concern at present is the exposure to radon gas, a radioactive decay product of radium,

a common radioactive element in soil and rock This is discussed in more detail below

Man-made sources of radiation

Man-made sources of radiation include medical X-rays and radioisotopes, ion sensors in smoke detectors, uranium used to provide the gleam in den-tures, mantles in camping lanterns, radioactive wastes, nuclear accidents (e.g., Chernobyl), and careless handling of nuclear materials Cesium-137 was discarded in a dump in Brazil and ultimately killed 4 people and con-taminated 249 Until fairly recently, radium was used to hand-paint luminous watch dials Workers used to “point” their brushes by running them between their lips, a practice that led to cases of radiation sickness

The cause of radiation

Elements that exist in an unstable form are continually decaying to more stable ones In the process, they give off energy in several ways Ionizing radiation arises when an unstable nucleus gives off energy An unstable nucleus is called a radionuclide In contrast, X-irradiation is a form of cosmic ray and also occurs when a suitable target such as tungsten is bombarded with electrons It does not arise from nuclear decay

Types of radioactive energy resulting from nuclear decay

1 A nucleus can eject two protons and two neutrons to lose mass and convert itself into another element The ejected components consti-tute an alpha-particle An alpha-particle is slow moving and will not penetrate skin, but it can cause dangerous ionization if ingested An alpha-particle is actually identical to the nucleus of helium

2 The neutron of a nucleus can lose an electron to become a positron The lost, negatively charged particle is called a beta-particle

3 Even after emitting alpha- or beta-particles, a nucleus may remain

in an agitated state It can rid itself of excess energy by giving off gamma-rays These are short, intense bursts of electromagnetic en-ergy with no electrical charge They can penetrate lead and concrete and can cause extensive tissue damage by ionization

4 Neutrons are ejected from the nucleus during nuclear chain reactions They collide and combine with the nuclei of other atoms and induce radioactivity of the above types This is the primary source of radio-activity following a nuclear explosion

Measurement of radiation

There are two different types of measurements of ionizing radiation One is concerned with the level of energy actually emitted from the source, and the other is concerned with the amount of tissue damage that can be produced

by a particular form of radiation The field is unfortunately further confused

by a more recent shift to the international (SI) system The equivalent values are shown in Table 13 They are not easily interchangeable

Measures of energy

1 As the nucleus of an atom decays, it gives off a burst of energy (the ionizing radiation) called a “disintegration.” The number of disinte-grations per unit time varies with the nature of the source Various counting instruments (scintillation counters, etc.) measure disinte-grations per minute (dpm) The basic unit of measuring radiation

Table 13 Equivalent Values, Old and New Systems

1 curie (Ci) 37 gigabecquerels (Gbq)

1 rem (rem) 10 millisieverts (mSv)

100 rem 1 Sievert (Sv)

1 rad (rad) 10 milligrays (mGy)

100 rad 1 gray (Gy)

1 roentgen (R) 258 millicoulombs/kilogram (mC/kg)

energy is the curie (Ci) and it is the number of disintegrations (3.7 × 1010) occurring in 1 second in 1 gram of radium-226 Radioiso-topes used in science are usually provided in milliCi (mCi) amounts The new unit is the Becquerel (Bq) A Bq represents one disintegration per second: 1 Ci = 37 × 109 Bq (37 gigabecquerels)

2 The first unit of radiation was the roentgen (R) It has a complicated definition based on the amount of X-rays or gamma-rays required to cause a standard degree of ionization in air

Measures of damage

1 The earlier measure of radiation damage is the rem, which stands for roentgen-equivalent-man It is that amount of ionizing radiation of any type that produces in humans the same biological effect as 1 R The new international unit is the sievert (Sv): 1 Sievert = 100 rem

2 The rad is the amount of radiation absorbed by 1 gram of tissue The new international unit is the gray (Gy): 1 gray = 100 rad

A rough scale of toxicity is as follows: 10,000 rem is rapidly fatal because

of damage to the central nervous system (CNS) Whole-body exposure to

300 rem is approximately the LD50 Between 100 and 300 rem, radiation damage occurs The assumption is made that the risk associated with radi-ation is linear all the way to zero When it comes to assessing carcinogenic potential, however, accuracy is extremely difficult Below 10 rem, effects are unclear due to confounding factors such as smoking, pollution, and diet Over 300 agents have been shown to be carcinogenic in animal tests Resi-dents of Denver have lower cancer death rates than those of New Orleans despite higher radiation exposures because of increased levels of cosmic radiation at their high altitude

Major nuclear disasters of historic importance

Hiroshima

The people from whom the most reliable data have been gathered concerning radiation hazards are the survivors of the atom bomb dropped on Hiroshima

at 8:15 a.m., August 6, 1945 In 1947, the Radiation Effects Research Foun-dation was established Exhaustive studies have shown that the heavily exposed people, called the “hibakusha,” had a 29% greater chance of dying from cancer than normal Excess numbers of leukemia cases began appearing

in the late 1940s and peaked in the early 1950s but by the early 1970s had dropped to levels near those of unexposed Japanese Now the surviving hibakusha have longer life expectancies than the overall population, perhaps because of closer medical supervision

One of the most feared hazards of radiation is that of congenitally deformed infants because of radiation-induced genetic defects in the mother

While such defects have been demonstrated experimentally, the Hiroshima study compared 8000 children of hibakusha with 8000 of unexposed Japa-nese and found an incidence of chromosome damage in 5/1000 of the former and 6/1000 of the latter Protective mechanisms may be functioning in humans (e.g., spontaneous early abortion) Fetuses exposed in utero are a different story Dozens of mentally retarded infants were born in the areas around Hiroshima and Nagasaki (target of the second atomic bomb) in the months following the blasts Spontaneous abortions were also numerous The fetus appears to be most vulnerable between 8 and 15 weeks

One recent development, however, is that a special panel formed by the U.S National Research Council recently released a report following a reas-sessment of the Hiroshima and Nagasaki data and concluded that the levels

of exposure were much lower than previously calculated Original estimates were based on tests at the Nevada nuclear test site using much flimsier build-ings than were actually present in those cities As a consequence, the estab-lished safe limits have had to be revised downward This is primarily a concern for people exposed to radiation on the job, but it has revived the controversy about whether there is any safe level of exposure Ironically, evidence is now surfacing that scientists and technicians who worked on the atomic bomb project during the war are showing up with elevated incidences of cancer which, when adjusted for exposure level, may be even greater than those of the hibakusha The new safe exposure limit is 20 mSv/yr averaged over 5 years with no more than 50 in any one year The old level was 50 mSv/yr

Chernobyl

The most recent and highly publicized nuclear disaster was Chernobyl in April of 1986 (a much worse but largely concealed disaster occurred in Russia in 1958) In this disaster, 20% of the plant’s radioactive iodine escaped, along with 10 to 20% of its radioactive cesium and other isotopes Approximately 135,000 people lived in a 30-km radius of the power plant There were 30 deaths and 237 cases of severe radiation injury Some 2000 children have been born to women who were living in the accident zone at the time of the disaster No abnormalities have yet been detected in them

An examination of about 700,000 people over a wider area has thus far not revealed any physical problems Russian scientists estimate an increase in the cancer rate of 0.04% over the next 20 years In western Europe, exposed

to the drift of radioactive dust, it is estimated that, over the next 50 years,

1000 additional cancer deaths will occur Normally, there would be 30,000,000 cancer deaths in this period, so the increase is 0.003% Aside from those people directly exposed to the effects of the explosion, the greatest risk of exposure appears to come from eating contaminated food Thousands

of reindeer had to be destroyed in northern Scandinavia because they had grazed on contaminated pasture

Dr Marvin Goldwin, chief of the Joint-U.S.-Soviet Medical Team, made the following points in a recent report

1 Everyone in the Northern Hemisphere received a small dose of ra-diation The degree of exposure of those people at highest risk cannot

be accurately identified

2 Radioactive iodine posed an early risk to the thyroid glands of ex-posed people

3 As of 1991 their was no detectable increase in the incidence of cancer but leukemia may yet show up and solid tumors may not show up for 10 years

Chernobyl provides another example of how psychological damage can often exceed physical damage in an environmental disaster Thousands of people received a radiation exposure that exceeded the maximum recom-mended lifetime allowance Because radiation levels in their locales have fallen to low levels similar to those of surrounding areas, it makes no medical

or scientific sense to relocate them Stress and fear, however, create an under-standable desire in these people to be moved out of the area, and their wishes are presently being acted upon

Three Mile Island

The partial core meltdown of the Unit 2 reactor at Three Mile Island in March

1979 was largely responsible for bringing the nuclear power program in the United States to a halt Of the “defense in depth” safety features, all but the outer water shield failed Some authorities claim that even a complete melt-down would not have breached this defense Despite concerns of nearby residents, only 15 Ci of radioactivity were actually released The news media exploited the event with sensational reports of “deadly clouds of radioactive gas” and made much of the potential for explosion of a large bubble of hydrogen in the reactor In fact, there was none because no oxygen was present The people at greatest risk from radiation were the workers who were involved in the clean-up U.S federal regulations limit the maximum exposure of workers in the nuclear industry to 12 rem per year Workers in the “hot” areas receive about 1 mrem per hour, the equivalent of one chest X-ray Totals for such workers in 1987 were about 710 millirem (see Chapter

2 for more on Three Mile Island)

The Hanford release

In contrast, massive amounts of iodine-131 were deliberately released (for purposes still classified as top secret) from the military nuclear facility at Hanford, Washington in the 1940s and 1950s Some residents may have received as much as 2295 rem Again, the greatest source of exposure may have been the consumption of contaminated meat and vegetables Multimil-lion-dollar studies have recently been commissioned to seek answers to the degree of risk and to assign responsibility Obviously, the potential for civil action is considerable

The consistent elements present in all of these disasters, including the discarding of cesium-137 in Brazil, have been human error, misjudgment, and negligence Deliberate callousness may have been involved at Hanford, where charges of unsafe practices and antiquated, dangerous equipment are still being made Conversely, the real danger to the public has routinely been overblown, and events have been exploited by the media and by antinuclear groups In the minds of many, nuclear reactors equate with nuclear bombs

Radon gas: the natural radiation

As noted above, 82% of the ionizing radiation to which North Americans are exposed comes from natural sources It has been estimated that there is enough natural radioactive material in the human body that, if it were a laboratory animal, it would have to be disposed of as hazardous waste The average, annual, natural background exposure in Great Britain is about 1 mSv (0.1 rem or 100 mrem) In Canada, it is somewhat higher because the Canadian Shield (the band of granite rock that spans mid-northern Ontario, Quebec, and Manitoba) is rich in uranium deposits Radon gas is by far the largest potential health hazard from natural radiation It is the decay product

of uranium and it seeps up through faults in the sub-strata of soil and may leak into houses through cracks in basement walls, drains, etc The advent

of airtight houses has increased the risk by trapping radon gas in the house

In the British study, it was calculated that 1,000,000 people were exposed to radon at levels of 5 to 15 mSv annually In contrast, only 5100 workers in nuclear industry were exposed to levels as high or higher This is the equiv-alent of 50 to 150 chest X-rays annually Radon homes are distributed very randomly, with highly contaminated homes located right beside radon-free ones The federal government conducted a survey of Canadian homes and found that Winnipeg had the highest radon levels of any Canadian city, with high levels also found in parts of Saskatchewan and northern Ontario and Quebec (see Table 14) Toronto (in Ontario) has low levels

A map of radon risk areas in the United States, published by National Geographic, shows a band running slightly east of North through the middle

of Ohio, and another running east-west through New York state A U.S federal study surveyed 20,000 homes in 17 states and found that 25% had potentially hazardous levels of radon Radon was described as the largest environmental radiation health hazard in America Debate over the degree

of risk plagues this area, as it does others The EPA study measured radon levels in basements where they would be highest Calculations of risk have estimated the lifetime risk of dying from radon-induced cancer at 0.4% for exposed individuals, which is the same as one’s chances of dying in a fire

or a fall If radon were a man-made carcinogen, it would unquestionably be banned, and most certainly would be the target of anti-nuclear activists Nevertheless, there is still controversy regarding the degree of risk, or per-haps more correctly, the degree of exposure A British study calculated that

6 to 12% of all myeloid leukemias might be attributed to radon exposure,

with levels rising to 23 to 43% in Cornwall, where the highest exposures occur Worldwide, their calculations suggested 13 to 25% of all myeloid leukemias in all age groups could be due to radon

In 1984, a construction worker at the Limerick nuclear generating station near Reading, Pennsylvania consistently set off radiation alarms despite the fact that he had never worked in a “hot” area An examination of his home subsequently revealed radon levels of 2600 pCi/L, the highest ever recorded North Dakota, with 63% of homes showing levels of 4 pCi/L or greater, leads the United States in radon exposure, followed by Minnesota (46%), Colorado (39%), Pennsylvania (37%), and Wyoming and Indiana (26%) After cigarette smoking, radon is probably the most common cause of lung cancer Radon-222 (222Rn) and 220Rn are the only gaseous decay products of uranium Their half-life is 3.8 days and they decay to particles (not gases), including radioactive poloniums, which actually are the alpha-emitting toxins that cause cell damage Alpha-particles, unlike gamma-rays, can only cause cell damage for a radius of about 70 µ, hence the risk of lung cancer The increased incidence of leukemias is difficult to explain on this basis

Tissue sensitivity to radiation

In general, tissues with a high rate of turnover are more susceptible to the effects of ionizing radiation Thus, thyroid, lung, breast, stomach, colon, and bone marrow have high sensitivity; brain, lymph tissue, esophagus, liver, pancreas, small intestine, and ovaries are intermediate; and skin, gall bladder, spleen, kidneys, and dense bone are low This order of sensitivity roughly parallels the frequency of primary cancer in these tissues If molecular dis-ruption is sufficient, the cell will die Because hair follicles and gastrointestinal mucosa have high turnovers, radiation sickness involves hair loss and severe diarrhea Because bone marrow cells also have a high turnover, repair of

Table 14 Radon Concentrations in Canadian Homes in, pCi/L Air

City

No Homes Tested

No Homes

>4.5 pCi/L %

Compiled from data reported by McGregor et al., Health Physics, 39, 285–299, 1980.

DNA may not be complete before replication occurs and the daughter cells may be malignant This is why leukemia is the commonest cancer associated with radiation injury

Questions regarding the safety of the nuclear industry continue to emerge In August 1989, two workers at the Pickering Nuclear Plant in Ontario were mistakenly given unshielded practice equipment to change a new type of fuel rod just recently introduced They received what were widely reported as the highest levels of radiation ever encountered by work-ers in the Canadian nuclear industry: 5.6 and 12.2 rem The annual allowable limit set by the Atomic Energy Commission is 2 rem The radiation exposure from an average chest X-ray is 15 mrem This has been calculated (theoret-ically) to cause one additional cancer per 100,000,000 population In other words, if the entire population of North America were to receive one chest X-ray, one could expect three additional cancer cases as a result These workers received about 700 times this amount, which would cause one additional cancer per 143,000 people This risk would be lessened if they were removed to areas where there is no possibility of additional exposure for at least 1 year The safety maxim that should apply in all situations involving exposure to radiation is ALARA (as little as reasonably achiev-able) It should be noted that, once again, human error was responsible for this accident

In Great Britain, a disturbing report was released in early 1990 to the effect that offspring of nuclear plant workers had an increased incidence of birth defects Because these children are not directly exposed to radioactive material, the conclusion, if the data are correct, is that exposure of the parents (mostly men) caused genetic damage These results contradict earlier studies, and it has been pointed out that clusters of birth defects occur geographically

in the absence of nuclear generators (or other identifiable causes) The data will be of concern until they can be explained or disproved Public pressure has resulted in the cancellation of 50 new nuclear power plants in the United States As a result, there is greater reliance on coal-fired generators Approx-imately 200 coal miners die each year in mine accidents and an equal number from black lung disease (pneumoconiosis) Recent (1992) mine accidents include 26 killed in Nova Scotia, 400 in Turkey, and 38 in Russia Such events rarely cause a ripple of concern among opponents of nuclear energy There

is also the problem of acid rain resulting from sulfur pollution of the atmo-sphere by coal-fired generators Has the public traded a potential but high-profile risk for a real and greater one that is less visible? Table 15 compares various sources of radiation encountered by Americans

Microwaves

Microwaves are the shortest waves in the radio portion of the electromag-netic spectrum (1 mm–30 cm) They are at very high frequencies (1000–300,000 megacycles/s) and are used in radar, for long-distance trans-mission of phone and TV signals, and, of course, in microwave ovens

Because of its high dielectric constant, water dissipates energy as heat when exposed to microwaves At high enough energies, thermal damage may occur in living tissues Recent concern has been expressed over possible carcinogenic effects of microwaves given off by cellular phones The energy level is so low (<5 watts), however, that no thermal effects can be detected Little information is available concerning non-thermal effects of microwaves, but evidence for carcinogenicity is scanty and anecdotal

Ultraviolet radiation

Ultraviolet radiation occupies the electromagnetic spectrum between 400 and 4 nm: UVA, 400–320 nm; UVB, 320–280 nm; UVC, <280 nm An increase

of 1 to 2% of UVB radiation is associated with an increase of 2 to 4% in skin cancer UVB is in the ionizing radiation range and can therefore damage DNA, leading to mutations and cancer The effect on melanocytes appears

to be more complicated Melanomas often appear first at sites not directly exposed to sunlight Tropical and subtropical areas have much higher inci-dences of skin cancer than temperate zones In North America, it has been claimed that the incidence of skin cancer has increased by 400% in recent years, presumably because of the destruction of the ozone layer

Medical uses of UV radiation

1 Photophoresis, in which blood is removed from a patient and ex-posed to UV light, then returned to the patient, is useful for treating mycosis fungoides, a complication of skin cancer, and it is promising for some leukemias

2 UVA is used in conjunction with a photosensitive drug called psorelen to treat the skin lesions of psoriasis The technique is called PUVA (psorelen-UVA)

Table 15 Average Annual U.S Doses

Building materials (brick and masonry) 3–4 mrem

Total average annual exposure ~200 mrem

a Much higher at downwind locations in Nevada, Utah, etc.