3.0 HOW CAN RADIATION EXPOSURE HARM PEOPLE?

Harm to people from radiation exposure starts with damage to cells in the body.   The cell damage arises from damage to constituents of the cell, especially DNAFigure 3.1 shows different types of damage to DNA produced by radiation (including ultra violet).

FIGURE 3.1

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Radiation damage to cells may occur directly from a radiation hit on the critical target or indirectly from free radicals (reactive chemicals) that are produced by radiation.  Key sources of knowledge on radiation effects are resented in Figure 3.2 and include knowledge gained from cell, animal, and epidemiological studies.

FIGURE 3.2

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3.1  Harm from Small Radiation Doses

3.1.1  Radiation Effects in Somatic and Germ Cells

Most of the cells in the body are somatic cells.  Somatic cells are cells other than the germ cells.

Germ cells are involved in reproduction (i.e., producing babies).  Skin and lung cells are examples of somatic cells and are not involved in reproduction.

Small radiation doses can affect cells biologically. The effects depend on the type and amount of radiation.  These effects include cell killing, altered genes, damaged chromosomes, and cells being temporarily held (arrested) at specific places in the cell cycle called checkpoints. 

DNA is checked for damage while cells are arrested at checkpoints, and the damage is most often correctly repaired. However, on rare occasions, the damage is not correctly repaired. 

Misrepair (incorrect repair) of DNA damage can lead to unstable DNA in the cell nucleus.  Unstable DNA in living cells is called genomic instability. 

Cells that survive with genomic instability can, over time, cause big problems for people.  Two such problems are cancer in irradiated persons and genetic effects in children of irradiated parents. 

Genetic effects arise from genomic instability in germ cells of the irradiated parents. 

Cancer arises from genomic instability in somatic cells in the irradiated person.

3.1.2  Genetic Effects

Small doses of ionizing radiation can permanently damage DNA in germ cells as previously indicated.  One type of permanent damage is gene mutation.  A mutation can be transmitted from one generation to another and therefore represents a genetic effect of irradiation.

Two specific germ-cell stages are considered important in evaluating the effects of radiation on the heredity of germ cells:

 
Spermatogonia continue to multiply throughout the reproductive life span of an individual.  However, oocytes are not replaced during adult life.

The genetic effects that could be caused by radiation are too numerous to be considered here individually. For radiation risk assessment, genetic disorders can be grouped as:

3.1.2.1  Dominant and X-Linked Single-Gene Disorders

Most cells from humans contain two sets of chromosomes with matched pairs of genes, one gene from each parent.  The matched gene partners can differ, with one gene being dominant over its recessive partner gene.

Achondroplastic dwarfism is an example of a dominant gene disorder that could be caused by ionizing radiation.

A recessive gene can only show its effect if both partner genes show the effect. If a bad gene is present on the X-chromosome, it will invariably produce an effect in males.  This is because males only have one X-chromosome.

Females have two X-chromosomes. If a female has one bad gene on an X-chromosome, but the other X-chromosome has a good gene partner, then the bad gene can behave as recessive (i.e., not having an influence).

Single-gene disorders associated with the X-chromosome are called X-linked effects.

Muscular dystrophy is an example of an X-linked effect that could be caused by ionizing radiation. 

There is no direct evidence that the above-indicated diseases have been induced by radiation in humans.  However, based on results of animal studies, the diseases are considered possible consequences of radiation exposure.

3.1.2.2  Chromosome Disorders

Chromosome damage in germ cells of parents can influence heredity. Most somatic cells of humans contain 23 pairs of chromosomes, with one member of each pair donated by the father and the other by the mother.

Radiation exposure of the parents could lead to an abnormal number of chromosomes (aneuploidy) in their offspring, which could severely affect the unborn or newborn child.

In most cases, aneuploidy will result in spontaneous loss of pregnancy.  In the remaining cases, a severely affected child would be expected.

Down’s syndrome is an example of a consequence of aneuploidy. People with aneuploidy have a significant reduction in their life expectancy, have abnormal body features, and have no children.

3.1.2.3  Multifactorial Disorders

Multifactorial disorders (diseases) involve complex patterns of inheritance. These disorders represent a very large class of genetic diseases.

A specific combination of mutant genes must be present for multifactorial diseases to occur.  Environmental factors can also be important.

Examples of multifactorial diseases include:

3.1.3  Late Somatic Effects

Late somatic effects of irradiation are those effects that occur in somatic cells years after brief exposure.  Cancer is the somatic effect of most concern in radiation risk assessment.  For chronic exposure to radiation over many years, the late somatic effects may occur during the irradiation period.

Cancer does not appear immediately after brief radiation exposure. It appears only after a delay (latent period).  For humans, the latent period may be many years for some cancers (e.g., lung cancer). 

Other factors such as cigarette smoking can also influence the cancer risk from radiation exposure.

Mechanisms presently considered to be involved in the induction of cancer by radiation include:

More than one mechanism could be involved for a given type of cancer.  However, the relative importance of the indicated mechanisms is not clear.

3.2  Harm from Large Radiation Doses

3.2.1  Harm from Short-term Exposure

Large radiation doses can destroy millions or more cells in tissues of the body. Because tissues of the body have important functions, destroying large numbers of cells in tissue can lead to impairment of organ function, morbidity, and death from organ failure.

Deterministic (nonstochastic) effects of irradiation are those health effects that arise only when large numbers of cells are destroyed by radiation.  For such effects, there is a threshold dose below which the health effect does not occur.

For deterministic effects, the severity of the health effect can increase as the radiation dose increases above the threshold. 

Deterministic health effects include:

Deterministic effects usually appear within a few months after brief (short-term) exposure to large radiation doses (e.g., from a nuclear weapon or nuclear accident).  The earliest effects seen are associated with what has been called the prodromal phase (acute radiation sickness phase).

The prodromal phase is made up of the symptoms and signs appearing in the first 2 days after brief exposure to radiation.  After super-lethal doses of several tens of Gy, all individuals begin to show all symptoms associated with the prodromal phase within about 15 minutes. 

Reactions during the prodromal phase are mediated via the autonomic nervous system.  They are expressed as gastrointestinal and neuromuscular symptoms.

The gastrointestinal symptoms are:

The neuromuscular symptoms are:

Other deterministic effects of irradiation include bleeding, infection, hair loss, temporary suppressed sperm counts, and permanently suppressed ovulation.  Morbidity can arise from damage to the skin, eye, thyroid, liver, lung, bone marrow, and other sites. 

Death can arise from severe damage to key organs (e.g., skin, intestines, bone marrow, lung, and liver).

Radiation-induced deterministic effects can adversely impact the performance of humans (i.e., performance degradation). Members of population exposed to a nuclear weapon could be severely impaired by deterministic effects of brief exposure to neutrons and gamma rays.

The US military uses a computer program called HPAC to evaluate performance degradation based on combinations (complexes) of radiation-induced symptoms and signs over time.  The time patterns of the symptom complexes depend on the type of exposure (e.g., brief, chronic, etc.).

Prodromal effects of irradiation can also arise from radiation exposure resulting from a nuclear accident or radiological incident. This happened to Russians involved in the Chernobyl accident that occurred in April 1986.

Three hundred Chernobyl accident victims suspected of suffering from the acute radiation sickness were sent to the specialized treatment center in Moscow and to hospitals in Kiev within the first 3 days following the start of the Chernobyl accident.  Over the subsequent days, some 200 additional people were admitted for examinations.

Acute radiation sickness was confirmed in 99 of the 128 people (firemen, Unit 4 reactor operators, turbine-room duty officer, and auxiliary personnel) admitted to the specialized treatment center in Moscow during the first 2 days of the Chernobyl accident and in 6 of the 74 victims hospitalized during the following 3 days.

3.2.2  Harm from Long-term Exposure

Deterministic effects also include radiation effects (other than cancer and genetic effects) that continue to occur after an extended period (e.g., years) of chronic (long-term) exposure.  Such chronic exposure can arise from long-lived radionuclides ingested via contaminated food or inhaled via contaminated air.

Russian nuclear workers at the Mayak plutonium production facility in the Chelyabinsk region (near the Urals Mountains) were exposed over years to neutrons plus gamma rays and to alpha radiation plus gamma rays.  Various deterministic effects were caused by these radiation exposures.  Other effects (e.g., cancer) were also induced.

In addition to cancer, genetic effects (in their children), and prodromal effects, two effects were seen in Mayak workers that were not previously reported in western literature:

Pneunosclerosis appears to be related to radiation pneumonitis and pulmonary fibrosis in the lung.

Chronic radiation disease (or chronic radiation sickness) was originally reported by the Russian physicians, A. K. Gus'kova and G. D. Baysogolov.  They described chronic radiation disease as being characterized by varying degrees of cardiovascular, gastrointestinal, and neural system disorders.

Chronic radiation disease occurred mainly in workers with total gamma-ray doses in excess of 1 Gy.

Both pneumosclerosis and chronic radiation disease can occur years after the start of chronic exposure to radiation.

3.3    Harm from Exposure to Radiation and Other Hazards (Chemical, Biological)

The number of different types of chemical/biological (CB) agents that potentially could be involved in combined nuclear/biological/chemical (NBC) exposure of humans in association with a terrorist incident is staggering.

Potential CB agents include the following biologicals:

Among the chemicals are:

Genetically engineered organisms also represent a possible future hazard. 

Out of concern about possible terrorist acts, smallpox vaccinations were initiated on January 24, 2003 among some key medical personnel in the U.S.

Little is known about dose-response relationships in humans for many of the individual agents of interest.  Even less is known about potential harm from combined exposures.

It is possible to adequately predict the consequences of combined exposure when the modes of action of individual agents are known. However, key knowledge is lacking at the present time about dosimetry and modes of action.