Nuclear Radiation and its Impacts

  Radiation Types


 

Isotopes of elements that emit ionising radiation are radioactive isotope or radio nuclides

Effect of alpha, beta is greatest when absorbed, ingested or deposited in or near living tissue

The penetration of alpha, beta and gamma increase from alpha to gamma but ionisation and local damage are of reverse order

  Effects of Radiations


Acute and Delayed Effects

A single accidental exposure to a high dose of radiation during a short period of time is referred to as an acute exposure, and may produce biological effects within a short period after exposure. These effects include:

The delayed effects of radiation are due to both acute exposure and continuous exposure (chronic exposure). In this case, the negative effects may not be apparent for years. The chronic exposure is likely to be the result of improper or inadequate protective measures.

In the case of inhalation or ingestion of radioactive materials, a single "acute" event may cause a long period "chronic" internal body exposure due to irradiation of tissue where radioactive material has been fixed

The most common delayed effects are various forms of cancer (leukaemia, bone cancer, thyroid cancer, lung cancer) and genetic defects (malformations in children born to parents exposed to radiation).

Latency period

In any radiological situation involving the induction of cancer, there is a certain time period between the exposure to radiation and the onset of disease. This is known as the "latency period" and is an interval in which no symptoms of disease are present. The minimum latency period for leukaemia produced by radiation is 2 years and can be up to 10 years or more for other types of cancer.

Effects of Radiation on Foetus

It is well known that the foetus is more sensitive to the effects of radiation than the adult human. If an irradiation occurs in the first 30 weeks of pregnancy, delayed effects may appear in the child. These include mental and behaviour retardation, with a delay period of approximately 4 years.

Because of these possible effects, dosimetry during pregnancy differs from the usual protocol. Special attention is paid to both external and internal irradiation.

It is not possible to accurately measure the dose to the foetus and so it must be inferred from the exposure to the mother.

 

  Dose – Effect Relationship


The connection between effects of exposure to radiation and dose (i.e., dose-response relationship) is classified into 2 categories, non-stochastic, and stochastic.

Non-stochastic effects

The non-stochastic effects, also referred to as deterministic or tissues and organs effects, are specific to each exposed individual. They are characterised by:

A certain minimum dose must be exceeded before the particular effect is observed. Because of this minimum dose, the non-stochastic effects are also called Threshold Effects. The threshold may differ from individual to individual

The magnitude of the effect increases with the size of the dose received by the individual

Stochastic effects

Stochastic effects are those that occur by chance. The main stochastic effects are cancer and genetic defects. Stochastic effects can also be caused by many other factors, not only by radiation. Since everybody is exposed to natural radiation, and to other factors, stochastic effects can arise in all of us regardless of the type of work (working with radiation or not).

 

  Radiation Exposure Pathways


Each of the different routes, or pathways, by which people can be exposed to radiation result in exposure to different parts of the body. Health physicists must analyze the potential for and effects of exposure via each of the three basic pathways, inhalation, ingestion, and direct exposure, when calculating exposures or estimating the effects of exposures.

  • Inhalation
  • Ingestion
  • Direct Exposure

Inhalation

Exposure by the inhalation pathway occurs when people breathe radioactive materials into the lungs. The chief concerns are radioactively contaminated dust, smoke, or gaseous radionuclides such as radon

What happens to inhaled radioactive materials?

Radioactive particles can lodge in the lungs and remain for a long time. As long as it remains and continues to decay, the exposure continues. For radionuclides that decay slowly, the exposure continues over a very long time.

Inhalation is of most concern for radionuclides that are alpha or beta particle emitters.

Alpha and beta particles can transfer large amounts of energy to surrounding tissue, damaging DNA or other cellular material. This damage can eventually lead to cancer or other diseases and mutations.

Ingestion

Exposure by the ingestion pathway occurs when someone swallows radioactive materials. Alpha and beta emitting radionuclides are of most concern for ingested radioactive materials. They release large amounts of energy directly to tissue, causing DNA and other cell damage.

What happens to ingested radioactive materials?

Ingested radionuclides can expose the entire digestive system. Some radionuclides can also be absorbed and expose the kidneys and other organs, as well as the bones. Radionuclides that are eliminated by the body fairly quickly are of limited concern. These radionuclides have a short biological half-life.

Direct (External) Exposure

The third pathway of concern is direct or external exposure from radioactive material. The concern about exposure to different kinds of radiation varies:

Limited concern about alpha particles. They cannot penetrate the outer layer of skin, but if you have any open wounds you may be at risk.

Greater concern about beta particles. They can burn the skin in some cases, or damage eyes.

Greatest concern is about gamma radiation. Different radionuclides emit gamma rays of different strength, but gamma rays can travel long distances and penetrate entirely through the body.

Gamma rays can be slowed by dense material (shielding), such as lead, and can be stopped if the material is thick enough. Examples of shielding are containers; protective clothing, such as a lead apron; and soil covering buried radioactive materials.

Effects of Radiation Type and Exposure Pathway

Both the type of radiation to which the person is exposed and the pathway by which they are exposed influence health effects. Different types of radiation vary in their ability to damage different kinds of tissue. Radiation and radiation emitters (radionuclides) can expose the whole body (direct exposure) or expose tissues inside the body when inhaled or ingested.

All kinds of ionizing radiation can cause cancer and other health effects. The main difference in the ability of alpha and beta particles and gamma and x-rays to cause health effects is the amount of energy they can deposit in a given space. Their energy determines how far they can penetrate into tissue. It also determines how much energy they are able to transmit directly or indirectly to tissues and the resulting damage.

Although an alpha particle and a gamma ray may have the same amount of energy, inside the body the alpha particle will deposit all of its energy in a very small volume of tissue. The gamma radiation will spread energy over a much larger volume. This occurs because alpha particles have a mass that carries the energy, while gamma rays do not.

  Risk Assessment Process


  Steps of Risk Assessment


Risk Assessment find the answers for all questions like:

Who/What/Where is at risk?

Individual, General population, Lifestages such as children, teenagers, pregnant/nursing women

Population subgroups - highly susceptible (for example, due to asthma, genetics, etc.) and/or highly exposed (for example, based on geographic area, gender, racial or ethnic group, or economic status)

What is the environmental hazard of concern?

Chemicals (single or multiple/cumulative risk), Radiation, Physical (dust, heat), Microbiological or biological

Nutritional (for example, diet, fitness, or metabolic state), Socio-Economic ( for example, access to health care)

Where do these environmental hazards come from?

Point Sources (for example, smoke or water discharge from a factory; contamination from a Superfund site)

Non-Point Sources (for example, automobile exhaust; agricultural runoff)

Natural sources

How does exposure occur?

Pathways (recognizing that one or more may be involved)

Air, Surface Water, Groundwater, Soil, Solid Waste, Food, Non-food consumer products, pharmaceuticals
Routes (and related human activities that lead to exposure)

Ingestion (both food and water), Contact with skin, Inhalation, Non-dietary ingestion (for example, "hand-to-mouth" behavior)

  Dose Response Relationship


Typically, as the dose increases, the measured response also increases.

At low doses there may be no response.

At some level of dose the responses begin to occur in a small fraction of the study population or at a low probability rate.

Both the dose at which response begin to appear and the rate at which it increases given increasing dose can be variable between different pollutants, individuals, exposure routes, etc.

Dose-response assessment is a two-step process

The first step is an assessment of all data that are available or can be gathered through experiments, in order to document the dose-response relationship(s) over the range of observed doses (i.e, the doses that are reported in the data collected). However, frequently this range of observation may not include sufficient data to identify a dose where the adverse effect is not observed (i.e., the dose that is low enough to prevent the effect) in the human population

The second step consists of extrapolation to estimate the risk (probably of adverse effect) beyond the lower range of available observed data in order to make inferences about the critical region where the dose level begins to cause the adverse effect in the human population

Non-linear dose-response assessment

Non-linear dose response assessment has its origins in the threshold hypothesis, which holds that a range of exposures from zero to some finite value can be tolerated by the organism with essentially no chance of expression of the toxic effect, and the threshold of toxicity is where the effects (or their precursors) begin to occur. It is often prudent to focus on the most sensitive members of the population; therefore, regulatory efforts are generally made to keep exposures below the population threshold, which is defined as the lowest of the thresholds of the individuals within a population. 

Linear dose-response assessment

Much background data is not available

In this type of assessment, there is theoretically no level of exposure for such a chemical that does not pose a small, but finite, probability of generating a carcinogenic response. 

The extrapolation phase use a straight line. The slope of this straight line, called the slope factor or cancer slope factor, is use to estimate risk at exposure levels that fall along the line

Cancer Risk = Exposure x Slope Factor

Total cancer risk is calculated by adding the individual cancer risks for each pollutant in each pathway of concern (i.e., inhalation, ingestion, and dermal absorption), then summing the risk for all pathways.

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