Nuclear Radiation and its Impacts
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
Why are there different ways to measure a dose of radiation?
When you think of a dose of medication, you think of an absolute measurement of the quantity you take. But radiation isn't measured by the quantity you take.
Radiation from medical examinations is similar to sunlight. The effect of sunlight on the skin depends on the light's intensity and how long a person stays in it.
Sunlight Effect Factors:
- Length of exposure
Radiation dose will tell us about an effect the radiation has on tissue. Radiation dose can be measured a number of ways
Types of Doses
Absorbed dose is used to assess the potential for biochemical changes in specific tissues
Equivalent dose is used to assess how much biological damage is expected from the absorbed dose. (Different types of radiation have different damaging properties)
Effective dose is used to assess the potential for long-term effects that might occur in the future
Types of Doses
Absorbed dose is the concentration of energy deposited in tissue as a result of an exposure to ionizing radiation, it means the energy absorbed by human tissue. X-rays, unlike sunlight, can penetrate deep into the body and deposit energy in internal organs. X-rays can even pass through a person's body.The unit of measurement for absorbed dose is the milligray (mGy).
If you have a CT of your upper abdomen, the absorbed dose to your chest is very low, because it has only been exposed to a small amount of scattered radiation. The absorbed dose to your stomach, pancreas, liver and other organs is greatest, because they have been directly exposed
Radiation Flux is the ratio of number of particles penetrating through a cross-sectional area
Absorbed radiation dose is the energy of ionising radiation absorbed by a unit mass of substance being irradiated. Energy absorbed by Kg of material
Unit is rad
1 Rad = 6.24*1013 eV/cm3
Equivalent dose is an amount that takes the damaging properties of different types of radiation into account. (Not all radiation is alike.)
Absorbed vs. equivalent dose
Absorbed dose tells us the energy deposit in a small volume of tissue.
Equivalent dose addresses the impact that the type of radiation has on that tissue.
Because all radiation used in diagnostic medicine has the same low-harm potential, the absorbed dose and the equivalent dose are numerically the same. Only the units are different.
For diagnostic radiation: The equivalent dose in milliSievert (mSv) = the absorbed dose in mGy
Effective dose is a calculated value, measured in mSv, that takes three factors into account:
- the absorbed dose to all organs of the body,
- the relative harm level of the radiation, and
- the sensitivities of each organ to radiation
Effective dose: The quantity of effective dose helps us take into account sensitivity.
Different body parts have different sensitivities to radiation. For example, the head is less sensitive than the chest.
Effective dose relates to the overall long-term risk to a person from a procedure and is useful for comparing risks from different procedures.
Effective dose is not intended to apply to a specific patient.
The actual risk to a patient might be higher or lower, depending on the size of the patient and the type of procedure.
Example of absorbed dose, equivalent dose and effective dose. If you have a CT of the abdomen, what is the dose to the abdomen?
Typical absorbed dose: 20 mGy
Typical equivalent dose: 20 mSv
Typical effective dose: 15 mSv
External Dose and Internal Dose
External Dose When the source of radiation is situated outside of the body, the irradiated person will receive what is referred to as an external dose
For beta radiation, the absorption in air can be very important (depending on the beta energy). For alpha radiation, the external dose is zero since alpha particles cannot penetrate past the layer of dead skin
Film badges or thermoluminescent dosimeters (TLD) are used to measure the external dose
Internally deposited radionuclides will irradiate a person's body from the inside. Typical routes for internal irradiation are inhalation or ingestion of radionuclides and are referred to as intake.
Radioactive materials are not completely retained in the body. The amount that is retained after an intake is called the uptake. Like Sr-90 inside the bones or I-131 and I-125 inside the thyroid, majority of an intake gets eliminated.
To most accurately estimate the internal dose, the percent of uptake, biological and natural half-life of the radionuclides, as well as the type of radiation, radiation energy, and preferred organ must be taken into consideration
After an intake radioisotopes present in the body can irradiate the internal organs for many years to come. The time that each radioisotope is present in the body depends on the physical and biological half-life, the organ, the chemical composition, etc. The equivalent dose resulted from the exposure of a certain organ for the next 50 years (or to age 70 years for children) is called committed equivalent dose. Similarly the effects of exposure to the whole body for the next 50 years is called committed effective dose. In practical application for radioisotopes with short half-life the period may be shorter.
Annual Limit of Intake (ALI)
Starting from the permitted dose, an annual limit of intake (ALI) can be calculated for each radionuclide. Defined as the activity of a radionuclide that will deliver a committed effective dose of 20 mSv. The ALI values vary with radionuclide, chemical composition, route of entry in the body, etc.
Natural and Artificial Dose
Due to the presence of numerous sources of ionizing radiation in the natural environment (i.e. around us and inside us), the dose received from these types of sources has been named the natural dose, or background. The natural dose varies around the world by more than a factor of ten. This variation arises from differences in soil composition, type of materials used for building, food and water, to the altitude (the higher the altitude, the larger the irradiation from cosmic sources) etc. Main contribution (over 50%) comes from inhalation of radioactive Radon gas products found in the air, than K-40 inside our bodies
This is due to artificially produced radiation and is referred to as the artificial dose. The main artificial dose comes from X-ray machines used in medical diagnoses.
Absorbed Dose, Equivalent dose etc can be described as a function of time. When doing so, the terms are called Absorbed Dose rate, Equivalent Dose rate, etc. The units of measure remain the same but, the values are divided by the time interval. For example we can have absorbed dose rate received in a minute is measured in mGy/min, effective dose received in year in mSv/year, etc.
Absorbed dose and equivalent dose measurements can be used to assess short-term risk to tissues. (Short term is weeks to months.)
For properly performed diagnostic examinations, there will be no short-term effects from the radiation exposure, so absorbed dose and equivalent dose are not very useful.
For patients, the most important dose quantity is effective dose, because it allows for simple comparisons of long-term risks.
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).
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.
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 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.
- Direct Exposure
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.
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.
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.
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)
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.
Terrestrial and Cosmic Radiation
Radioactivity in nature comes from two main sources, terrestrial and cosmic
Terrestrial radioisotopes are found on the earth that came into existence with the creation of the planet. Although some are long gone, some radioisotopes take a long time to decay and become non-radioactive (on the order of hundreds of millions of years) and are still around today
Radioactive elements found in rock, soil, water, air, and in food from the earth make there way in our bodies when we drink water, breath air or eat foods which contain them. These naturally occurring radioisotopes such as carbon-14, potassium-40, thorium-223, uranium-238, polonium-218, and tritium(hydrogen-3) expose us to radiation from within our bodies
By far, the largest contributor to our daily exposure of radiation is the natural world, and the major form of natural radiation is radon gas. Radon-222 is a naturally occuring decay product of uranium-238 which is commonly found in soils and rocks. Radon-222 is a gas which is odorless, colorless, tasteless and chemically nonreactive. As it escapes from the soils and rocks of which it is trapped, it enters the water we drink and the air we breath
Since distribution of uranium in the earth's crust varies from place to place, so does the prevalence of radon gas. In areas where surface rocks contain a high concentration of uranium, radon gas could enter a home through a crack in the foundation.
Cosmic - Another source of natural radiation comes from the interaction of cosmic rays with the earth's upper atmophere. Cosmic rays permeate all of space and are composed of highly energized, positively charged particles as well as high energy photons. Approaching the earth at near the speed of light, most cosmic rays are blocked by the earth's protective atmosphere and magnetic field. As a byproduct of the interaction between cosmic rays (i.e. particles) and the atmosphere, many radioactive isotopes are formed such as carbon-14
The higher you are in altitude, the more you are exposed to cosmic radiation. In fact, the average amount of exposure to cosmic radiation that a person gets in the Unites States roughly doubles for every 6,000 foot increase in elevation
Third source of radiations are the nuclear radiation from human activities; Nuclear Power Plants