What is Radiation?

Radiation is energy in the form of moving waves or streams of particles. Ionizing radiation has energy capable of removing electrons as it passes through matter (such as air, water or living tissue), which can lead to change of chemical composition of the material. Examples include particles (alpha particles, beta and neutrons) and electromagnetic waves (gamma rays and X-ray).

Non-Ionizing radiation is radiation of lower energy than ionizing radiation; it does not possess enough energy to electrons from matter. However, it can excite molecules and atoms causing them to vibrate faster. Examples include, visible light, ultraviolet light, infrared and radio waves.

  Radiation in the Electromagnetic Spectrum (Source: EPA website)

Sources of Ionizing Radiation

Radiation is part of our universe. We are all exposed to natural background radiation. Radiation is present in soil, rocks, the air we breathe, the water we drink, and even in our own bodies.  We are also exposed to natural ionising radiation that comes from outer space and passes through the atmosphere of the planet, the cosmic radiation. This natural radiation sources make up the bulk of radiation we are exposed to daily.

We are also exposed to man-made radiation from various activities such as: medical uses for diagnosis and treatment (diagnostic x-ray procedures, radiotherapy, nuclear medicine – which uses radioactive material to diagnose and treat cancer), x-ray baggage scanners, the nuclear fuel cycle, fallout from nuclear weapons testing and accidents around the world, as well as commercial products like smoke detectors.

Sources of Non-Ionizing Radiation (NIR)

We use and are exposed to non-ionizing radiation sources every day. This form of radiation does not carry enough energy to ionize atoms or molecules. The sources of NIR include:

  • Ultraviolet (UV) radiation from the sun
  • Radiofrequency (RF) radiation used in broadcast and communications applications
  • Power transmission lines
  • Microwaves used in the home kitchen
  • Infrared radiation used in heat lamps
  • Ultraviolet (UV) from tanning beds
  • Ultraviolet from sterilization devices

What are the health effects of Ionizing Radiation?

Our body cells comprise of DNA molecules. DNA consists of two long chains of nucleotides twisted together into a double helix; it is the molecular compound in the nucleus of a cell that forms the blueprint for the structure and function of the cell.

The primary way ionizing radiation affects our health is through breakage of DNA molecules. When one is exposed to ionizing radiation, three things can happen:

  1. The DNA is repaired properly, and the cell continues to function normally.
  1. Deterministic effect

The DNA damage is severe causing the cell to die. When exposed to very high dose of radiation within a short period of time, the cell may be damaged beyond repair or die. If this happens to large number of cells in a body organ or tissue, it can impair functions of vital organs. Health effects includes nausea, skin and deep tissue burns, and impairment of the body’s ability to fight infection may result within hours, days or weeks. This effect is called deterministic effect. Most deterministic effects occur when one is exposed above radiation dose thresholds specific to each exposed tissue. By limiting doses below thresholds, deterministic effects will be prevented.

3.     Stochastic (probabilistic) effects

Stochastic effects are those that occur by chance and consist primarily of cancer and genetic/mutation effectsStochastic effects often show up years after exposure. There are no types of cancers that are formed only as a result of radiation. The timing of the effects or their severity does not depend on the dose. Some types of cancers, however, show a bigger rate increase for a given radiation dose than others. Cancer risks are also known to vary with age at exposure and attained age, with risks being higher for those exposed as children.

What is Radiation Dose?

The term dose is a common in medical applications, e.g. we talk of dose of medicine. However, in radiation protection, the term dose means different from dose in medical applications. When ionizing radiation interacts with an object or body, it deposits energy.  In radiation protection, dose refers to is the energy to that object or body e.g. body tissue. The radiation dose can be described in three ways:

  1. Absorbed dose

it is the energy of ionizing radiation absorbed per unit mass of any material. The absorbed dose is measured in a unit called the gray (Gy). A dose of one gray is equivalent to a unit of energy (joule) deposited in a kilogram of a substance

  1. Equivalent dose

Absorbed dose tells us the energy deposit in a small volume of tissue. It does not give full information on biological effects of different types of radiation. One type of radiation can cause more observed biological effects, given the same amount of absorbed dose, than others. For example, 1 Gy of neutron radiation is more harmful to tissue than 1 Gy of gamma radiation.

A factor called radiation weighting factor (WR) is used to equate different types of radiation with different biological effects. A weighted absorbed dose is called Equivalent Dose. The Equivalent dose is expressed in a unit called Sievert (Sv). This means 1 Sv of neutron radiation will have the same biological effect as 1 Sv of gamma radiation

  1. Effective dose

Different body organs and tissue have different radiation sensitivities, and different radioactive materials inside the body tend to concentrate in different organs, giving a different pattern of equivalent dose. Some organs or tissues are more sensitive to ionizing radiation than others. Effective dose of a tissue is obtained by multiplying its tissue weighting factor (WT) with the Equivalent dose. The unit is Sievert (Sv)

The concept of effective dose is used in cases where radiation exposure to different body organs or tissues is not inform. For example, ingestion of radionuclides. In this case, Effective dose will be the sum of the effective doses of all exposed organs or tissues.

Dose Limits in Kenya

The Kenya Nuclear Regulatory Authority adopts the recommendations of the International Commission on Radiological Protection  and radiation safety standards and guides of  International Atomic Energy Agency in setting the limits of amount of radiation the members of public and occupationally exposed workers may receive may receive.

In Kenya, the effective dose limit for the member of public is 1 mSv in one calendar year. The effective dose limit for occupationally exposed worker is set at 20 mSv in any one year and 100 mSv in five consecutive years.

The dose limit for female radiation worker who has notified pregnancy or is breast-feeding, the embryo or foetus or the breastfed infant is afforded the same broad level of protection as is required for members of the public. An effective dose of 1 mSv in a year.

CATEGORY OF WORKERS WHOLE BODY (mSv) SKIN (mSv) LENS (mSv) EXTREMITY (mSv)
Worker over the age of 18 years 20 500 20 500
Trainees and students of age 16 to 18 who use sources in the course of their studies 6 150 20 150

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Radiation Protection Principles

The aim of radiation protection is to prevent reliably the deterministic effects of radiation and to reduce the risk of stochastic effects to a reasonably achievable level.

In order to achieve radiation protection, there are three principles recommended by the International Commission on Radiological Protection (ICRP) and have been incorporated into our legislation:

  • Justification – The expected benefits from an activity associated with radiation should outweigh the harm resulting from the activity.
  • Optimization – Even if an activity that is connected with radiation exposure is justified, optimization ensures exposure to ionizing radiation should be as low as reasonably achievable (ALARA), taking into account economic and societal factors.

This means that unnecessary exposure to radiation must be avoided and unavoidable exposure must be kept as low as possible.

  • Dose limitation – The levels of radiation dose to individuals in planned exposure situations (in normal circumstances) that must not be exceeded.