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Health Effects from Radiation

published  First Published: 16/03/2011
Article written by: Nigel Brookson

What are the health effects from nuclear radiation?

Before looking at the effects of being exposed to nuclear radiation, we need to know about the different types of radiation, and how radiation is measured.

 

Types of radiation

Nuclear radiation arises from hundreds of different kinds of unstable atoms. While many exist in nature, the majority are created in nuclear reactions. Ionizing radiation which can damage living tissue is emitted as the unstable atoms (radio nuclides) change ('decay') to become different kinds of atoms.

 

The fundamental kinds of ionizing radiation are:

  • Alpha particles
    Alpha particles are helium nuclei consisting of two protons and two neutrons and are emitted from naturally-occurring heavy elements such as uranium and radium. Although alpha particles are intensely ionizing, they cannot penetrate human skin, so are dangerous to us only if emitted inside the body.
  • Beta particles
    Beta particles are fast-moving electrons emitted by many radioactive elements. Beta particles are more penetrating than alpha particles, but can be easily shielded; for example they can be stopped by a few millimetres of wood or aluminium. Beta particles can penetrate a little way into human tissue but are generally less dangerous to people than gamma radiation. Exposure to beta particles produces an effect like sunburn, but which is slower to heal. Beta-radioactive substances are also safe if kept in appropriate sealed containers.
  • Gamma rays
    Gamma rays are high-energy beams simi liar to X-rays. Gamma rays are emitted in many radioactive decays and are very penetrating, so require more substantial shielding. Gamma rays are the main hazard to people dealing with sealed radioactive materials used, for example, in industrial gauges and radiotherapy machines. Radiation dose badges are worn by workers in potentially exposed situations to detect them and hence monitor exposure. We all receive about 0.5-1 mSv per year of gamma radiation from cosmic rays and from rocks, and in some places, much more. Gamma activity in a substance (e.g. rock) can be measured with a Geiger counter.

 

Units of radiation and radioactivity

In order to quantify how much radiation we are exposed to in our daily lives and assess potential health impacts as a result, it is necessary to establish a unit of measurement.

The basic unit of radiation dose absorbed in tissue is the gray (Gy), where one gray represents the deposition of one joule of energy per kilogram of tissue.
However, since neutrons and alpha particles cause more damage per gray than gamma or beta radiation, another unit, the sievert (Sv) is used in setting radiological protection standards.This unit of measurement takes into account biological effects of different types of radiation.

One gray of beta or gamma radiation has one sievert of biological effect, one gray of alpha particles has 20 Sv effect and one gray of neutrons is equivalent to around 10 Sv (depending on their energy).
Since the sievert is a relatively large value, dose to humans is normally measured in millisieverts (mSv), one-thousandth of a sievert.

The becquerel (Bq) is a unit or measure of actual radioactivity in material (as distinct from the radiation it emits, or the human dose from that), with reference to the number of nuclear disintegrations per second (1 Bq = 1 disintegration/sec). Quantities of radioactive material are commonly estimated by measuring the amount of intrinsic radioactivity in becquerels one Bq of radioactive material is that amount which has an average of one disintegration per second, i.e. an activity of 1 Bq.
One curie was originally the activity of one gram of radium-226, and represents 3.7 x 1010 disintegrations per second (Bq).

The Working Level Month (WLM) has been used as a measure of dose for exposure to radon and in particular, radon decay products.

 

Natural background radiation

Naturally occurring background radiation is the main source of exposure for most people, and provides some perspective on radiation exposure from nuclear energy.

The average dose received by all of us from background radiation is around 2.4 mSv/yr, which can vary depending on the geology and altitude where people live; ranging between 1 and 10 mSv/yr, but can be more than 50 mSv/yr.

The highest known level of background radiation affecting a substantial population is in Kerala and Madras states in India where some 140,000 people receive doses which average over 15 millisievert per year from gamma radiation, in addition to a similar dose from radon. Comparable levels occur in Brazil and Sudan, with average exposures up to about 40 mSv/yr to many people.

In Australia we experience one of the lowest levels of background radiation of around 1.4 mSv/yr.

Several places are known in Iran, India and Europe where natural background radiation gives an annual dose of more than 50 mSv and up to 260 mSv (at Ramsar in Iran). Lifetime doses from natural radiation range up to several thousand millisievert. However, there is no evidence of increased cancers or other health problems arising from these high natural levels.

Radon gas has decay products that are alpha emitters. People everywhere are typically exposed to around 0.2 mSv/yr, and often up to 3 mSv/yr, from inhaled radon without apparent ill-effect. However, in industrial situations its control is a high priority.

 

Public exposure to natural radiatione

Source of exposure Annual effective dose (mSv) and Average Typical range:

  • Cosmic radiation Directly ionizing and photon component 0.28
  • Neutron component 0.10
  • Cosmogenic radio nuclides 0.01
  • Total cosmic and cosmogenic 0.39 0.31.0e
  • External terrestrial radiation Outdoors 0.07
  • Indoors 0.41
  • Total external terrestrial radiation 0.48 0.3-1.0e
  • Inhalation Uranium and thorium series 0.006
  • Radon (Rn-222) 1.15
  • Thoron (Rn-220) 0.1
  • Total inhalation exposure 1.26 0.2-10e
  • Ingestion K-40 0.17
  • Uranium and thorium series 0.12
  • Total ingestion exposure 0.29 0.2-1.0e
  • Total 2.4 1.0-13

 

Radiation Effects and Doses

Unfortunately, our knowledge of radiation effects comes primarily from people who have received high doses. The risk associated with large radiation doses is relatively well established, however, the risks associated with doses under about 200 mSv are less obvious because of the large underlying incidence of cancer caused by other factors.

Radiation protection standards assume that any dose of radiation, no matter how small, involves a possible risk to human health. However, available scientific evidence does not indicate any cancer risk or immediate effects at doses below 100 mSv a year. At low levels of exposure, the body's natural repair mechanisms seem to be adequate to repair radiation damage to cells soon after it occurs.

 

Some comparative radiation doses and their effects

  • 2 mSv/yr Typical background radiation experienced by everyone (average 1.5 mSv in Australia, 3 mSv in North America).
  • 1.5 to 2.0 mSv/yr Average dose to Australian uranium miners, above background and medical.
  • 2.4 mSv/yr Average dose to US nuclear industry employees.
  • Up to 5 mSv/yr Typical incremental dose for aircrew in middle latitudes.
  • 9 mSv/yr Exposure by airline crew flying the New York Tokyo polar route.
  • 10 mSv/yr Maximum actual dose to Australian uranium miners.
  • 20 mSv/yr Current limit (averaged) for nuclear industry employees and uranium miners.
  • 50 mSv/yr Former routine limit for nuclear industry employees. It is also the dose rate which arises from natural background levels in several places in Iran, India and Europe.
  • 100 mSv/yr Lowest level at which any increase in cancer is clearly evident. Above this, the probability of cancer occurrence (rather than the severity) increases with dose.
  • 350 mSv/lifetime Criterion for relocating people after Chernobyl accident.
  • 1,000 mSv cumulative Would probably cause a fatal cancer many years later in 5 of every 100 persons exposed to it (i.e. if the normal incidence of fatal cancer were 25%, this dose would increase it to 30%).
  • 1,000 mSv single dose Causes (temporary) radiation sickness such as nausea and decreased white blood cell count, but not death. Above this, severity of illness increases with dose.
  • 5,000 mSv single dose Would kill about half those receiving it within a month.
  • 10,000 mSv single dose Fatal within a few weeks.

 

Limiting exposure

Public dose limits for exposure from uranium mining or nuclear plants are usually set at 1 mSv/yr above background.
In most countries the current maximum permissible dose to radiation workers is 20 mSv per year averaged over five years, with a maximum of 50 mSv in any one year. This is over and above background exposure, and excludes medical exposure. The value originates from the International Commission on Radiological Protection (ICRP), and is coupled with the requirement to keep exposure as low as reasonably achievable (ALARA) taking into account social and economic factors.

Radiation protection at uranium mining operations and in the rest of the nuclear fuel cycle is tightly regulated, and levels of exposure are monitored.

There are four ways in which people are protected from identified radiation sources:

  1. Limiting time. In occupational situations, dose is reduced by limiting exposure time.
  2. Distance. The intensity of radiation decreases with distance from its source.
  3. Shielding. Barriers of lead, concrete or water give good protection from high levels of penetrating radiation such as gamma rays. Intensely radioactive materials are therefore often stored or handled under water, or by remote control in rooms constructed of thick concrete or lined with lead.
  4. Containment. Highly radioactive materials are confined and kept out of the workplace and environment. Nuclear reactors operate within closed systems with multiple barriers which keep the radioactive materials contained.

 

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