Radiation exposure has many harmful effects on the body, the severity of which depends on the level of exposure.  Any case of radiation exposure to a living cell will have one of three outcomes:

1. The cell dies as a result of damage incurred.  Depending on the organ, the cell may be repaired and have no lasting negative effect.

2. The cell is damaged but is able to repair itself without incurring lingering damage.

3. The cell is damaged but is unable to repair itself correctly with harmful consequences arising from the poor repair job.

In the end though, the exact method by which radiation causes cancer and other health effects is not fully known.  What is known is that ionising radiation is able to affect change on molecules within a target cell, including DNA.  In most cells the result of damaged DNA would be minimal- with any changes limited to the cell and any cells replicated from it (although in the case of this change resulting in cancer the damage is far from minimal).  It should be noted that mutations with extremely negative effects are rare compared to those with benign effects.  However, if a person's gonads are exposed to radiation the hereditary material contained within them may be irreparably damaged.  Studies of atomic bomb survivors have shown that this risk is minimal.  Studies based on extensive examinations of survivors of the bombs dropped on Hiroshima and Nagasaki published in the early nineties found that there were no detectable effects on children born to survivors.  In addition to damaging DNA, ionising radiation can produce free radicals (oxidative species formed when a molecule is ionised, losing an electron- forcing it to "steal" an electron from another molecule).  Free radicals can disrupt the cellular structure and machinery, with the net result ranging from cell death to temporary cellular injury, with the effects possibly extended to other nearby cells.  The effects of radiation on quickly dividing cells are most pronounced as the damage to the cell's structure interferes with its ability to divide.  This fact makes radiation so effective at killing cancer cells while potentially leaving surrounding normal cells intact.

Levels of Radiation Exposure:

There are two extremes of radiation exposure- chronic and acute. 

Chronic Radiation Exposure

Ionising radiation exposure over an extended period of time is termed chronic exposure.  Everyone experiences chronic radiation exposure in the form of background radiation, which is considered a low level exposure.  The results of low level exposure to ionising radiation are the most difficult to model and predict.  This stems from the fact that the body is adapted to coping with a regular background radiation level, and as such the result of a small increase in exposure is difficult to detect.  The body's capability to deal with a small increase in background radiation levels is apparent when one examines the variation in background radiation levels across the globe (noting that background radiation levels can vary by up to two orders of magnitude). 

Occupational exposure remains an extremely variable source of chronic radiation.  Although occupational exposure levels are regulated, limiting whole body exposure to around 50 mSv for a normal adult worker.  In working environments involving radiation exposure the standard safety model is to keep exposure "as low as reasonably achievable".  This is achieved via five main routes:

1. Time:  A person's dose of radiation is directly related to the amount of time they spend in the presence of the source.  Thus limiting the amount of time one spends near a source limits exposure.

2. Distance:   Radiation intensity follows what is called an "inverse square law", that is to say- the intensity is inversely proportional to the square of the distance between the source and the person.  So standing four feet from a source reduces a person's dose to a quarter of that received by someone two feet from the same source. 

3. Shielding:  Placing a barrier between a person and a source can greatly reduce the level of radiation that reaches them.

4. Containment:  Radioactive sources are typically kept in a compact container and stored away from the working environment.

5. Cleanliness:  Workers are expected to wash their hands and wear protective clothing to prevent carrying radioactive materials into other areas on site as well as prevent secondary exposure.

Non-cancerous symptoms of long term radiation exposure include fibrosis, hair loss and dryness.  Fibrosis is a condition caused by scarring and characterised by a loss of elasticity in tissues exposed to ionising radiation.  This condition is typically seen in radiotherapy patients exposed to radiation in the same area of the body over an extended time period.  Hair loss typically occurs in area exposed to higher doses of radiation and is likely to be permanent.  In cases where tear, salivary glands or sweat glands are affected by radiation the glands may lose some or all function- a problem particularly prominent for radiation therapy patients where treatment involves the head or neck areas.

Acute Radiation Exposure

An exposure to ionising radiation over a brief time period is referred to as an acute exposure.  Of greatest concern are situations where a dosage is greater then 500 mSv (more conservative estimates may put this far lower).  The symptoms of an acute exposure of ionising radiation are called radiation sickness.  In radiation therapy the acute side effects include skin damage (general dryness and sometimes breakdown), swelling, infertility and general fatigue.  The symptoms associated with increasing dose are displayed in the table below (reprinted from a wikipedia entry) .

0.05–0.2 Sv

No symptoms.

0.2–0.5 Sv

No noticeable symptoms. Red blood cell count decreases temporarily.

0.5–1 Sv

Mild radiation sickness with headache and increased risk of infection due to disruption of immunity cells. Temporary male sterility is possible.

1–2 Sv

Light radiation poisoning, 10% fatality after 30 days. Typical symptoms include mild to moderate nausea (50% probability at 2 Sv), with occasional vomiting, beginning 3 to 6 hours after irradiation and lasting for up to one day. This is followed by a 10 to 14 day latent phase, after which light symptoms like general illness, anorexia and fatigue appear (50% probability at 2 Sv). The immune system is depressed, with convalescence extended and increased risk of infection. Temporary male sterility is common. Spontaneous abortion or stillbirth will occur.

2–3 Sv

Severe radiation poisoning, 35% fatality after 30 days. Nausea is common (100% at 3 Sv), with 50% risk of vomiting at 2.8 Sv. Symptoms onset at 1 to 6 hours after irradiation and last for 1 to 2 days. After that, there is a 7 to 14 day latent phase, after which the following symptoms appear: loss of hair all over the body (50% probability at 3 Sv), fatigue and general illness. There is a massive loss of white blood cells, greatly increasing the risk of infection. Permanent female sterility is possible. Convalescence takes one to several months.

3–4 Sv

Severe radiation poisoning, 50% fatality after 30 days Other symptoms are similar to the 2–3 Sv dose, with uncontrollable bleeding in the mouth, under the skin and in the kidneys (50% probability at 4 Sv) after the latent phase.

4–6 Sv 

Acute radiation poisoning, 60% fatality after 30 days.  Fatality increases from 60% at 4.5 Sv to 90% at 6 Sv (unless there is intense medical care). Symptoms start half an hour to two hours after irradiation and last for up to 2 days. After that, there is a 7 to 14 day latent phase, after which generally the same symptoms appear as with 3-4 Sv irradiation, with increased intensity. Female sterility is common at this point. Convalescence takes several months to a year. The primary causes of death (in general 2 to 12 weeks after irradiation) are infections and internal bleeding.

6–10 Sv

Acute radiation poisoning, 100% fatality after 14 days. Survival depends on intense medical care. Bone marrow is nearly or completely destroyed, so a bone marrow transplantation is required. Gastric and intestinal tissue are severely damaged. Symptoms start 15 to 30 minutes after irradiation and last for up to 2 days. Subsequently, there is a 5 to 10 day latent phase, after which the person dies of infection or internal bleeding. Recovery would take several years and probably would never be complete.

10–50 Sv 

Acute radiation poisoning, 100% fatality after 7 days.  An exposure this high leads to spontaneous symptoms after 5 to 30 minutes. After powerful fatigue and immediate nausea caused by direct activation of chemical receptors in the brain by the irradiation, there is a period of several days of comparative well-being, called the latent (or "walking ghost") phase. After that, cell death in the gastric and intestinal tissue, causing massive diarrhoea, intestinal bleeding and loss of water, leads to water-electrolyte imbalance. Death sets in with delirium and coma due to breakdown of circulation. Death is inevitable; the only treatment that can be offered is pain therapy.

50–80 Sv

Immediate disorientation and coma in seconds or minutes. Death occurs after a few hours by total collapse of nervous system.

More than 80 Sv

U.S. military forces expect immediate death. A worker receiving 100 Sv in an accident at Wood River, Rhode Island, USA on 24 July 1964 survived for 49 hours after exposure, and an operator receiving 120 Sv to his upper body in an accident at Los Alamos, New Mexico, USA on 30 December 1958 survived for 36 hours.