March 2011

Elements that contain unstable nuclei are radioactive; they are called radionuclides. They decay by releasing mostly alpha and beta particles accompanied by gamma rays. An alpha particle has low-energy, is positively charged and consists of two protons and two neutrons, i.e., a helium atom without its 2 electrons; it can be stopped by tissue paper or human skin.  A beta particle is a high-energy, negatively charged electron (negatron) or a positively charged positron; it can be stopped by a sheet of aluminum. Gamma rays are high energy, short-wavelength, electromagnetic radiation; they can be stopped by concrete or lead.

The energy released by radionuclides may knock electrons out of their orbits around an atom’s nucleus. This process is called ionizing radiation. Ionizing radiation damages living tissues, leads to changes in constituents of the cell, including the DNA of chromosomes, and results in changes in structure and function of the cells and organ systems. Understanding the potential for ionizing radiation to effect changes to living tissues requires knowing how much radioactive energy is absorbed by the tissues. 


The extent and time period of exposure to a dose is important to determine the likely biological damage to a human body. A healthy adult body has a given capacity to repair damage from radiation. Thus a full body exposure to a big dose over a short time is generally more harmful because the body cannot keep up with repairs, than a full body exposure to a small dose over a long time which the body usually can repair as it occurs. 

Ingesting and inhaling radioactive particulate, such as radioactive dust blown by the wind from a nuclear plant fire or atomic bomb test, is harmful. As the radioactive particulate enters the cell, it damages the DNA which affects the expression of chromosomes which, in some cases, does not show up for decades as a tumor or cancer, making it difficult to establish cause and effect.

Exposure to other DNA-altering contaminants in the environment, such as urban pollution, lead from paints and gasoline, radon in stone buildings, herbicides, pesticides, industrial waste and agricultural run-off, and lifestyle exposures, such as from smoking, drugs, alcohol and pollutants in the workplace further complicates any cause and effect analyses.

If a significant quantity of radioactive particulate stays in parts of the body, such as radioactive iodine in the thyroid or radioactive polonium (in cigarette smoke) in the lungs, it may cause DNA damage that leads to:

- tumors (thyroid, ovaries, breasts, prostate, lungs, etc.) that may become cancerous 
- leukemia, i.e., cancer of the blood and bone marrow
- birth defects 
- neurological defects that may hinder future mental development resulting in lower IQs 

As a significant part of the US soil, water, fauna (includes people) and flora was exposed to radioactive isotopes from atomic testing in Nevada, mostly during the 40s, 50s and 60s, adverse public health impacts, some lasting more than one generation, have occurred. 


Multiple-layer, aluminized suits with face masks and leaded-glass goggles provide skin protection against alpha and beta particles. The addition of breathing filters provide protection against ingesting airborne, gamma-ray-emitting radionuclides; inhaling alpha and beta particles emitted by radionuclides and the associated gamma rays may cause cancer. Contaminated air filters need to be frequently replaced.

There is no such portable protection against the gamma rays emitted from radio nuclides-contaminated surfaces and spilt liquids. 

Advising people to stay indoors is of marginal value, because houses have air leakage of about 1 to 4 air changes per hour (ACH), depending on outdoor wind and temperature conditions; a minimum of 0.5 ACH is needed for health requirements.


There are three measurement units for radioactivity: the becquerel (Bq) measures radioactivity, the gray (Gy) measures the absorbed dose and the Sievert (Sv) measures the biological effects of the absorbed dose.

The Bq measures the activity of the radioactive source, meaning the number of atoms which, within a particular time frame, transform and emit radiation.
1 Bq = 1 disintegration per second (dps); disintegration energy levels vary by isotope.
The Bq is a very small unit; multiples are often used:
1 MBq = 1 mega becquerel = 1,000,000 Bq
1 GBq = 1 giga becquerel = 1,000,000,000 Bq
1 TBq = 1 tera becquerel = 1,000,000,000,000 Bq
The radioactivity of an environment, a material or a foodstuff is given in Bq per kilogram or per liter.

The Gy measures the absorbed dose, meaning the energy transferred by one or more isotopes to the material by ionizing radiation upon encountering it.
1 Gy = 1 joule of ionizing energy per kilogram; Sub-multiples are often used:
1 mGy = 1 milligray = 0.001 Gy
1 uGy = 1 microgray = 0.000001 Gy 
1 nGy = 1 nanogray = 0.000000001 Gy

The Sv evaluates the effects of ionizing radiation on living material. At equal Gy doses, the effects of radioactivity on living tissue depends on the type and energy level of the radiation (alpha, beta, gamma, neutron, etc.), on the tissue type and on the length of exposure.
The Sv is a very large unit; sub-multiples are often used:
1 mSv = 1 millisievert = 0.001 Sv
1 uSv = 1 microsievert = 0.000001 Sv


International System of units (SI Units) and corresponding Common Units 

- Bq is the unit of radioactivity that corresponds to the curie (Ci) 
- Gy = 1 joule/kg is the unit of Absorbed Dose that corresponds to the rad 
- Sv = 1 J/kg x We x Wt is the unit of Equivalent Dose that corresponds to the rem
- coulomb/kilogram (C/kg) is the unit of exposure that corresponds to the roentgen (R)

1 Joule = 6,200 billion mega electronvolt (MeV) = 1 watt.second
kBq/sq m = 1,000 Bq of radioactive particulate over an area of 1 sq m 
1 TBq = 27 Curies, or 1 pCi = 0.037 Bq 
1 GBq = 27 mCi 
1 MBq = 27 uCi  
37 GBq = 1 Ci

1 rad = 0.01 Gy
1 rem = 0.01 Sv
1 roentgen (R) = 0.000258 coulomb/kilogram (C/kg)
1 coulomb = 1 amp.second; it is a flow of one amp of electric charge for one second.


There are several models to predict the long-term, biological damage caused by ionizing radiation. Three of them are discussed below. The collection and analyses of data to validate one model over another is an ongoing process. Because the current data is inconclusive, scientists disagree on which method should be used. As noted above, any radiation dose needs to be adjusted with energy and tissue weighing factors and subjective factors, such as for a pregnant woman, to determine cancer risks.

Linear No Threshold Model: The linear no threshold model (LNT) is a method for predicting the long-term, biological damage caused by ionizing radiation and is based on the assumption that the risk is directly proportional to the dose at ALL dose levels, i.e., the sum of several very small doses have the same effect as one larger dose. The LNT model does not hold for low-dose ionizing radiation, because a healthy body is able to repair the damage as it occurs. Only when the damage exceeds the repair capability will permanent damage occur.

Threshold Model: The threshold model assumes very small doses of ionizing radiation have negligible harmful effects below a certain threshold and do have harmful effects above it. The model is widely used in toxicology.

The LNT model predicts higher cancer risks than the threshold model. However, there is little evidence that the LNT model applies in case of cumulative doses totaling less than 100 mSv/yr, i.e., a healthy adult body can repair the damage of these doses as they occur. 

At doses greater than100 mSv/yr, if suddenly applied, a healthy adult body may not be able to cope with the damage; cancer risks may increase as the dose increases.

However, the above may not be the case for the fetus of pregnant women, newborn infants, young children, sickly/weak/old people, etc. 

Radiation Hormosis Model: The radiation hormosis model holds that chronic low doses of ionizing radiation, in addition to background radiation, are beneficial by activating repair mechanisms that protect against disease; they are not activated in absence of the additional ionizing radiation. One way to increase one’s chronic ionizing radiation exposure is to move from a low-lying area to the high mountains. The model predicts the least cancer risks by assuming that radiation is beneficial for very low doses, while still recognizing that it is harmful in large doses.


The external radiation dose, such as from soil, air, water and food, in Bqs, can be measured using appropriate instrumentation. The Bqs measured are much greater than the Gys encountered by a person because of personal protection and distance from the radiation source; the intensity of radiation is reduced by the square of the distance from the source.

Dose conversion factors, DCFs, have been calculated using computer programs by the various government agencies. The DCFs take into account the energy of multiple isotopes, multiple exposure events, isotope residence times, radioactive daughters, tissue types, distance from the source, etc. DCFs (units Sv/Bq) are used to convert the Bqs to Svs. 

Because DCFs exist for all radionuclides, the total Sv dose received from all radionuclides taken into the body during a year or a lifetime can be calculated and compared with public Sv dose limits set by government agencies. The website below has several examples using Bqs and DCFs to calculate Svs for ingestion, inhalation and immersion.


Absorbed Dose: Gy is a unit of ionizing radiation dose absorbed by biological matter, either through the skin, inhaled or ingested. 

To gauge biological effects the Absorbed Dose is multiplied by weighing factor We, which is dependent on the type of ionizing radiation. Such measurement of biological effect is called "Equivalent Dose" and is measured in Sv. 

Equivalent Dose = Gy x energy weighing factor We = Sv

For x-rays, gamma rays, electrons, positrons, muons: We = 1, and 1 Gy x 1 = 1 Sv
For neutrons of different energy levels: We varies from 5 to 20, and 1 Gy varies from 5 to 20 Sv 
For alpha particles, fission fragments, heavy nuclei: We = 20, and 1 Gy x 20 = 20 Sv

Example: The Equivalent Dose of mixed radiation may be 0.3 mGy x (We = 5, slow neutron) + 6 mGy x (We = 1, gamma rays) + 0.1 mGy x (We = 20, fast neutron) = 9.5 mSv

To gauge biological effects the Equivalent Dose is multiplied by weighing factor Wt, which is dependent on the tissue type. Such measurement of biological effect is called "Effective Dose" and is measured in Sv. 

Effective Dose = Gy x We x tissue weighing factor Wt = Sv 

For bone surface, skin: Wt = 0.01
For bladder, breast, liver, esophagus, thyroid: Wt = 0.05
For bone marrow, colon, lung, stomach: Wt = 0.12
For gonads (testes, ovaries): Wt = 0.20

Example: The above calculated Effective Dose for a bladder may be 9.5 mSv x (Wt = 0.05, bladder) = 0.475 mSv 

During an X-ray test, the dense bone tissue absorbs radiation energy causing some instant ionizing damage, such as creating free radicals inside bones, whereas the radiation energy easily passes through the less dense fleshy tissues to the film in old X-ray systems, to the digital sensor in new X-ray systems.

Ingestion and inhalation of radioactive particles cause much greater ionizing damage to body tissues for longer periods of time than high energy electromagnetic waves, such as X-rays. 


Background radiation comes from outer space (cosmic, solar), the earth (radon, potassium, uranium, thorium), food, and even other people. US natural background radiation exposure is an average of 3.6 mSv/yr; Australia 2.4 mSv/yr; Ramsar (Iran) 260 mSv/yr

Manmade average exposure is 2.6 mSv/yr, of which CT scans 55%, other diagnostic & therapeutic 24%, other 21%

US total radiation exposure, background plus manmade, is an average of 3.6 + 2.6 = 6.2 mSv/yr per person, increased from 3.6 mSv/yr about 20 years ago when CT scans were much less common. 

The 6.2 mSv/yr average is misleading, because the majority of people have only x-rays during their lifetime, whereas a small percentage of people have CT scans, cancer treatments with radioactive isotopes, angiograms, stent implants, etc. These people have exposures several times greater than 6.2 mSv/yr during their treatment periods.

Example: On October 1, 2011, radiation at a hospital entrance (people walking in and out) near Fukushima in Japan was measured at 0.51 microSv/hr. Someone working at the entrance would be exposed to 0.51 x 2,000 hr/yr = 1.02 mSv/yr which is well within (background + manmade) radiation range. This radiation exposure has to be typed, converted to dose and adjusted with factors to estimate any health impact.

Notable Radiation Events: According to UN and US National Academy of Sciences Reports:

- More than 500 atmospheric atomic device detonations released about 70 billion curies; almost all of it is from instantaneous, short-life, gammy radiation, little from medium and long-life isotopes.

- Chernobyl, 1986, released about 100 million curies; most of it spread as medium and long-life isotopes over a very large geographical area; the plant had no concrete containment vessel, as many other former USSR plants.

Radioactive iodine concentrates in the thyroid which may cause thyroid cancer 2-3 years after exposure. Of all the children exposed by drinking milk from 1986 to 2002, 16 years, about 4,000 were diagnosed with thyroid cancer. As of September 2005, 15 had died, with more to come in future years.

- Fukushima Daiichi, 2011, released about 10 million curies; most of it spread as medium and long-life isotopes by the prevailing winds over the Pacific Ocean.

- Three Mile Island, 1979, released about 50 curies; the plant has a concrete containment vessel, as do all other US nuclear power plants.

Note: Worldwide, nuclear plants without proper containment vessels should be decommissioned and dismanteld, i.e., no more Chernobyls!

1 curie = 37 billion atomic disintegrations per second = 37 billion Becqerel

High Radiation Exposure Occupations: Examples of industries with significant occupational radiation exposure, IN ADDITION to the above background + manmade exposure:

- Airline crew (the most exposed population), 4.6 mSv/yr
- Industrial radiography
- Medical radiology and nuclear medicine
- Uranium mining
- Nuclear power plant and nuclear fuel reprocessing plant workers, 3.6 mSv/yr
- Research laboratories (government, university and private)

Note: Pilots are more likely to get colon, rectal, prostate and brain cancers; female crew members are twice as likely to suffer breast cancer, and, if pregnant, increase the risk of Down's syndrome and leukemia for their unborn children; the fetus statutory limit is 1 mSv/yr. An explanation for the pilots may be their sedentary working conditions, the poor airline food, the radio headset and the instrument and radar radiation in the cockpit.

Here is a URL which calculates radiation doses for various isotopes, distance from the source, shielding, etc.


All foods are slightly radioactive, some more than others. All food sources combined expose a person to about 0.4 mSv per year on average. 

The average radioactivity of bananas from potassium-40 (half life 1.3 billion years, decay energy 1.3 MeV) is 130 Bq/kg, or about 19.2 Bq per 150 gram banana. It contains about 450 mg of potassium of which the potassium-40 makes up 0.0117%, or about 53 ugram.

The Effective Dose of 1,000 bananas = DCF (5.02 Sv/1 billion Bq; for potassium-40) x 19.2 Bq/banana x 1,000 bananas = 0.096 mSv. 

Eating 1,000 bananas, or 40 tablespoons of peanut butter, or smoking 1.4 cigarettes equals a dose of about 0.1 mSv, or one millimort. Cigarette smoke does radioactive damage to a person’s body, especially the lungs.

Bananas are radioactive enough to regularly cause false alarms on radiation sensors used to detect illegal smuggling of nuclear material at US ports. 

For comparison:

Carrots: 126 Bq/kg from potassium-40; half life 1.3 billion years, decay energy 1.3 MeV
Banana: 130 Bq/kg from potassium-40
Brazil nuts: 207 Bq/kg from potassium-40, plus 37-259 Bq/kg from radium-226; half-life 1,620 years, decay energy 4.9 MeV
US tobacco: 19.1 Bq/kg from polonium-210; half-life 138 days, decay energy 5.3 MeV 

Cigarette smoke inhaled from one pack per day (5.475 kg tobacco/yr): 25 mBq from polonium-40/pack x 365 d/yr = 9.1 Bq/yr. This dose is about 50 times that of a non-smoker and mostly impacts the lungs. 


Airborne radioactive isotopes from the Chernobyl and Fukushima Nuclear Power Plant fires were spread by the weather and have entered the soil, water, and the fauna and flora. The isotopes are most harmful if they enter the human body through inhalation, ingestion or open wounds.

The isotopes of greatest concern for drinking water and food (including seafood and kelp) are: 

tritium: half-life 12.3 years, 0.018 MeV beta emitter, does not collect in body, is eliminated with urine.*
strontium-90: half-life 29 years, 0.546 MeV gamma-ray emitter, collects in bones and teeth
iodine-131: half-life 8.1 days, 0.4 MeV beta and 0.4 MeV gamma-ray emitter, collects in thyroid
cesium-137: half-life 30.2 years, 0.3 MeV beta and 0.66 MeV gamma-ray emitter, collects in fleshy tissue, such as kidneys
radium-226: half-life 1,620 years, 4.9 MeV alpha emitter, collects in bones, liver, breast; a major source is flyash from coal plants

** Tritium has a biological half-life of about 10 days due to taking in and eliminating of water. The radiation fraction in the body of an ingested dose = biological half-life/isotope half-life = 10 days/ (365 days/yr x 12.3 years) = 0.0022. Tritium is a least dangerous isotope.

Grazing cows concentrate iodine-131 in their milk, causing milk consumers, such as infants, to be excessively exposed, and concentrate cesium-137 in their flesh. Pregnant women, nursing mothers, fetuses and young children face the greatest danger from iodine-131, because it accumulates in the thyroid. 

Children are at much higher risk than adults because they are growing, and their thyroid glands are more active and in need of iodine. The gland is smaller in children than in adults, so a given dose of iodine-131 will deliver a higher dose of radiation to a child’s thyroid and potentially do more harm.

According to the Centers for Disease Control and Prevention, if an adult and a newborn ingest the same dose of radioactive iodine, the thyroid dose will be 16 times higher to a newborn than to an adult; for a less than 1-year-old, eight times the adult dose; for a 5-year-old, four times the adult dose.

Pregnant women take up more iodine-131 in the thyroid, especially in the first trimester. The iodine crosses the placenta and reaches the fetus; its thyroid takes up more iodine as pregnancy progresses. During the first week after birth a baby’s thyroid activity increases up to fourfold and stays at that level for a few days, so newborns are especially vulnerable.

Potassium iodide can protect the thyroid by saturating it with normal iodine. People in Japan have been advised to take it. 


- A luminous EXIT sign (1970s) contains about 1,000,000 million Bq (1 TBq), or 27 Curies of tritium. They often end up in landfills causing the leacheate to be contain up to 250,000 pCi/liter, which may be similar to some nuclear plant tritium leaks.

- NRC limit for a wristwatch = 25 mCi of tritium/watch = 25,000,000 pCi of tritium/watch = 925,000 Bq


                        100 Bq: Japan maximum iodine-131/liter for drinking water (babies)
                        300 Bq: Japan maximum iodine-131/liter for drinking water (older children, adults)
                        740 Bq: EPA maximum tritium/liter for drinking water, or 20,000 pCi of tritium/liter
                     1,000 Bq: one kg of coffee
                     1,000 Bq: one kg of granite (such as a kitchen countertop)
                     2,000 Bq: one kg of coal ash
                     2,000 Bq: Japan maximum iodine-131/kg of fish and vegetables
                     3,000 Bq: radon in a 100 sq meter Australian home
                     3,000 Bq: I.A.E.A. maximum iodine-131/liter for drinking water (older children, adults)
                     5,000 Bq: one kg superphosphate fertilizer
                     7,000 Bq: human adult (100 Bq/kg x 70 kg)
                     7,000 Bq: Canada (Ontario) maximum tritium/liter for drinking water
                   10,000 Bq: Switzerland maximum tritium/liter for drinking water
                   30,000 Bq: household smoke detector with americium
                   30,000 Bq: radon in a 100 sq meter European home
                 500,000 Bq: one kg uranium ore (Australian, 0.3%)
                 925,000 Bq: tritium in one wristwatch
                  1 million Bq: one kg of low level radioactive waste
                25 million Bq: one kg of uranium ore (Canadian, 15%)
                70 million Bq: radioisotope for medical diagnostic purposes
    1,000,000 million Bq: one luminous EXIT sign with tritium (1970s) = 27 Curies
  10,000,000 million Bq: one kg of 50-yr-old, vitrified, high-level nuclear waste
100,000,000 million Bq: radioisotope source for medical therapy = 2,700 Curies


The below table indicates additional radiation above average background exposure:  

          0.001 mSv: one backscatter wave scan at an airport for about 10 seconds 
          0.007 mSv: one bitewing X-ray, F-speed film for about 0.4 second 
            0.01 mSv: living near a nuclear plant for one year 
          0.075 mSv: Airport whole-body backscatter scan        
          0.014 mSv: one dental X-ray, Panorex, digital for about 18 seconds
            0.02 mSv: sleeping next to another person for one year 
    0.03-0.05 mSv: one airplane cross-country flight of about 6 hours
          0.036 mSv: eating one banana per day for a year 
            0.05 mSv: nuclear plant design standard at perimeter fence for one year
              0.1 mSv: living in a brick house instead of a wood-frame house for one year
        0.1-0.2 mSv: one skull X-ray for about 0.5 seconds 
        0.1-0.5 mSv: one chest X-ray for about 0.5 seconds
              0.4 mSv: one mammogram for about 0.5 seconds 
        0.6-1.7 mSv: one abdomen X-ray for about 0.5 seconds 
              1.5 mSv: EPA maximum for an average adult for one year
              2.2 mSv: airline crew member, short flights for one year 
              2-4 mSv: one head CT scan for about 10 minutes
              3-6 mSv: airline crew member, cross-country flights, 900 hrs/yr 
              3-8 mSv: one barium X-ray for about 0.5 seconds
               10 mSv: cooking with natural gas (radon) for a year
            5-15 mSv: one whole-body CT scan for about 20 minutes
            6-18 mSv: one chest CT scan for about 10 minutes
                 9 mSv: airline crew member, polar flights, such as Tokyo-NYC, 900 hrs/yr
               13 mSv: smoking one pack of cigarettes per day for a year
               20 mSv: nuclear plant worker, maximum 5-year average*+
          40-50 mSv: cardiac catheterization, coronary angiogram, heart x-ray studies for about 1 hour
               50 mSv: nuclear plant worker, maximum total exposure in one year
        50-100 mSv: changes in blood chemistry
             100 mSv: lowest clearly carcinogenic level; 1 millimort
               0.25 Sv: temporary sterility in men
       0.50-0.55 Sv: nausea, fatigue within hours
       0.70-0.75 Sv: vomiting and hair loss in 2-3 weeks
                 1-2 Sv: for about an hour, 0 to 5% fatal
                 2-6 Sv: external-immediate severe skin burns, internal-50% fatal
               8-30 Sv: for about an hour, 100% fatal

Note: MRIs and EKGs: no radiation         

The above list shows that an airline crew member smoking a pack of cigarettes per day will significantly increase his/her chances of developing cancer and that female crew members should not be flying, smoking and cooking with natural gas during pregnancy.

* Workers exposed to ionizing radiation, such as nuclear plant workers, usually wear personal dosimeters that total various exposures for a period. The total exposure is not to exceed a 5-year average of 20 mSv, with any one year not to exceed 50 mSv. The actual exposures usually are significantly less.

+ Annual dose limits declined from 150 mSv in the 1950s, to 20 mSv at present.


A graphical presentation of radiation doses from various sources was prepared by Randall Munroe and Ellen, Senior Reactor Operator at the Reed Research Center. 

Note: some data do not indicate the exposure period.

Sources for the graphic:


Much is written about the dangers of nuclear energy. However, it is the safest source of energy for producing electric power, in accordance with studies by the World Health Organization and the european study EXTERNE based on data from past decades. Any deaths due to future global warming, partially the result of the CO2 from fossil fuels, was not considered by these studies.

The USA: 30,000 deaths/yr from coal pollution of 2,000 TWh/yr, or 15 deaths/yr/TWh, a ratio that will likely remain about the same over the years.

China: 500,000 deaths/yr from coal pollution of 1,800 TWh/yr, or 278 deaths/yr/TWh, a ratio that will likely decline, as China implements safer mining practices and more efficient, cleaner-burning coal power plants over the years.

Energy Source Mortality Rates; Deaths/yr/TWh

Coal - world average, 161
Coal - China, 278
Coal - USA, 15
Oil - 36
Natural Gas - 4
Biofuel/Biomass - 12
Peat - 12
Solar/rooftop - 0.44-0.83
Wind - 0.15
Hydro - world, 0.10
Hydro - world*, 1.4
Nuclear - 0.04

* Includes the 170,000 deaths from the failure of the Banquao Reservoir Dam in China in 1975