The effects of radiation depend on the dose and dose rate. There are some widely accepted thresholds now for high dose rate. For whole body irradiation and for doses delivered over a short time, the following applies:

20 Sv: Severe gastrointestinal effects, nervous system damage followed by rapid death

10 Sv: Death within weeks if whole body exposed, severe damage to the organ exposed

3 Sv: Radiation sickness, 50% chance of death for whole body exposure

1 Sv: Radiation sickness for whole body exposure

500 mSv: leukopoenia

200 mSv: detectable chromosomal aberrations

You can find more information in the UNSCEAR reports, they are freely downloadable from the internet.

Some other pretty easy reading if you are interested

www.nrc.gov/reading-rm/basic-ref/teachers/09.pdf

www-pub.iaea.org/MTCD/publications/PDF/te_934_prn.pdf

www.iaea.org/Publications/Booklets/RadPeopleEnv/pdf/chapter_5.pdf

There is also some other interesting reading and tools on this site http://www.remm.nlm.gov/

I agree with Dr. Radu A. Vasilache and I kindly add the parameter of age of the person irreadiated or the phase of pregnancy for the case of a fetus. Another quite new factor in radiobiology is the personalized index of readiosensitivity. It is easily obtained by chromosomal aberrations for each person, under standard condiotions, as all the people do not exhibit the same resistance to radiation. Also, there are many papers related to low doses exposure and the theory of hormesis. According to this opinion, very very very briefly, we could say that after a few exposures to low dose radioation, it is quite possible for someone to develop resistance to a future higher dose exposure.

Best Regards

According to the IAEA, ionizing radiation has a direct action on the complex vital molecules (for example the DNA) within the cell by breaking the bonds between the atoms. Ionization in non-vital molecules (for example, water molecules) produces very active chemicals (free radicals) which attack vital molecular systems.The damage may change the coded information in the cell nucleus, disrupt the cell’s chemistry and function or physically rupture membranes some of which contain the digestive enzymes. Natural mechanisms are capable of identifying and repairing limited damage to improve the cell’s chance of survival.

However, incorrect or incomplete repairs are also a possibility: these may affect the cell’s longer term viability or performance.

At very high whole body doses in excess of 15 Gy, swelling (oedema) of the brain and generalized shock affecting the cardiovascular system leads to coma and death.

The range of doses associated with death from acute exposure of the blood system, the gastrointestinal tract and the central nervous system is based upon sparse human data, supplemented by knowledge of the dose-response relationship derived from animal experiments. No individual would be expected to die after receiving an acute whole body dose at or below 1 Gy unless the person was seriously ill before irradiation. In an exposed population of 100 people, about 5 individuals would probably die after receiving about 2 Gy and about 50 would die within 60 days of receiving a homogeneous whole body dose of 3.5 Gy. This is called the Lethal Dose,

LD50/60. When the correct treatment is provided by a specialized hospital, the survival rate improves and the LD50/60 increases to between 4 and 5 Gy. Most individuals would be expected to die after receiving an acute exposure to a whole body dose of between 6 and 10 Gy unless they receive treatment to prevent infection and bleeding. Above about 10 Gy death is most likely, even after attempts to stimulate the bone marrow or administration of a bone marrow transfusion from a compatible donor.

From a low level atomic and nuclear point of view, regarding the damage radiation can do to DNA there is a key difference between single and double strand damage. Damage to a single strand still leaves the DNA with information to regenerate on the second strand, however if both strands are damage the sequence information is lost. For this reason radiation from ions such as alpha particles is more damaging than say x-rays, since the alpha particle has many interactions with atoms in the body before it loses its energy. X-rays on the other hand tend to have fewer interactions or only one interaction if it is absorbed by an inner shell electron, making it less likely to create double DNA strand damage. This is one reason why hadron therapy with say energetic protons or carbon ions is more effective at treating cancer by killing off the cancer cells.

All answers are relevant. I taught this subject to Masters students in Radiation Physics during 1980s. The best book in my view is RADIATION BIOLOGY by Alison P. Casaret, available on Amazon.com. U can find it EASY to comprehend, with Target Models to calculate the damage and dose delivered..

https://www.researchgate.net/publication/236035267_Human_Nuclear_Physiology_1?ev=prf_pub 

Hello! Subject complicated, but interesting. We investigated nuclear human physiology. Hopefully, after reading the work, you will find some answers, All the best!

Article Human Nuclear Physiology 1

Professor Mary Helen Barcellos-Hoff, Biological effects of radiation (IRPA13 Refresher Course):

http://www.irpa.net/irpa13/Audio/Refresher-Courses/IRPA13-RC1-Barcello.zip

Regards, Viktor

Biological effectiveness of gamma-rays differs among its quality, dose, dose rate and time course after irradiation, and provided below is an outline. If you are especially interested in some of these effects, rsvp then I will be happy to give you more details.

There are early occurring effects and late occurring effects. For radiation protection purposes, biological effects are grouped into two categories. One is stochastic effects (cancer and heritable effects) where the linear no threshold model is applied, and the other is tissue reactions (formerly called nonstochastic or deterministic effects) with a threshold below which no effect would occur. Acute doses up to approximately 100 mGy produce no functional impairment of tissues and stochastic risks are principal risks, but risks of tissue reactions (especially, cataracts and circulatory disease) become increasing important at >0.5 Gy.

Acute exposure, fractionated/protracted, and chronic exposure scenarios need to be considered. For gamma-rays, although depending on biological endpoints, the lower the dose rate, the lower the biological effectiveness, so called, a sparing dose rate effect. For radiation protection purposes, ICRP has employed the dose and dose rate effectiveness factor of 2. 

In addition to somatic effects (effects occurring in exposed individuals themselves), ICRP has prudently assigned detriment for heritable effects (effects occurring in progeny of exposed individuals), although no scientific evidence is available for heritable effects in humans.

60Co gamma-rays and 137Cs gamma-rays are frequently used. The maximum relative biological effectiveness value (RBEM) of 137Cs gamma-rays versus 60Co gamma-rays for induction of dicentrics (unstable type chromosome aberrations) was 1.63 (Table 6.3 on page 141 in http://ncrponline.org/wp-content/themes/ncrp/PDFs/Docs_in_Review/NCRPM1701.pdf )

For radiation protection purposes, three exposure situations are considered (i.e., planned, existing and emergency). Dose limits and dose constraints are used in planned exposure situations, whilst reference levels are used in existing and emergency situations. Effective dose limits aim to keep the occurrence of stochastic health effects below unacceptable levels, whereas equivalent dose limits aim to avoid tissue reactions. Dose constraints and reference levels are in conjunction with the optimization of protection to restrict individual doses.

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Please see the following references.

Article The biological effects of radiation

Article Biological Effects of Ionizing Radiation.

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