Ionizing radiation is an IARC category 1 human carcinogen. RF and ELF non ionizing radiation are category 2B.

see IARC monographs  75,78, 80,100D,102.

Plus BEIR v report 1990 NRC and ICRP reports.

One very good article mention in detail about effect of radiation. 

Preliminary evidence has been presented that irradiation targeted to the cytoplasm yields a significant increase in the frequency of mutations. Finally, genetic events including the induction of mutations and changes in gene expression may occur in neighboring cells that receive no direct radiation exposure at all. This `bystander effect' involves gap junction mediated cell–cell communication, and activation of the p53 damage response pathway. The possible role of these phenomena in radiation carcinogenesis is discussed.

Check This Article.

http://carcin.oxfordjournals.org/content/21/3/397.full.pdf+html

In a nutshell...answer is yes !! radiation is always a source of carcinogen however it depends from body to body how your cells respond to any radiations...for most humans only 'certain radiations' of specific properties may bring mutations in genes and trigger cancer. And again these 'certain radiations' may only be specific to few people causing cancer and obviously exposure time is critical  (thats what I essentially mean by saying 'body to body').  Otherwise all men who put phones in their pant pockets would have had sever testicular cancer just after using it for few days :P....... 

It was also said low dose of exposure to radiation can improve the mechanism of body's anti-irradiation. It was reported that in some higher dose of radiation background area, the residents were not in higher risk of cancer. The environment makes the body fit unpleasant condition during a long period.

No.  It is a dose issue.  At very low dose, it is not a carciongen.  Everyone receives radioationn from different sources everyday, but not everyone develops cancers.

Life has evolved in a world with significant levels of ionizing radiation and our bodies have adapted to it.

• Radiation has always been present and is all around us in many natural forms. Life has evolved in a world with significant levels of ionizing radiation.

• Many radioisotopes are naturally occurring, and originated during the formation of the solar system and through the interaction of cosmic rays with molecules in the atmosphere. Tritium is an example of a radioisotope formed by this interaction.

• Radioisotopes such as polonium-210, carbon-14 and potassium-40 naturally occur within the human body.

• Potassium-40 is present in many common foods including red meats, white potatoes, carrots, bananas, lima beans and Brazil nuts.

• The annual average effective dose from natural background radiation is approximately 2.4 mSv worldwide.

However, for thyroid, low dose of radiation is a risk factor for malignant transformation of the thyroid.

Ionizing radiation has been present in the environment since before the dawn of life and is time variable and location dependent. There is a rapid increase with altitude due to cosmic rays and occasional solar particle events and air crew can get up to 6 mSv per year before they are stood down from flying. It is possible that an extreme solar event (1 in 100 years) could give 20 to 50 mSv at aircraft altitudes. The biological effects at low doses are not well known and a conservative linear no threshold extrapolation from high dose data is used. Epidemiology is unlikely to resolve the low dose question and better understanding of cell and organ response is needed. Our electronics is increasingly vulnerable to single event effects from radiation and less capable of self repair than we are.

Yes it is carcinogenic and increase risk of cancer but not always.

A broad spectrum of tissue types contributes to the overall cancer risk that is observed with low-LET ( low-linear energy transfer) radiation , including lung, colorectal, breast, stomach, liver, and bladder cancers as well as several types of leukemia, including acute myeloid leukemia and acute lymphatic lymphoma (NCRP, 2000; Preston et al., 2003; BEIR, 2006). It is not known whether the same spectrum of tumors will occur for high-LET radiation as with low-LET radiation, and some differences should be expected. Relative biological effectiveness (RBE) factors  describe the ratio of a dose of high-LET radiation to that of the X rays or gamma rays that produce the identical

biological effect.

yes it is carcinogenic especially ultraviolet B radiation induces skin cancers

Yes, I agree fully with dr. Hassanein.

Radiation is definitely a carcinogen , this does not mean however that it immediately  cause cancer . There are three main steps from cellular exposure to a carcinogen to a fully blown malignancy. Initiation: a reversable change in the DNA after exposure to a carcinogen e.g. radiation, Promotion: The changes become irreversable and Progression when the  changes becomesfixed and active such as to produce and re-produce  malignancy. The severity of a carcinognic effect depends on the degree, intensity, and time of exposure (younger and dividing cells being more vulnerable). as well as on the presence of co-factors and other injuriuos chemicals.

Some carcinogenes or carcinogenic doses may lead to all three steps co-inciding within a short period of time 

Radiation always cause DNA damage therefore increases the risk of cancer but the appearance of the malignant process itself (cancer) is not always immediately   evident and depends on the above mentioned and other factors. Think for e.g. on mammographies, dental and other X-ray exposures for diagnostic or treatment purposes.

Radiation during imaging by X-ray or CT-Scan is carcinogenic particularly in children. Also radiation is a risk factor of cataract. 

@all contributors: thank you for all your answers. My question was inspired by a discussion by a professor in physics concerning epidemiology research related to radiation, especially radioactive waste. Some publications say that radioactive waste is harmless below a safe level, but what is a safe level? It is just a matter of consensus and politely say that noone wiil get cancer under certain radiactive environment. To me this can turn into public lie. What do you think?

An student in physical therapy became the star in a world congress of the speciality around 1970 when he proposed that physical therapy was vibration therapy, the only difference from one class of vibration to another being the frequency in terms of Hertz, i.e. cycles per second, so both Physiotherapy/Rehabilitation medicine and Tomotherapy being part of the same class

There is precious little evidence that non-ionizing radiation is carcinogenic

Great question and an opportunity to shed some light on a myth that has plagued mankind for 60 years!

First of all, high-dose, high-dose-rate radiation (whole body) kills or causes cancer.  Low-dose, low-dose-rate radiation ...not so much.  The human body is designed to repair damage (from ANY source...chemical, radiation, trauma, etc.).  It's intuitively obvious, when you think about it.  The myth stems from 1946 when Dr. Herman Mueller stated that radiation risk extrapolates down to zero exposure, that there is no safe level of exposure to ionizing radiation (the so-called Linear No Threshold Theory).  There was no science to back up that statement, but it reflected a lot that was going on back then.  If readers would like to learn more, go to:

http://radiationeffects.org/ (Scientists for Accurate Radiation Information)

and browse some of the many articles that go into greater detail (some links below)

http://radiationeffects.org/2014/11/08/radiation-effects-on-humans-and-organisms-and-reasons-for-the-fear/

http://radiationeffects.org/2014/10/18/lnt-the-guide-for-the-perplexed/

http://radiationeffects.org/2014/08/21/hormesis-by-low-dose-radiation-effects-low-dose-cancer-risk-modeling-must-recognize-up-regulation-of-protection/

http://radiationeffects.org/2014/08/22/the-road-to-linearity-why-linearity-at-low-doses-became-the-basis-for-carcinogen-risk-assessment-2/

The erroneous myth extends down to low, innocuous levels of other insults like chemicals as well.

Radiation in any dose is carcinogenic ,but to cause cancer depends on the individuals genetic make up,that is how much prone an  individual is for radiation induced damage and ultimately to get cancer.some individuals having some deficient genetic make are more prone to get radiation induced cancers like persons with xeroderma pigmentosum,and to cut it short we should not forget 

Last but not the least we should not forget the stochastic and nonstochastic effects of radiation.

@Joe, John, Mark and Shaqul: thank you for your answers. Best wishes

A high radiation dose is harmful. A low radiation dose produces beneficial health effects. See: Commentary on Fukushima and Beneficial Effects of Low Radiation

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3834738/ and Remedy for Radiation Fear at:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4036393/ and Chapter 2, at:

https://www.novapublishers.com/catalog/product_info.php?products_id=54053 

Answer: We don't know. You can find an equal number of scientific papers and arguments that state that low dose RT is harmful as that it is beneficial. Much belief and little and conflicting evidence. If science were a democracy, the truth comes from the votes of the majority: low dose RT it is neither harmful nor beneficial. However, overrated fear for radiation is harmful: From the Chernobyl (1996) rescue workers, 28 men died from a radiation overdose. All had received doses above 2.1 Sievert. Remarkably, there was no increased number of radiation related cancers amongst the survivors (There were more thyroid cancers in young children). However, since most of the rescue workers were 'contaminated', they had difficulty in finding a new job, a wife, or get a mortgage or life insurance. Many became social outcasts and suffered from psychological distress and got addicted to alcohol and tobacco; several hundreds of them died from related diseases. Conclusion: The biological risk of low dose radiation is negligible, the economic and psychological hazards of nuclear disasters are huge.  

With regards to ionizing radiation, I don't think there is a dose below which one can say definitively that no harm or biological damage will be caused.  Even doses as small as a few rad or a fraction of a rad given per year over many years has shown to cause biological changes to DNA.  While the significance of these changes is unknown, the possibility remains that the risk of carcinogenesis may be increased for individuals who already have pre-disposing conditions, or other genetic mutations.

As a radiation oncologist, we use ionizing radiation for the treatment of cancers.  Here, the benefits of treating a potentially life-threatening condition far outweigh the risk of developing a second cancer.  However, patients do need to be counselled about the risks of treatment.

As far as non-ionizing radiation goes (radio waves, microwaves, etc), the energy is not sufficient to cause breaks in chemical bonds.  This does not mean that biological changes do not occur.  However, a link has not been established between non-ionizing radiation and carcinogenesis.

FYI - amounts of radiation from various things:

http://xkcd.com/radiation/

 For radiation protection purposes, all human health effects are classified into either stochastic effects or non-stocshastic effects (also called deterministic effects, or more recently, tissue reactions). As regards the shape of the dose-response curve, the linear non-threshold (LNT) type is assumed for stochastic effects and threshold type is assumed for tissue reactions. Stochastic effects include cancer and heritable cancer. For the exact shape of the dose response curve is not known at low dose below 100-200 mSv. Evidence for beneficial effects has not been accumulated enough to safely assume that health risks assuming an LNT are lower than those assuming beneficial effects. The same applies to low-dose hypersensitivity or non-targeted effects such as bystander effects and genomic instability.

Article Classification of radiation effects for dose limitation purp...