Cancer is a stochastic effect of radiation, meaning that it only has a probability of occurrence, as opposed to deterministic effects which always happen over a certain dose threshold. The consensus of the nuclear industry, nuclear regulators, and governments, is that the incidence of cancers due to ionizing radiation can be modeled as increasing linearly with effective radiation dose at a rate of 5.5% per sievert. Individual studies, alternate models, and earlier versions of the industry consensus have produced other risk estimates scattered around this consensus model. There is general agreement that the risk is much higher for infants and fetuses than adults, higher for the middle-aged than for seniors, and higher for women than for men, though there is no quantitative consensus about this. This model is widely accepted for external radiation, but its application to internal contamination is disputed. For example, the model fails to account for the low rates of cancer in early workers at Los Alamos National Laboratory who were exposed to plutonium dust, and the high rates of thyroid cancer in children following the Chernobyl accident, both of which were internal exposure events. The European Committee on Radiation Risk calls the ICRP model "fatally flawed" when it comes to internal exposure.

Radiation can cause cancer in most parts of the body, in all animals, and at any age, although radiation-induced solid tumors usually take 10–15 years, and can take up to 40 years, to become clinically manifest, and radiation-induced leukemias typically require 2–10 years to appear. Some people, such as those with nevoid basal cell carcinoma syndrome or retinoblastoma, are more susceptible than average to developing cancer from radiation exposure. Children and adolescents are twice as likely to develop radiation-induced leukemia as adults; radiation exposure before birth has ten times the effect.

Radiation exposure can cause cancer in any living tissue, but high-dose whole-body external exposure is most closely associated with leukemia, reflecting the high radiosensitivity of bone marrow. Internal exposures tend to cause cancer in the organs where the radioactive material concentrates, so that radon predominantly causes lung cancer, iodine-131 is most likely to cause thyroid cancer, etc.

The associations between ionizing radiation exposure and the development of cancer are based primarily on the "LSS cohort" of Japanese atomic bomb survivors, the largest human population ever exposed to high levels of ionizing radiation. However this cohort was also exposed to high heat, both from the initial nuclear "flash" of infrared light and following the blast due their exposure to the firestorm and general fires that developed in both cities respectively, so the survivors also underwent Hyperthermia therapy to various degrees. Hyperthermia, or heat exposure following irradiation is well known in the field of radiation therapy to markedly increase the severity of free-radical insults to cells following irradiation. Presently however no attempts have been made to cater for this confounding factor, it is not included or corrected for in the dose-response curves for this group.

Additional data has been collected from recipients of selected medical procedures and the 1986 Chernobyl disaster. There is a clear link (see the UNSCEAR 2000 Report, Volume 2: Effects) between the Chernobyl accident and the unusually large number, approximately 1,800, of thyroid cancers reported in contaminated areas, mostly in children.

For low levels of radiation, the biological effects are so small they may not be detected in epidemiological studies. Although radiation may cause cancer at high doses and high dose rates, public health data regarding lower levels of exposure, below about 10 mSv (1,000 mrem), are harder to interpret. To assess the health impacts of lower radiation doses, researchers rely on models of the process by which radiation causes cancer; several models that predict differing levels of risk have emerged.

Studies of occupational workers exposed to chronic low levels of radiation, above normal background, have provided mixed evidence regarding cancer and transgenerational effects. Cancer results, although uncertain, are consistent with estimates of risk based on atomic bomb survivors and suggest that these workers do face a small increase in the probability of developing leukemia and other cancers. One of the most recent and extensive studies of workers was published by Cardis, "et al." in 2005 . There is evidence that low level, brief radiation exposures are not harmful.

According to the linear no-threshold model, any exposure to ionizing radiation, even at doses too low to produce any symptoms of radiation sickness, can induce cancer due to cellular and genetic damage. Under the assumption, survivors of acute radiation syndrome face an increased risk of developing cancer later in life. The probability of developing cancer is a linear function with respect to the effective radiation dose. In radiation-induced cancer, the speed at which the condition advances, the prognosis, the degree of pain, and every other feature of the disease are not believed to be functions of the radiation dosage.

However, some studies contradict the linear no-threshold model. These studies indicate that some low levels of radiation do not increase cancer risk at all, and that there may exist a threshold dosage of ionizing radiation below which exposure should be considered safe. Nonetheless the 'no safe amount' assumption is the basis of US and most national regulatory policies regarding "man-made" sources of radiation.

There are two major databases that track radiation accidents: The American ORISE REAC/TS and the European IRSN ACCIRAD. REAC/TS shows 417 accidents occurring between 1944 and 2000, causing about 3000 cases of acute radiation syndrome, of which 127 were fatal. ACCIRAD lists 580 accidents with 180 ARS fatalities for an almost identical period. The two deliberate bombings are not included in either database, nor are any possible radiation-induced cancers from low doses. The detailed accounting is difficult because of confounding factors. ARS may be accompanied by conventional injuries such as steam burns, or may occur in someone with a pre-existing condition undergoing radiotherapy. There may be multiple causes for death, and the contribution from radiation may be unclear. Some documents may incorrectly refer to radiation-induced cancers as radiation poisoning, or may count all overexposed individuals as survivors without mentioning if they had any symptoms of ARS. The table below attempts to catalog some cases of ARS. Many of these incidents involved additional fatalities from other causes, such as cancer, which are excluded from this table.

Chronic radiation syndrome is a constellation of health effects that occur after months or years of chronic exposure to high amounts of ionizing radiation. Chronic radiation syndrome develops with a speed and severity proportional to the radiation dose received, i.e., it is a deterministic effect of radiation exposure, unlike radiation-induced cancer. It is distinct from acute radiation syndrome in that it occurs at dose rates low enough to permit natural repair mechanisms to compete with the radiation damage during the exposure period. Dose rates high enough to cause the acute form (> ~0.1 Gy/h) are fatal long before onset of the chronic form. The lower threshold for chronic radiation syndrome is between 0.7 and 1.5 Gy, at dose rates above 0.1 Gy/yr. This condition is primarily known from the Kyshtym disaster, where 66 cases were diagnosed, and has received little mention in Western literature. A future ICRP publication, currently in draft, may recognize the condition but with higher thresholds.

In 2013, Alexander V. Akleyev described the chronology of the clinical course or CRS while presenting at ConRad in Munich, Germany. In his presentation, he defined the latent period as being 1-5 years, and the formation coinciding with the period of maximum radiation dose. The recovery period was described as being 3-12 months after exposure ceased. He concluded that "CRS represents a systemic response of the body as a whole to the chronic total body exposure in man." In 2014, Akleyev's book "Comprehensive analysis of chronic radiation syndrome, covering epidemiology, pathogenesis, pathoanatomy, diagnosis and treatment" was published by Springer.

In the 1960s, the incidence 5 years after a radical mastectomy varied from 0.07% to 0.45%.

Today, it occurs in 0.03% of patients surviving 10 or more years after radical mastectomy.

Early detection is key. Untreated patients usually live 5 to 8 months after diagnosis.

The incidence of squamous cell carcinoma continues to rise around the world. A recent study estimated that there are between 180,000 and 400,000 cases of SCC in the United States in 2013. Risk factors for squamous cell carcinoma varies with age, gender, race, geography, and genetics. The incidence of SCC increases with age and the peak incidence is usually around 60 years old. Males are affected with SCC at a ratio of 2:1 in comparison to females. Caucasians are more likely to be affected, especially those with fair Celtic skin and chronically exposed to UV radiation. Squamous cell carcinoma of the skin is the most common among all sites of the body. Solid organ transplant recipients (heart, lung, liver, pancreas, among others) are also at a heightened risk of developing aggressive, high-risk SCC. There are also a few rare congenital diseases predisposed to cutaneous malignancy. In certain geographic locations, exposure to arsenic in well water or from industrial sources may significantly increase the risk of SCC.

Radiation burns are caused by exposure to high levels of radiation. Levels high enough to cause burn are generally lethal if received as a whole-body dose, whereas they may be treatable if received as a shallow or local dose.

Smoking and alcohol abuse as the major risk factors. Viral causes has recently been taken under consideration as one of the risk factors. Viruses such as Epstein-Barr virus (EBV) (majorly involved in causing nasopharyngeal carcinoma) and human papilloma virus are included in this category. Chewing of betel nut ("Areca catechu") quid has been directly associated to cause oral cancers. It has also been stated under the FDA poisonous plant data base by the U.S Food and Drug Administration

An unbalanced diet, deficit in fruits and vegetables has shown to increase the risk of cancer.

Fluoroscopy may cause burns if performed repeatedly or for too long.

Similarly, Computed Tomography and traditional Projectional Radiography have the potential to cause radiation burns if the exposure factors and exposure time are not appropriately controlled by the operator.

A study of radiation induced skin injuries has been performed by the Food and Drug Administration (FDA) based on results from 1994, followed by an advisory to minimize further fluoroscopy-induced injuries. The problem of radiation injuries due to fluoroscopy has been further investigated in review articles in 2000, 2001, 2009 and 2010.

People who have received solid organ transplants are at a significantly increased risk of developing squamous cell carcinoma due to the use of chronic immunosuppressive medication. While the risk of developing all skin cancers increases with these medications, this effect is particularly severe for SCC, with hazard ratios as high as 250 being reported, versus 40 for basal cell carcinoma. The incidence of SCC development increases with time posttransplant. Heart and lung transplant recipients are at the highest risk of developing SCC due to more intensive immunosuppressive medications used. Squamous cell cancers of the skin in individuals on immunotherapy or suffering from lymphoproliferative disorders (i.e. leukemia) tend to be much more aggressive, regardless of their location. The risk of SCC, and non-melanoma skin cancers generally, varies with the immunosuppressive drug regimen chosen. The risk is greatest with calcineurin inhibitors like cyclosporine and tacrolimus, and least with mTOR inhibitors, such as sirolimus and everolimus. The antimetabolites azathioprine and mycophenolic acid have an intermediate risk profile.

Radiation-induced lung injury is a general term for damage to the lungs which occurs as a result of exposure to ionizing radiation. In general terms, such damage is divided into early inflammatory damage ("radiation pneumonitis") and later complications of chronic scarring ("radiation fibrosis"). Pulmonary radiation injury most commonly occurs as a result of radiation therapy administered to treat cancer.

The lungs are a radiosensitive organ, and radiation pneumonitis can occur leading to pulmonary insufficiency and death (100% after exposure to 50 gray of radiation), in a few months. Radiation pneumonitis is characterized by:

- Loss of epithelial cells

- Edema

- Inflammation

- Occlusions airways, air sacs and blood vessels

- Fibrosis

Tonsillar carcinoma can be either HPV related or HPV unrelated. It is shown that cases which are HPV positive have a better prognosis than those with HPV negative oropharyngeal cancer.

Occupational lung diseases include asbestosis among asbestos miners and those who work with friable asbestos insulation, as well as black lung (coalworker's pneumoconiosis) among coal miners, silicosis among miners and quarrying and tunnel operators and byssinosis among workers in parts of the cotton textile industry.

Occupational asthma has a vast number of occupations at risk.

Bad indoor air quality may predispose for diseases in the lungs as well as in other parts of the body.

Occupational skin diseases are ranked among the top five occupational diseases in many countries.

Occupational skin diseases and conditions are generally caused by chemicals and having wet hands for long periods while at work. Eczema is by far the most common, but urticaria, sunburn and skin cancer are also of concern.

Contact dermatitis due to irritation is inflammation of the skin which results from a contact with an irritant. It has been observed that this type of dermatitis does not require prior sensitization of the immune system. There have been studies to support that past or present atopic dermatitis is a risk factor for this type of dermatitis. Common irritants include detergents, acids, alkalies, oils, organic solvents and reducing agents.

The acute form of this dermatitis develops on exposure of the skin to a strong irritant or caustic chemical. This exposure can occur as a result of accident at a workplace. The irritant reaction starts to increase in its intensity within minutes to hours of exposure to the irritant and reaches its peak quickly. After the reaction has reached its peak level, it starts to heal. This process is known as decrescendo phenomenon. The most frequent potent irritants leading to this type of dermatitis are acids and alkaline solutions. The symptoms include redness and swelling of the skin along with the formation of blisters.

The chronic form occurs as a result of repeated exposure of the skin to weak irritants over long periods of time.

Clinical manifestations of the contact dermatitis are also modified by external factors such as environmental factors (mechanical pressure, temperature, and humidity) and predisposing characteristics of the individual (age, sex, ethnic origin, preexisting skin disease, atopic skin diathesis, and anatomic region exposed.

Another occupational skin disease is Glove related hand urticaria. It has been reported as an occupational problem among the health care workers. This type of hand urticaria is believed to be caused by repeated wearing and removal of the gloves. The reaction is caused by the latex or the nitrile present in the gloves.

High-risk occupations include:

- Hairdressing

- Catering

- Healthcare

- Printing

- Metal machining

- Motor vehicle repair

- Construction

Leukemia is rarely associated with pregnancy, affecting only about 1 in 10,000 pregnant women. How it is handled depends primarily on the type of leukemia. Nearly all leukemias appearing in pregnant women are acute leukemias. Acute leukemias normally require prompt, aggressive treatment, despite significant risks of pregnancy loss and birth defects, especially if chemotherapy is given during the developmentally sensitive first trimester. Chronic myelogenous leukemia can be treated with relative safety at any time during pregnancy with Interferon-alpha hormones. Treatment for chronic lymphocytic leukemias, which are rare in pregnant women, can often be postponed until after the end of the pregnancy.

OPA has been found in most countries where sheep are farmed, with the exception of Australia and New Zealand. OPA has been eradicated in Iceland.

No breed or sex of sheep appears to be predisposed to OPA. Most affected sheep show signs at 2 to 4 years of age.

OPA is not a notifiable disease, and therefore it is difficult to assess its prevalence.

Some people have a genetic predisposition towards developing leukemia. This predisposition is demonstrated by family histories and twin studies. The affected people may have a single gene or multiple genes in common. In some cases, families tend to develop the same kinds of leukemia as other members; in other families, affected people may develop different forms of leukemia or related blood cancers.

In addition to these genetic issues, people with chromosomal abnormalities or certain other genetic conditions have a greater risk of leukemia. For example, people with Down syndrome have a significantly increased risk of developing forms of acute leukemia (especially acute myeloid leukemia), and Fanconi anemia is a risk factor for developing acute myeloid leukemia. Mutation in SPRED1 gene has been associated with a predisposition to childhood leukemia.

Chronic myelogenous leukemia is associated with a genetic abnormality called the Philadelphia translocation; 95% of people with CML carry the Philadelphia mutation, although this is not exclusive to CML and can be observed in people with other types of leukemia.

In rare cases where large tumors infringe on the brainstem which controls motor nerves, with or without surgery, paralysis or death can result. This occurs in less than 1% of large tumors.

Taste disturbance and mouth dryness are frequent for a few weeks following surgery. In a few patients this disturbance is longer or permanent.

An increasing number of people are now surviving cancer, with improved treatments producing cure of the malignancy (cancer survivors). There are now over 14 million such people in the US, and this figure is expected to increase to 18 million by 2022. More than half are survivors of abdominal or pelvic cancers, with about 300,000 people receiving abdominal and pelvic radiation each year. It has been estimated there are 1.6 million people in the US with post-radiation intestinal dysfunction, a greater number than those with inflammatory bowel disease such as Crohn's disease or ulcerative colitis.

All divers should be free of conditions and illnesses that would negatively impact their safety and well-being underwater. The diving medical physician should be able to identify, treat and advise divers about illnesses and conditions that would cause them to be at increased risk for a diving accident.

Some reasons why a person should not be allowed to dive are as follows:

- Disorders that lead to altered consciousness: conditions that produce reduced awareness or sedation from medication, drugs, marijuana or alcohol; fainting, heart problems and seizure activity.

- Disorders that substantially increase the risk of barotrauma injury conditions or diseases that are associated with air trapping in closed spaces, such as sinuses, middle ear, lungs and gastrointestinal tract. Severe asthma is an example.

- Disorders that may lead to erratic and irresponsible behavior: included here would be immaturity, psychiatric disorders, diving while under the influence of medications, drugs and alcohol or any medical disorder that results in cognitive defects.

Conditions that may increase risk of diving disorders:

- Patent foramen ovale

- Diabetes mellitus — No serious problems should be expected during dives due to hypoglycaemia in divers with well-controlled diabetes. Long-term complications of diabetes should be considered and may be a contrindication.

- Asthma

Conditions considered temporary reasons to suspend diving activities:

- Pregnancy—It is unlikely that literature research can establish the effect of scuba diving on the unborn human fetus as there is insufficient data, and women tend to comply with the diving industry recommendation not to dive while pregnant.

Ionizing radiation is classified as a neurotoxicant. A 2004 cohort study concluded that irradiation of the brain with dose levels overlapping those imparted by computed tomography can, in at least some instances, adversely affect intellectual development. Prenatal exposure to ionizing radiation at the 8-15 and 16–25 weeks after ovulation was found to induce severe mental retardation as well as variation in intelligence quotient (IQ) and school performance. It is uncertain, if there exist a threshold, under which one or more of these effects, of prenatal exposure to ionizing radiation, do not exist. Cumulative equivalent doses above 500 mSv of ionizing radiation to the head were proven with epidemiological evidences to cause cerebro-vascular atherosclerotic damage. The equivalent dose of 500 mGy x-rays is 500 mSv.

Radiation therapy was found to cause cognitive decline. Cognitive decline was especially apparent in young children, between the ages of 5 to 11. Studies found, for example, that the IQ of 5-year-old children declined each year after treatment by additional several IQ points, thereby the child's IQ decreased and decreased while growing older.