This was the most serious accident in U.S. commercial nuclear power plant operating history, although its small radioactive releases had no detectable health effects on plant workers or the public. The approximately 2 million people around TMI-2 during the accident are estimated to have received an average radiation dose of only about 1 millirem above the usual background dose. However, comprehensive investigations and assessments by several well respected organizations, such as Columbia University and the University of Pittsburgh, have concluded that in spite of serious damage to the reactor, the actual release had negligible effects on the physical health of individuals or the environment.

No injuries, deaths or direct health effects were caused by the accident. Experts determined that the approximately 2 million people in the nearby area during the accident were exposed to small amounts of radiation. It was determined that very low levels could be attributed to the accident and that the radioactive release had negligible effects on the physical health of the individuals or the environment.

This report contains a preliminary assessment of the radiation dose and potential health impact of the accident on March 28, 1979 at the Three Mile Island Nuclear Station. The **estimated dose that might have been received by an individual is less than 100 mrem**. The **collective dose received by the 2,164,000 people estimated to live within 50 miles of the reactor site is calculated to be 3,300 person-rem** (with a range of 1600 to 5300 person-rem). This corresponds to an **average dose of approximately 1.5 mrem**. The **potential number of fatal cancers that is projected to occur as a result of the accident is less than 1**. This potential impact would be **undetectable compared to the 325,000 cancer deaths that would normally be expected** to occur in a population of 2,164,000. The **estimated total health impact, including fatal and non-fatal cancers and genetic effects to all future generations is approximately 2 health effects**.

The Committee on Federal Research into the Biological Effects of Ionizing Radiation (BEIR), through its Three Mile Island Follow-up Research Subcommittee, reviewed follow-up studies on biological and health effects resulting from the Three Mile Island accident. The report states that, based on the low estimated radiation doses, the accident "would produce no detectable health effects" in the exposed population and agrees with the President's Commission that any health effects from the accident "would be negligible." The committee also noted that the principal health impact identified was psychological stress rather than physical health damage.

In a section titled "Columbia TMI Study" the slides describe the design as a comparison of cancer rates through 1985 for 69 small geographic areas by dispersion-model-based estimates of radioactive emissions, focusing on leukemia and childhood malignancies, and list the findings/conclusions as: "No clear associations of estimated emissions with radiosensitive cancer types and population subgroups – Hence, no convincing evidence that accident releases influenced cancer risk (Hatch et al., 1991)." The slides summarize Pennsylvania Department of Health and University of Pittsburgh studies as finding "No impact on cancer mortality overall" with some dose‑response trends for specific cancers, and "No increase overall" in cancer incidence with an "Increased risk of leukemia in men" in one study, indicating largely low or non‑detectable overall population health effects with some areas of scientific uncertainty.

There were no injuries or adverse health effects from the Three Mile Island accident. Investigations by the U.S. Nuclear Regulatory Commission and several independent agencies concluded that the average radiation dose to people living within 10 miles of the plant was about 0.08 millisieverts (8 millirems), and that this was a small fraction of the annual background dose. Epidemiological studies and health assessments have not shown any statistically significant increase in cancer or other health problems that could be attributed to the accident.

This joint NRC–EPA report investigated reported problems with plants and animals that might be related to the operation of and accident at Three Mile Island. It concludes: "No reasonable connection could be made between the operation of TMI and the health problems of livestock and pets that were brought to the attention of the staff of the Nuclear Regulatory Commission. The same holds true for the aftermath of the accident." The report notes that the worst-case calculated radiation exposure at Middletown (100 mrem) was less than one‑thousandth of that which might have caused clinically detectable effects in the exposed animal population, and states that the Pennsylvania Department of Agriculture "has found no evidence to connect" reported animal health problems with TMI.

Previous studies concluded that there was no evidence that the 1979 nuclear accident at Three Mile Island (TMI) affected cancer incidence in the surrounding area; however, there were logical and methodological problems in earlier reports that led us to reconsider data previously collected. Results support the hypothesis that radiation doses are related to increased cancer incidence around TMI. These associations would not be expected, based on previous estimates of near-background levels of radiation exposure following the accident.

This government report by the Ad Hoc Population Dose Assessment Group, including experts from the NRC, HHS, and EPA, evaluated radiation doses and potential health impacts from the Three Mile Island accident. The authors estimated that the average individual dose to the surrounding population was on the order of a few millirem and concluded that there were "no immediate health effects" and that latent or long‑term effects "if any, would be minimal" and likely undetectable against background rates of disease. The report emphasizes that the low magnitude of releases and resulting doses is the primary reason that any potential health effects would be expected to be very small.

Several studies of the population living near the Three Mile Island nuclear power plant have failed to show consistent or large increases in cancer that could be linked to the 1979 accident. In our analysis of cancer mortality and morbidity for the period 1979–1998, we observed no overall excess of cancer deaths in the counties closest to the plant compared with expected rates, although small increases in certain cancer sites in specific subgroups could not be definitively attributed to radiation exposure from the accident.

In this Pennsylvania Department of Health study (Talbott et al.), investigators examined mortality among residents living near the Three Mile Island nuclear power plant from 1979 to 1998. The abstract reports that "Overall, no consistent evidence was found that the accident increased risks of death from cancer" for the population as a whole, although the authors observed some dose‑response trends for specific cancers, such as breast cancer and lymphatic/hematopoietic cancers, that warranted further investigation. The study concludes that while the findings do not indicate a large overall impact on cancer mortality, they cannot entirely rule out small increases in risk for certain subgroups.

Because of concerns about radiation-induced health effects, the Pennsylvania Department of Health maintained for 18 years a registry of more than 30,000 people who lived within five miles of Three Mile Island at the time of the accident. The state's registry was discontinued in June 1997 **without any evidence of unusual health trends**. Indeed, **more than a dozen major, independent health studies of the accident showed no evidence of any abnormal number of cancers around TMI years after the accident**. The only detectable effect was **psychological stress during and shortly after the accident**. The studies found that the **radiation releases during the accident were minimal, well below any levels that have been associated with health effects from radiation exposure**. The **average radiation dose to people living within 10 miles of the plant was eight millirem, with no more than 100 millirem to any single individual**.

This University of Pittsburgh study (Han et al.) assessed cancer incidence from 1982 to 1995 among residents near Three Mile Island, using modeled radiation exposure categories. The authors report that "No overall increase in cancer incidence was observed in the study cohort" when comparing higher‑ and lower‑exposure groups, but they found "an increased risk of leukemia among men" that showed a positive trend with estimated dose. They caution that the leukemia finding was based on small numbers and potential exposure misclassification, and they conclude that the data provide limited evidence of increased leukemia risk in a subgroup but do not support a substantial overall increase in cancer attributable to the accident.

Within a short time, there was a well-recognized impact of the TMI accident by public health officials and academicians upon mental health of nearby residents—particularly pregnant women. Despite long-term and detailed follow-up of this population for cancer outcomes, community concerns continue. Overall, the accumulated epidemiologic evidence has not demonstrated large increases in cancer incidence or mortality that can be clearly attributed to radiation from the TMI accident, though some studies have reported small elevations in specific cancers and ongoing uncertainties remain.

This technical summary describes major health studies following the Three Mile Island accident, including work by the Ad Hoc Population Dose Assessment Group (NRC, HHS, EPA) and the BEIR follow‑up committee. It states that the federal population dose assessment concluded there were "no immediate health effects, and that latent or long‑term effects, if any, would be minimal," and that the BEIR follow‑up study "concluded that the accident would produce no detectable health effects." The article notes that the Hatch–Susser Columbia University study found that in all instances "the level of exposure was deemed to be very low" (average ~10 millirem, projected maximum 100 millirem) and that its expectation of "no excess cancer" was confirmed. It adds that the Pennsylvania Department of Health and subsequent National Cancer Institute work found no increased cancer risks to residents near TMI, and that the most important health effect identified was mental stress.

In the case of Three Mile Island, subsequent studies showed that the amount of radiation released was small and the resultant doses to the public were comparable to natural background fluctuations. Health surveys and epidemiological research did not demonstrate measurable increases in cancer or other diseases in the surrounding population that could be linked to the accident, although fear and mistrust of nuclear technology increased substantially.

Officials say the amount of radiation released into the atmosphere as a result of the partial meltdown was well within accepted levels – far less exposure than a person might receive during a medical procedure. A 1990 Pennsylvania Department of Health study did not find an overall increase in cancer deaths in the area that could be attributed to the accident, though some residents and researchers have continued to question whether small clusters of cancer might be related.

By the 2000s, reviews by U.S. regulatory agencies, public health departments, and independent academic groups generally converged on the conclusion that the off-site radiation doses from Three Mile Island were low and that any resulting increases in cancer risk, if present, would be too small to detect epidemiologically. A minority of researchers have argued for possible underestimation of doses and associations with cancer clusters, but these findings have not shifted the overall consensus view in radiological protection and epidemiology that the health and environmental impacts of the accident were very limited.