1. THORIUM A CARCINOGEN Kaitlyn Hepp
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Running header: THORIUM A CARCINOGEN
A Review of Thorium Exposure Health Risks
Abstract
The naturally occurring radioactive material thorium causes cancer in humans. Studies on the effect
of medical injections of thorium in the commercial form of thorotrast found connections to hepatic
cancers due to deposits formed in the liver. Occupational exposure studies discovered an increased
occurrence of lung cancer in thorium-exposed workers. Assessments of commercial products and the
thorium released from industrial sites found public exposure to exist, but was inconclusive on the
significance of exposure levels. Future research is necessary to assess public exposures to allow for
the correct amount of regulation on thorium’s use in commercial products to be established.
Introduction
Thorium appears in today’s market in products ranging from metals in the aerospace industry
to ceramics and is being explored for additional potential uses of its radioactive nature. The most
invested-in possibilities for thorium tech include nuclear reactors and radiopharmaceutical
technologies. Workers exposed to thorium through the production of thorium-tech and patients
exposed purposefully constitute the main group concerned in the examination of thorium-related
health effects. Concern around thorium first arose from findings that thorotrast (commercial name for
a thorium compound that was popularly used as an x-ray contrast medium during 1930s to 1950s)
was connected to hepatic cancers. Since then, additional studies were conducted around exposure due
to the mining and processing of thorium and consumer products that contain this radioactive material.
This review will examine the connection between thorium exposures and the risk of hepatic and lung
cancers as found in various studies.
Research Review
Studies on the health effects of thorotrast injections found a connection to deposits in the
liver and hepatic cancers. Manabu Fukumoto addressed the effects of thorotrast injection through

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studying archival materials from thorotrast patients (Fukumoto, 2014). The archival materials
included tissues from thorotrast induced intrahepatic cholangiocarcinoma (T-ICC) tumors and non-
cancerous tissues from the same thorotrast injected patients. By analyzing said tissues, as well as
sections of non-thorotrast induced intrahepatic cholangiocarcinoma (non-T-ICC) tumors that weren’t
exposed to radiation or chemotherapy, Fukumoto isolated several characteristics of T-ICC tumors.
This included a lower rate of K-ras gene mutations in T-ICC compared to non-T-ICC and a higher
rate of p53 mutations with indications that the p53 mutations weren’t a direct result of the α-particles
released from the thorium, but an effect from thorotrast deposits. Through the examination of the p53
mutations, Fukumoto provided a strong case for thorotrast inducing genetic instability and clonal
mutations. Via reviewing the cases of thorotrast patients, he also discovered an incubation period of
at least 20 years for the development of the cancer. The total dose of thorotrast wasn’t an important
factor in the incubation period, but dose rates did affect the time lapse with higher dose rates leading
to shorter incubations. The study concluded cancer inducement to be affected more by the incubation
period than the total dosage. The minimum 20-year incubation period was supported in a case study
performed by Rakov, Smalldon, and Derman from the Kingston Hospital (1963). They reviewed
reports from examinations, collected history of employment and previous environments, and
examined necropsy reports on a white male injected with thorotrast in 1936 who later died of hepatic
cancer in 1960. As well as exhibiting a 23-year incubation period, the patient demonstrated deposits
of thorotrast in the liver and a resulting hemangioendotheliosarcoma. Although the patient fell into
the at-least-20-year incubation period supported by Fukumoto, Rakov et al. (1963) cited a 1960 study
by Looney that reported an incubation period of 15 years. In all studies, there was a clear resultant of
hepatic cancers from the injection of thorotrast.
Beyond the injection of thorotrast, occupational exposure to thorium is a point of interest for
research. Studies found the exposure to thorium in the workplace to center around inhalation of
thorium-contaminated dust and the resulting health effects to be concentrated in the lungs. Chen,

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Cheng and Rong completed a case control study on miners in China who experienced occupational
exposure to thorium-bearing dust (Chen et al., 2005). They performed physicals on 136 randomly
selected miners, all men, 64 from high-dust producing workshops and 72 from low-dust producing
workshops that acted as an internal control. They also gathered information on death rates from lung
cancer for all mine employees. The difference in thorium lung burdens between the two groups was
1.13± .22 Bq, with the high-dust generating workshop miners having the higher lung burdens at 1.71
± 0.18 Bq. Severe breathlessness was significantly higher in the 64 miners from the crushing
workshop although chronic coughing and phlegm had no significant difference between the groups.
The groups had a highly significant difference in incidences of pneumoconiosis of stage 0+; the
crushing group with an 18.75% occurrence rate, while the control group had 1.4%. In studying the
hepatic parameters for the groups, all average values were within a normal range. The depositing of
thorium in the liver of thorium inhaled is thought to be very small due to the rate of thorium dioxide
(the compound used in thorotrast) absorption into the blood being only 0.0001 d-1
. The study on lung
cancer mortalities in the mine staff showed a raised rate in exposed workers in comparison to
unexposed workers attributed to the inhalation of dusts containing thorium, silica, and thoron
progeny. A 2009 systematic review of 133 articles by David Vearrier, John Curtis, and Michael
Greenberg (Vearrier et al., 2009) looked at radiation exposures to employees working with
technologically enhanced naturally occurring radioactive materials (TENORM). The exposure to ore
was estimated to be between 1 and 10 kBq/kg, which would result in an effective dose of 1-2 mSv
each year. Workers making gas mantles received between 1 and 10 mSv/year where the use of
thorium oxide welding rods lead to .15 mSv/year and the recycling of thorium lamps and welding
rods caused .3 mSv/year. Most occupational exposures to TENORM are minimal and not significant
enough for a conclusive link, but cancer has been connected to certain exposures of which workers
are at risk.

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Public exposure to thorium through average air concentrations and thorium in food and water
supplies is considered to be negligible according the CDC’s 1990 public health statement (CDC,
1990). That being noted, scientists attempted to identify the public exposure to thorium resulting
from its use in consumer products and dispersal from TENORM activities. Do Hyeon Yoo and his
colleagues performed measurements of five different consumer paints and then calculated annual
effective doses using the Monte Carlo method to assess the risks of increasing appearances of
TENORM in consumer products (Yoo et al., 2015). The most detected radionuclides in the paints
were 228-Ac and 212-Pb produced from the thorium decay chain. The radioactivity emitted from all
the radionuclides in the paint was calculated into the annual effective dose to humans in three
different positions: standing in the center of the room, standing 30 cm from the wall, and lying in the
center of the room. Standing 30cm from the wall was twice the effective dose than standing in the
center, but all effective doses for all paints were under the 1 mSv y-1
public dose limit. In the Vearrier
et al. study, there was an assessment of population exposures to TENORM (Vearrier et al., 2009).
They cited the global average for effective doses from natural sources to be 2.4 mSv/year. They
found that external exposure to gamma radiation to populations located near TENORM industrial
sites wasn’t significant. Internal exposure differed based on the material and what type of production
was called into question. Inhalation of radionuclides from gas mantles used 20 hrs/year can result in
exposures of 100 μSv to children and 50 μSv to adults. Population-wide exposure is estimated to
average 1–10 μSv/year from all TENORM materials; data on solely thorium exposure wasn’t
available. Both studies found the public exposure to thorium to be minimal.
Analysis of the Findings
It’s conclusive that high thorium exposures are a human carcinogen. There is an abundance
of research connecting thorotrast injections to hepatic cancers. Although there are discrepancies in
the believed necessary incubation period, all studies found it to take over a 15 year latent period for
cancer to develop. This conclusion makes the differentiation between negligible exposure and

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exposure where there are no observable adverse effects in the time studied crucial in the
understanding and interpretation of research on thorium. Further research should work to assess what
exposure can be considered negligible as there is no current consensus beyond the public safety limit
recommendations from the National Council on Radiation Protection and International Council on
Radiation Protection, which are known to differ from each other. A uniform understanding of the
impact of exposure to thorium would also benefit to additional studies in occupational and public
exposures. Studies in occupation had a narrow focus on mining and ore processing industries without
providing conclusive data on other industrial fields, and the public exposures don’t have sufficient
data to conclusively identify if there is an impact on the populace from thorium exposure and if so
what that impact is. The studies surrounding general populace exposures are working on the
feasibility of measurement methods, not real-world measurements and implications. Bringing the
methods found accurate to the usage in research on said implications would be a beneficial next step.
Conclusion
Through the research discussed in this paper it’s clear that thorium is a carcinogen. The
injection of thorium into the body, as done in the use of thorotrast, increases the risk of hepatic
cancers while occupational exposures are most likely to be due to inhalation and result in increased
lung cancer. Further study needs to be done on occupational exposures in industries outside of
mining and ore processing. The exposure to the public is the area most lacking in conclusive data and
also needs to be further researched. An understanding of negligible exposure levels will assist in
efficient and effective regulations. Currently, US regulations are minimal and enacted by the state
governments (Vearrier et al. 2009). Industries and products thought to be below the public safety
limit don’t undergo active regulation. The development of safety controls on consumer products is
needed with the admonition that it will need to be amended to reflect further research on general
exposure amounts and impacts.

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References
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Fukumoto, M. (2014). Radiation pathology: From thorotrast to the future beyond
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Rakov, H. L., Smalldon, T. R., & Derman, H. (1963). Hepatic hemangioendotheliosarcoma: Report
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