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PUBLIC HEALTH STATEMENT 

THORIUM 

CAS#: 7440-29-1 

Division of Toxicology October 1990 

This Public Health Statement is the summary 

chapter from the Toxicological Profile for Thorium. 

It is one in a series of Public Health Statements 

about hazardous substances and their health effects. 

A shorter version, the ToxFAQsTM, is also 

available. This information is important because 

this substance may harm you. The effects of 

exposure to any hazardous substance depend on the 

dose, the duration, how you are exposed, personal 

traits and habits, and whether other chemicals are 

present. For more information, call the ATSDR 

Information Center at 1-888-422-8737. 

This Statement was prepared to give you 

information about thorium and to emphasize the 

human health effects that may result from exposure. 

At this time, thorium has been found above 

background levels at 16 out of 1177 National 

Priorities List (NPL) hazardous waste sites. We do 

not know how many of the 1,177 NPL sites have 

been evaluated for thorium. As EPA evaluates more 

sites, the number of sites at which thorium is found 

at above background levels may change. Because 

these sites are potential or actual sources of human 

exposure to thorium and because thorium may 

cause harmful health effects, this information is 

important for you to know. 

When a radioactive chemical is released from a 

large area, such as an industrial plant, or from a 

container, such as a drum or bottle, it enters the 

environment as a radioactive chemical emission. 

This emission, which is also called a release, does 

not always lead to exposure. You are exposed only 

when you come into contact with the radioactive 

chemical. You can come into contact with it in the 

environment through breathing air, eating, drinking 

or smoking substances containing the radioactive 

chemical. Exposure may also result from skin 

contact with the radioactive chemical alone, or with 

a substance containing it. Exposure can also occur 

by being near radioactive chemicals at 

concentrations that may be found at hazardous 

waste sites or industrial accidents. 

If you are exposed to a hazardous chemical, several 

factors determine whether harmful effects will 

occur and the type and severity of those health 

effects. These factors include the dose (how much), 

the duration (how long), the pathway by which you 

are exposed (breathing, eating, drinking, or skin 

contact), the other chemicals to which you are 

exposed, and your individual characteristics such as 

age, sex, eating habits, family traits, life style, and 

state of health. 

1.1 WHAT IS THORIUM? 

Thorium is a naturally-occurring, radioactive metal. 

Small amounts of thorium are present in all rocks, 

soil, above-ground and underground water, plants, 

and animals. These small amounts of thorium 

contribute to the weak background radiation for 

such substances. Soil commonly contains an 

average of about 6 parts of thorium per million parts 

(ppm) of soil. Rocks in some underground mines 

may also contain thorium in a more concentrated 

form. After these rocks are mined, thorium is 

usually concentrated and changed into thorium 

dioxide or other chemical forms. Thorium-bearing 

rock that has had most of the thorium removed from 

it is called "depleted" ore or tailings. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

THORIUM 

CAS#: 7440-29-1 

Division of Toxicology October 1990 

More than 99% of natural thorium exists in the form 

(isotope) thorium-232. Besides this natural thorium 

isotope, there are more than 10 other different 

isotopes that can be artificially produced. In the 

environment, thorium-232 exists in various 

combinations with other minerals, such as silica. 

Most thorium compounds commonly found in the 

environment do not dissolve easily in water and do 

not evaporate from soil or water into the air. 

The thorium isotope-232 is not stable. It breaks 

down into two parts. This process of breaking down 

is called decay. The decay of thorium-232 produces 

a small part called "alpha" radiation and a large part 

called the decay product. The decay product of 

thorium-232 also is not stable. Like thorium-232, it 

in turn breaks down to an unstable isotope and the 

process continues until a stable product is formed. 

During these decay processes, the parent thorium232, its decay products, and their next decay 

products produce a series of new substances 

(including radium and radon), alpha and beta 

particles, and gamma radiation. The alpha particles 

can travel only very short distances through most 

materials and cannot go through human skin. The 

gamma radiation can travel farther and can easily go 

through human skin. The decay of thorium-232 into 

its decay products happens very slowly. In fact, it 

takes about 14 billion years for half the thorium-232 

to change into new forms. Fourteen billion years is 

called the radioactive half-life of thorium-232. 

Due to the extremely slow rate of decay, the total 

amount of natural thorium in the earth remains 

almost the same, but it can be moved from place to 

place by nature and people. For example, when 

rocks are broken up by wind and water, thorium or 

its compounds becomes a part of the soil. When it 

rains, the thorium-containing soil can be washed 

into rivers and lakes. Also, activities such as 

burning coal that contains small amounts of 

thorium, mining or milling thorium, or making 

products that contain thorium also release thorium 

into the environment. Smaller amounts of other 

isotopes of thorium are produced usually as decay 

products of uranium-238, uranium-235, and 

thorium-232, and as unwanted products of nuclear 

reactions. 

Thorium is used to make ceramics, lantern mantles, 

and metals used in the aerospace industry and in 

nuclear reactions. Thorium can also be used as a 

fuel for generating nuclear energy. More than 30 

years ago thorium oxides were used in hospitals to 

make certain kinds of diagnostic X-ray 

photographs. 

1.2 HOW MIGHT I BE EXPOSED TO 

THORIUM? 

Since thorium is found almost everywhere, you will 

be exposed to small amounts of it in the air you 

breathe and in the food and water you eat and drink. 

Scientists know, roughly, the average amounts of 

thorium in food and drinking water. Most people in 

the United States eat some thorium with their food 

every day. Normally, very little of the thorium in 

lakes, rivers, and oceans gets into the fish or 

seafood we eat. The amounts in the air are usually 

so small that they can be ignored. 

There may be more thorium than normal near an 

uncontrolled hazardous waste site in which thorium 

has not been disposed of properly. Consequently, 

you may be exposed to slightly more thorium if you 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

THORIUM 

CAS#: 7440-29-1 

Division of Toxicology October 1990 

live near one of these sites because you could 

breathe windblown dust containing thorium or eat 

food grown in soil contaminated with thorium. 

Children playing near a waste site could get thorium 

into their bodies if they eat contaminated soil. You 

could also be exposed to more thorium than normal 

if you work in an industry that mines, mills, or 

manufactures products containing thorium, or work 

in a research laboratory performing experiments 

with thorium. Larger-than-normal amounts of 

thorium might also enter the environment through 

accidental releases from thorium processing plants. 

1.3 HOW CAN THORIUM ENTER AND 

LEAVE MY BODY? 

Only a small amount of the thorium that you 

breathe or swallow in food, water, or soil enters 

your blood. One animal study has shown that 

thorium can enter the body if it is placed on the 

skin. After breathing thorium, you will usually 

sneeze, cough, or breathe out some of it within 

minutes. Some forms of thorium can stay in your 

lungs for long periods of time. However, in most 

cases, the small amount of thorium left in your 

lungs will leave your body in the feces and urine 

within days. After you eat or drink thorium, almost 

all of it leaves your body in the feces. The small 

amount of thorium left in your body may enter your 

bones from the blood and stay there for many years. 

The main way thorium will enter your body is by 

breathing dust contaminated with thorium. 

1.4 HOW CAN THORIUM AFFECT MY 

HEALTH? 

Studies of thorium workers have shown that 

breathing thorium dust may cause an increased 

chance of developing lung disease and cancer of the 

lung or pancreas many years after being exposed. 

Changes in the genetic material of body cells have 

also been shown to occur in workers who breathed 

thorium dust. Liver diseases and effects on the 

blood have been found in people injected with 

thorium in order to take special X-rays. 

Many types of cancer have also been shown to 

occur in these people many years after thorium was 

injected into their bodies. Since thorium is 

radioactive and may be stored in bone for a long 

time, bone cancer is also a potential concern for 

people exposed to thorium. 

Animal studies have shown that breathing in 

thorium may result in lung damage. Other studies in 

animals suggest drinking massive amounts of 

thorium can cause death from metal poisoning. The 

presence of large amounts of thorium in your 

environment could result in exposure to more 

hazardous radioactive decay products of thorium, 

such as radium and thoron, which is an isotope of 

radon. Thorium is not known to cause birth defects 

or to affect the ability to have children. 

1.5 WHAT LEVELS OF EXPOSURE HAVE 

RESULTED IN HARMFUL HEALTH 

EFFECTS? 

Thorium is odorless and tasteless, so you cannot tell 

if you are being exposed to thorium. We know very 

little about specific exposure levels of thorium that 

result in harmful effects in people or animals. High 

levels of exposure have been shown to cause death 

in animals, but no direct cause of death could be 

determined and no other health effects have been 

reported. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov 
 

 

 

 

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PUBLIC HEALTH STATEMENT 

THORIUM 

CAS#: 7440-29-1 

Division of Toxicology October 1990 

1.6 IS THERE A MEDICAL TEST TO 

DETERMINE WHETHER I HAVE BEEN 

EXPOSED TO THORIUM? 

Special tests that measure the level of radioactivity 

from thorium or thorium isotopes in your urine, 

feces, and air you breathe out can determine if you 

have been exposed to thorium. These tests are 

useful only if run within several days to a week 

after exposure. The tests cannot, however, tell you 

if your health will be affected by the exposure. The 

tests can be run only with special equipment and are 

probably not available at your local clinic or 

hospital. 

1.7 WHAT RECOMMENDATIONS HAS THE 

FEDERAL GOVERNMENT MADE TO 

PROTECT HUMAN HEALTH? 

The EPA has set a drinking water limit of 15 

picocuries per liter (15 pCi/L) of water for gross 

alpha particle activity and 4 millirems per year for 

beta particles and photon activity (for example, 

gamma radiation and X-rays). 

The federal recommendations have been updated as 

of July 1999. 

1.8 WHERE CAN I GET MORE 

INFORMATION? 

If you have any more questions or concerns, please 

contact your community or state health or 

environmental quality department or: 

Agency for Toxic Substances and Disease Registry 

Division of Toxicology 

1600 Clifton Road NE, Mailstop F-32 

Atlanta, GA 30333 

Information line and technical assistance: 

Phone: 888-422-8737 

FAX: (770)-488-4178 

ATSDR can also tell you the location of 

occupational and environmental health clinics. 

These clinics specialize in recognizing, evaluating, 

and treating illnesses resulting from exposure to 

hazardous substances. 

To order toxicological profiles, contact: 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

Phone: 800-553-6847 or 703-605-6000 

Reference 

Agency for Toxic Substances and Disease Registry 

(ATSDR). 1990. Toxicological profile for thorium. 

Atlanta, GA: U.S. Department of Health and 

Human Services, Public Health Service. 

DEPARTMENT of HEALTH AND HUMAN SERVICES, Public Health Service
 

Agency for Toxic Substances and Disease Registry 

www.atsdr.cdc.gov/ Telephone: 1-888-422-8737 Fax: 770-488-4178 E-Mail: atsdric@cdc.gov