Public Health Statement for Selenium
Spanish: Selenio
CAS#: 7782-49-2
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This Public Health Statement is the summary chapter from the Toxicological Profile for Selenium. It is one in a series of Public Health Statements about hazardous substances and their health effects. A shorter version, the ToxFAQs™, 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 1-800-232-4636.
This public health statement tells you about selenium and the effects of exposure.
The Environmental Protection Agency (EPA) identifies the most serious hazardous waste sites in the nation. These sites make up the National Priorities List (NPL) and are the sites targeted for long-term federal cleanup activities. Selenium has been found in at least 508 of the 1,623 current or former NPL sites. However, the total number of NPL sites evaluated for selenium is not known. As more sites are evaluated, the sites at which selenium is found may increase. This information is important because exposure to selenium at high levels may harm you and because these sites may be sources of exposure. A minimum dietary level of selenium is required for good health.
When a substance 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. This release
does not always lead to exposure. You are exposed to a substance
only when you come in contact with it. You may be exposed
by breathing, eating, or drinking the substance, or by skin
contact.
If you are exposed to selenium, many
factors determine whether you'll be harmed. These factors
include the dose (how much), the duration (how long), and
how you come in contact with it/them. You must also consider
the other chemicals you're exposed to and your age, sex, diet,
family traits, lifestyle, and state of health.
What is selenium?
Selenium is a naturally occurring, solid
substance that is widely but unevenly distributed in the earth's
crust. It is also commonly found in rocks and soil. Selenium,
in its pure form of metallic gray to black crystals, is often
referred to as elemental selenium or selenium dust. Elemental
selenium is commercially produced, primarily as a by-product
of copper refining. Selenium is not often found in the environment
in its elemental form, but is usually combined with other
substances. Much of the selenium in rocks is combined with
sulfide minerals or with silver, copper, lead, and nickel
minerals. Selenium also combines with oxygen to form several
substances that are white or colorless crystals. Some selenium
compounds are gases. Selenium and its compounds are used in
some photographic devices, gun bluing (a liquid solution used
to clean the metal parts of a gun), plastics, paints, anti-dandruff
shampoos, vitamin and mineral supplements, fungicides, and
certain types of glass. For example, selenium sulfide is used
in anti-dandruff shampoos by the common trade name Selsun
Blue. Selenium is also used to prepare drugs and as a nutritional
feed supplement for poultry and livestock. More information
on the chemical and physical properties, production, and uses
of selenium are found in Chapters 4 and 5.
What happens to selenium when it enters the environment?
Selenium occurs naturally in the environment.
As an element, selenium cannot be created or destroyed, although
selenium can change forms in the environment. Weathering of
rocks and soils may result in low levels of selenium in water,
which may be taken up by plants. Weathering also releases
selenium into the air on fine dust-like particles. Volcanic
eruptions may release selenium in air. Selenium commonly enters
the air from burning coal or oil. Selenium that may be present
in fossil fuels combines with oxygen when burned, which may
then react with water to form soluble selenium compounds.
Airborne particles of selenium, such as in ash, can settle
on soil or surface water. Disposal of selenium in commercial
products and waste could also increase the amount of selenium
in soil. The forms and fate of selenium in soil depend largely
on the acidity of the surroundings and its interaction with
oxygen. In the absence of oxygen when the soil is acidic,
the amount of selenium that can enter plants and organisms
should be low. Elemental selenium that cannot dissolve in
water and other insoluble forms of selenium are less mobile
and will usually remain in the soil, posing smaller risk of
exposure. Selenium compounds that can dissolve in water are
sometimes very mobile. Thus, there is an increased chance
of exposure to these compounds. Selenium may enter surface
water in irrigation drainage waters. Some evidence indicates
that selenium can be taken up in tissues of aquatic organisms
and possibly increase in concentration as the selenium is
passed up through the food chain. Selenium concentrations
in aquatic organisms have been a problem as a result of irrigation
runoff in some dry areas of the United States. Chapter 6 contains
more information on what happens to selenium in the environment.
How might I be exposed to selenium?
People are exposed to low levels of selenium
daily through food, water, and air. Selenium is also an essential
nutrient for humans and animals. However, selenium can be
harmful when regularly taken in amounts higher than those
needed for good health. People receive the majority of their
daily intake of selenium from eating food, and to a lesser
extent, from water intake. Estimates of the average intake
of selenium from food for the U.S. population range from 71
to 152 millionths of a gram of selenium per person per day.
Low levels of selenium can also be found in drinking water.
Selenium levels are less than 10 parts of selenium in a billion
parts of water (10 ppb) in 99.5% of drinking water sources
tested. People may be exposed to higher-than-normal levels
of selenium at hazardous waste sites by swallowing soil or
water, or by breathing dust. In some parts of the United States,
especially in the western states, some soils naturally have
higher levels of selenium compounds. Some plants can build
up selenium to levels that harm livestock feeding on them.
In these areas, people could be exposed to too much selenium
if they eat a lot of locally grown grains and vegetables or
animal products that have built up high levels of selenium.
People may also be exposed to selenium from industrial sources.
Humans are normally not exposed to large amounts of selenium
in the air, unless selenium dust or volatile selenium compounds
are formed in their workplace. Occupations in which humans
may be exposed to selenium in the air are the metal industries,
selenium-recovery processes, paint manufacturing, and special
trades. Chapter 6 contains more information on how people
can be exposed to selenium.
How can selenium enter and leave my body?
Selenium from the environment mainly
enters the body when people eat food containing selenium.
The human body easily absorbs the organic selenium compounds
(for example, selenoamino acids) when eaten, and makes them
available where needed in the body. The selenium in drinking
water is usually in the form of inorganic sodium selenate
and sodium selenite; these forms of selenium are also easily
absorbed from the digestive tract. The human body can change
these inorganic selenium compounds into forms that it can
use. Selenium in the air may also enter your body when you
breathe it.
Hazardous waste sites at which selenium
is present could represent a major source of exposure. The
way that selenium can enter the body from a particular site
depends on such factors as whether vegetables are grown in
soil in which selenium from the site has been deposited, whether
water at the site contains selenium and is able to flow into
drinking water supplies, and whether selenium dust blows into
the air. As mentioned earlier, specific conditions at a site
can greatly influence which selenium compounds form and whether
they can move in the environment to places where people might
be exposed. Therefore, it is important to know that the presence
of selenium at a site does not necessarily mean that people
are being exposed to it. Specific tests of locally grown food,
drinking water, and air must be done to find out whether exposure
is occurring. You should also be aware that selenium compounds,
including those used in some medicated dandruff shampoos,
are not easily absorbed through the skin.
Most of the selenium that enters the
body quickly leaves the body, usually within 24 hours. Beyond
what the body needs, selenium leaves mainly in the urine,
but also in feces and breath. Selenium in the urine increases
as the amount of the exposure goes up. Selenium can build
up in the human body, however, if exposure levels are very
high or if exposure occurs over a long time. The amount that
builds up in the body depends on the chemical form of the
selenium. It builds up mostly in the liver and kidneys but
also in the blood, lungs, heart, and testes. Selenium can
build up in the nails and in hair, depending on time and amount
of exposure. Chapter 3 contains more information on how selenium
enters and leaves the human body.
How can selenium affect my health?
To protect the public from the harmful
effects of toxic chemicals and to find ways to treat people
who have been harmed, scientists use many tests.
One way to see if a chemical will hurt
people is to learn how the chemical is absorbed, used, and
released by the body; for some chemicals, animal testing may
be necessary. Animal testing may also be used to identify
health effects such as cancer or birth defects. Without laboratory
animals, scientists would lose a basic method to get information
needed to make wise decisions to protect public health. Scientists
have the responsibility to treat research animals with care
and compassion. Laws today protect the welfare of research
animals, and scientists must comply with strict animal care
guidelines.
The general public rarely breathes high
levels of selenium, although some people may be exposed to
selenium dust and selenium compounds in workplace air. Dizziness,
fatigue, and irritation of mucous membranes have been reported
in people exposed to selenium in workplace air at concentrations
higher than legal levels. In extreme cases, collection of
fluid in the lungs (pulmonary edema) and severe bronchitis
have been reported. The exact exposure levels at which these
effects might occur are not known, but they become more likely
with increasing amounts of selenium and with increasing frequency
of exposure.
The normal intake of selenium by eating
food is enough to meet the Recommended Daily Allowance (RDA)
for this essential nutrient. However, as discussed in Chapters
2 and 3 of this profile, selenium compounds can be harmful
at daily dietary levels that are higher than needed. The seriousness
of the effects of excess selenium depends on how much selenium
is eaten and how often. Intentional or accidental swallowing
of a large amount of sodium selenate or sodium selenite (for
example, a very large quantity of selenium supplement pills)
could be life-threatening without immediate medical treatment.
Even if mildly excessive amounts of selenium are eaten over
long periods, brittle hair and deformed nails can develop.
In extreme cases, people may lose feeling and control in arms
and legs. These health effects, called selenosis, were seen
in several villages in China where people were exposed to
foods high in selenium for months to years. No human populations
in the United States have been reported with long-term selenium
poisoning, including populations in the western part of the
country where selenium levels are naturally high in the soil.
Because most people in the United States eat foods produced
in many different areas, overexposure to selenium in food
is unlikely to occur.
In some regions of China where soil levels
of selenium are very low, not eating enough selenium has resulted
in health effects. Selenium is used by the body in antioxidant
enzymes that protect against damage to tissues done by oxygen,
and in an enzyme that affects growth and metabolism. Not eating
enough selenium can cause heart problems and muscle pain.
Muscle pain has also been noted in people fed intravenously
for a long time with solutions that did not contain selenium.
Babies born early may be more sensitive to not having enough
selenium, and this may contribute to lung effects. In the
United States, selenium in food is sufficient to meet the
RDA and prevent harmful effects from not enough selenium.
Upon contact with human skin, industrial
selenium compounds have been reported to cause rashes, redness,
heat, swelling, and pain. Brief, acute exposure of the eyes
to selenium dioxide as a dust or fume in workplace air may
result in burning, irritation, and tearing. However, only
people who work in industries that process or use selenium
or selenium compounds are likely to come into contact with
levels high enough to cause eye irritation.
Studies of laboratory animals and people
show that most selenium compounds probably do not cause cancer.
In fact, some studies of cancer in humans suggest that lower-than-normal
selenium levels in the diet might increase the risk of cancer.
Other studies suggest that dietary levels of selenium that
are higher than normal might reduce the risk of cancer in
humans. However, taking selenium so that your daily amount
is greater than that required might just increase your risk
of selenium poisoning.
Based on studies done until 1987, the
International Agency for Research on Cancer (IARC) determined
that selenium and selenium compounds could not be classified
as to their ability to cause cancer in humans. However, since
then, the EPA has determined that one specific form of selenium,
called selenium sulfide, is a probable human carcinogen. Selenium
sulfide is the only selenium compound shown to cause cancer
in animals. Rats and mice that were fed selenium sulfide daily
at very high levels developed cancer. Selenium sulfide is
not present in foods, and it is a very different chemical
from the organic and inorganic selenium compounds found in
foods and in the environment. Also, if introduced into the
environment, selenium sulfide does not dissolve readily in
water and would probably bind tightly to the soil, further
reducing any chance of exposure. Because selenium sulfide
is not absorbed through the skin, the use of anti-dandruff
shampoos containing selenium sulfide is generally considered
safe.
Very high amounts of selenium have caused
decreased sperm counts, increased abnormal sperm, changes
in the female reproductive cycle in rats, and changes in the
menstrual cycle in monkeys. The relevance of the reproductive
effects of selenium exposure in animals studied to potential
reproductive effects in humans is not known. Selenium compounds
have not been shown to cause birth defects in humans or in
other mammals.
Chapter 3 contains more information on
the health effects of selenium and selenium compounds in humans
and animals.
How can selenium affect children?
This section discusses potential health
effects from exposures during the period from conception to
maturity at 18 years of age in humans.
Children living near selenium waste sites
or coal burning plants are likely to be exposed to higher
environmental levels of selenium through breathing, touching
soil, and eating contaminated soil. Children living in areas
of China with high selenium in the soil had higher levels
of selenium in the blood than adults from that area. Very
few studies have looked at how selenium can affect the health
of children. Children need small amounts of selenium for normal
growth and development. Children will probably show the same
sort of health effects from selenium exposure as adults, but
some studies suggest that they may be less susceptible to
health effects of selenium than adults.
We do not know if exposure to selenium
could result in birth defects in people. Selenium compounds
have not been shown to cause birth defects in humans or in
other mammals. We have no information to suggest that there
are any differences between children and adults in where selenium
is found in the body or in how fast it enters or leaves the
body. Studies in laboratory animals have shown that selenium
crosses the placenta and enters the fetus. Studies in humans
show that infants are supplied with selenium through breast
milk, and therefore, women who were exposed to selenium by
living near a waste site might transfer selenium to their
babies. However, babies in areas of China with high selenium
in the soil did not show any signs of health effects due to
selenium, even though some of their parents did.
How can families reduce the risk of exposure to selenium?
If your doctor finds that you have been
exposed to significant amounts of selenium, ask whether your
children might also be exposed. Your doctor might need to
ask your state health department to investigate.
Since selenium occurs naturally in the
environment, we cannot avoid exposure to it. Certain dietary
supplements and anti-dandruff shampoos contain selenium in
high levels. You should not exceed the recommended dosages
when using these products.
Children living near selenium waste sites
or coal burning plants are likely to be exposed to higher
environmental levels of selenium through breathing, touching
soil, and eating contaminated soil. Some children eat a lot
of dirt. You should discourage your children from eating dirt.
Make sure they wash their hands frequently and before eating.
Discourage your children from putting their hands in their
mouths or from other hand-to-mouth activity.
The primary route of human exposure to
selenium is through eating food. People who irrigate their
home gardens with groundwater containing high levels of selenium
may grow and eat plants that contain high levels of selenium
because this element is taken up in some plants. Fishermen
and hunters of waterfowl who regularly eat fish and game from
waterways with high selenium content may also consume above
average levels of selenium. To reduce your family's exposure
to selenium, obey any wildlife advisories issued by your state.
Information on fish and wildlife advisories in your state
is available from your state public health or natural resources
department.
Is there a medical test to determine whether I have been exposed to selenium?
Selenium can be measured in the blood,
urine, and fingernails or toenails of exposed individuals.
However, since selenium is an essential nutrient normally
present in foods, low levels of selenium are normally found
in body tissues and urine. Tests for selenium are most useful
for people who have recently been exposed to high levels.
Samples of blood, urine, or nails can be properly collected
in a physician's office and sent to a laboratory that has
the special equipment needed to measure selenium. Urine can
be used to determine short-term exposure. Because red blood
cells last about 120 days before they are replaced by newly
made red blood cells, the presence of selenium in red blood
cells can show whether a person was exposed to selenium during
the 120 days before testing, but not if exposed more than
120 days before testing. Toenail clippings can be used to
determine longer-term exposure.
Many methods are available to measure
selenium levels in human tissue and the environment. However,
none of the methods that are routinely available can measure
or detect each selenium compound in one test, and better tests
that measure lower levels of different selenium compounds
are needed. Also, these tests cannot determine the exact levels
of selenium you may have been exposed to or predict whether
health effects will occur, even though very high amounts of
selenium in blood are clearly related to selenosis. Some human
as well as animal studies suggest that when people are exposed
over a long period to higher-than-normal amounts of selenium,
their bodies adjust to the higher amounts. Chapter 3 contains
more information on studies that have measured selenium in
blood and other human tissues.
The length of time that selenium stays
in the body after exposure stops depends on the form of selenium
to which the person was exposed. Thus, it is difficult to
predict how useful a test will be if some time has gone by
since exposure stopped. Chapter 7 contains more information
on the methods available to measure selenium in human tissues
and in the environment.
What recommendations has the federal government made to protect human health?
The federal government develops regulations and recommendations to protect public health. Regulations can be enforced by law. Federal agencies that develop regulations for toxic substances include the Environmental Protection Agency (EPA), the Occupational Safety and Health Administration (OSHA), and the Food and Drug Administration (FDA). Recommendations provide valuable guidelines to protect public health but cannot be enforced by law. Federal organizations that develop recommendations for toxic substances include the Agency for Toxic Substances and Disease Registry (ATSDR) and the National Institute for Occupational Safety and Health (NIOSH).
Regulations and recommendations can be expressed in not-to-exceed levels in air, water, soil, or food that are usually based on levels that affect animals; then they are adjusted to help protect people. Sometimes these not-to-exceed levels differ among federal organizations because of different exposure times (an 8 hour workday or a 24 hour day), the use of different animal studies, or other factors.
Recommendations and regulations are also periodically updated as more information becomes available. For the most current information, check with the federal agency or organization that provides it. Some regulations and recommendations for selenium include the following:
The EPA Office of Drinking Water regulates the amount of selenium allowed in drinking water. Public water supplies are not allowed to exceed 50 ppb total selenium.
The FDA regulations allow a level of 50 ppb of selenium in bottled water. OSHA is responsible for setting regulations on selenium levels allowable in the workplace. The exposure limit for selenium compounds in the air for an 8 hour period is 0.2 mg selenium/m3. Chapter 8 contains other regulations and guidelines for selenium.
References
Agency for Toxic Substances and Disease Registry (ATSDR). 2003. Toxicological profile for Selenium. Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service.
Where can I get more information?
If you have questions or concerns, please contact your community or state health or environmental quality department or:
For more information, contact:
Agency for Toxic Substances and Disease Registry
Division of Toxicology and Human Health Sciences
4770 Buford Highway
Chamblee, GA 30341-3717
Phone: 1-800-CDC-INFO 888-232-6348 (TTY)
Email: Contact CDC-INFO
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.