What is radiation?
Radiation is the emission (sending
out) of energy from any source. X-rays are an example of radiation, but so is
the light that comes from the sun and the heat that is constantly coming off
our bodies.
When talking about radiation and
cancer, many people think of specific kinds of radiation such as x-rays or the
radiation made by nuclear reactors. But there are different types of radiation,
and many of them are not linked to cancer.
Types of radiation
Radiation exists across a spectrum
from very high-energy (high-frequency) radiation to very low-energy
(low-frequency) radiation. This is sometimes referred to as the electromagnetic
spectrum. From highest to lowest energy, the main forms of radiation are:
- Gamma rays
- X-rays
- Ultraviolet (UV) rays
- Visible light
- Infrared rays
- Microwaves
- Radiofrequency (radio) waves
- Extremely low-frequency (ELF) radiation
An important distinction that
affects the health risks from radiation is whether the energy is ionizing or
non-ionizing.
Ionizing radiation is high-frequency radiation that has enough energy to
remove an electron from (ionize) an atom or molecule. Ionizing radiation has
enough energy to damage the DNA in cells, which in turn may lead to cancer.
Gamma rays, x-rays, some high-energy UV rays, and some sub-atomic particles
such as alpha particles and protons are forms of ionizing radiation.
Non-ionizing radiation is low-frequency radiation that does not have enough energy
to remove electrons or directly damage DNA. Low-energy UV rays, visible light,
infrared rays, microwaves, and radio waves are all forms of non-ionizing
radiation. Aside from UV rays, these types of radiation are not known to
increase cancer risk.
It is important to understand the
difference between these types of radiation. For example, the non-ionizing
radiation given off by a cell phone or a television screen is not the same as
the ionizing radiation you might get from x-rays taken in the hospital.
The types of radiation exposures
discussed here include:
- Ionizing radiation from sources such as medical and
other man-made forms of radiation, and natural background radiation from
the earth and sky.
- Ultraviolet (UV) radiation from natural and man-made
sources.
- Some sources of non-ionizing radiation, including power
lines, cell phones, TV and computer screens, and microwaves.
Ionizing radiation
Ionizing radiation has enough energy
to knock electrons off of atoms or molecules. This is called ionization.
Ionized molecules are unstable and quickly undergo chemical changes.
If ionizing radiation passes through
a cell in the body, it can lead to mutations (changes) in the cell's DNA, the
part of the cell that contains its genes (blueprints). This could contribute to
cancer, or to the death of the cell. The amount of damage in the cell is
related to the dose of radiation it receives. The damage takes place in only a
fraction of a second, but other changes such as the beginning of cancer may
take years to develop.
Types of ionizing radiation include
x-rays, gamma rays, some high-energy UV rays, and particles given off by
radioactive materials such as alpha particles and protons. These forms of
radiation have different energy levels and can penetrate cells to different
extents, but all are capable of causing ionization.
Does
ionizing radiation cause cancer?
Ionizing radiation is a proven human
carcinogen (cancer causing agent). The evidence for this comes from many
different sources, including studies of atomic bomb survivors in Japan, people
exposed during the Chernobyl nuclear accident, people treated with high doses
of radiation for cancer and other conditions, and people exposed to high levels
of radiation at work, such as uranium miners.
Most studies on radiation and cancer
risk have looked at people exposed to very high doses of radiation in the
settings above. It is harder to measure the much smaller increase in cancer
risk that might come from much lower levels of radiation exposure. Most studies
have not been able to detect an increased risk of cancer among people exposed
to low levels of radiation. For example, people living at high altitudes, who
are exposed to more natural background radiation from cosmic rays than people living
at sea level, do not have noticeably higher cancer rates.
Still, most scientists and
regulatory agencies agree that even small doses of ionizing radiation increase
cancer risk, although by a very small amount. In general, the risk of cancer
from radiation exposure increases as the dose of radiation increases. Likewise,
the lower the exposure is, the smaller the increase in risk. But there is no
threshold below which ionizing radiation is thought to be totally safe.
Although radiation exposure affects
the occurrence of various types of cancer, it does not affect their
aggressiveness (tendency to grow and spread).
Types
of cancer linked to ionizing radiation
Ionizing radiation increases the
risk of certain types of cancer more than others.
The thyroid gland and bone marrow
are particularly sensitive to radiation. Leukemia, a type of cancer that arises
in the bone marrow, is the most common radiation-induced cancer. Leukemias may
appear as early as a few years after radiation exposure.
Other types of cancer can also
result from radiation exposure, although they may take longer to develop
(usually at least 10 to 15 years). Some of the other cancers most strongly
linked to radiation exposure in studies include:
- Lung cancer
- Skin cancer
- Thyroid cancer
- Multiple myeloma
- Breast cancer
- Stomach cancer
These are not necessarily the only
cancer types that may be linked to radiation, however.
The types of cancer linked to
radiation are also affected by the part of the body that is exposed. For
example, people who get pelvic radiation therapy would not be expected to have
higher rates of cancers in the head and neck because these areas weren't
exposed to radiation.
Other factors may also play a role
in how likely a person exposed to radiation is to develop cancer. Age is one
such factor - children's growing bodies are more sensitive to radiation than
adults. A person may also have gene changes that make their cells more
vulnerable to radiation damage, which might in turn raise their risk more than
in someone without these gene changes.
Sources
of ionizing radiation
People may be exposed to ionizing
radiation from 3 main sources:
- Natural background radiation comes from cosmic rays from our solar system and
radioactive elements normally present in the soil. This is the major
contributor to worldwide radiation exposure.
- Medical radiation
comes in the form of diagnostic x-rays and other tests, as well as from
radiation therapy. Radiation therapy is currently used to treat some types
of cancer and involves dosages many thousand times higher than those used
in diagnostic x-rays.
- Non-medical, man-made radiation can come from workplace and other sources, and is also
a result of above ground nuclear weapons testing that took place before
1962.
Natural
background radiation
We are all exposed to some amount of
radiation just from being on this planet. This is known as background
radiation. For most people, background radiation accounts for most of their
exposure to ionizing radiation during the year. It comes from several different
sources.
Cosmic rays: Cosmic rays are radioactive particles that hit the earth
from outer space. They come from the sun and from other stars. The earth's
atmosphere blocks a portion of these rays, but some of them reach the ground.
Because the atmosphere blocks some cosmic rays, exposure is greater at higher
altitudes. For example, people who live in Denver, Colorado, which is at a high
elevation, are exposed to slightly more cosmic rays than people living at sea
level. People are also exposed to higher levels of cosmic rays during airplane
flights. Airline pilots and flight attendants, who spend many hours at high
elevations, are exposed to more of these rays, but it is not clear if they have
an increased risk of cancer because of it.
Radiation in the earth: People are also exposed to small amounts of radiation from
radioactive elements that occur naturally in rocks and soil. Some of these may
end up in building materials used in houses and other structures. Tiny amounts
of radiation may even be found in drinking water and in some plant-based foods
as a result of being in contact with the soil. For people who smoke, tobacco
can account for a significant portion of the yearly radiation they receive.
Radon: The largest source of natural background radiation for most
people is radon. This is an odorless, colorless gas that is formed from the
breakdown of radioactive elements in the ground. Radon levels are usually
higher inside buildings and homes, especially in levels closer to the ground
such as basements. Radon levels can vary a great deal, depending on where you
live. For more detailed information on radon and its possible health effects,
see our document, Radon.
Medical
radiation
Ionizing radiation is used in the
diagnosis and treatment of some medical conditions. This can be in the form of
radiation that penetrates from outside the body, or radioactive particles that
are swallowed or inserted into the body.
Imaging tests: Certain types of imaging tests, such as x-rays, CT scans,
and nuclear medicine tests (such as PET scans and bone scans) expose people to
low levels of radiation in order to create internal pictures of the body. (MRI
and ultrasound exams do not use ionizing radiation.)
The increased risk of cancer from
exposure to any single test is likely to be very small. Still, concerns have
been raised in recent years as the average amount of radiation a person is
exposed to from medical tests has risen. Children's growing bodies are
especially sensitive to radiation.
Because of the very small but real
risk, and the fact that radiation exposure from all sources can add up over
one's lifetime, imaging tests that use radiation should only be done if there
is a good medical reason to do so. The usefulness of the test must always be
balanced against the possible risks from exposure to the radiation. In some
cases, other imaging tests such as ultrasound or MRI may be an option. But if
there is a reason to believe that an x-ray or CT scan is the best way to look
for cancer or other diseases, the patient will most likely be helped more than
the small dose of radiation can hurt.
For more detailed information on the
possible radiation risks from imaging tests, see our document, Imaging (Radiology) Tests.
Radiation therapy: Ionizing radiation is an effective way to treat certain
kinds of cancer. During radiation therapy, high doses of ionizing radiation
(much higher than those used for imaging tests) are directed at the cancer,
resulting in the death of the cancer cells. However, this can lead to DNA
mutations in other cells that survive the radiation, which may eventually lead
to the development of a second cancer.
Overall, radiation therapy alone
does not appear to be a very strong cause of second cancers. This is probably
due to the fact that doctors try to focus the radiation on the cancer cells as
much as possible, which means few normal cells are exposed to radiation. Still,
some studies have linked radiation therapy with an increased risk of leukemia,
thyroid cancer, early-onset breast cancer, and some other cancers. The amount
of increased risk depends on a number of factors, include the dose of
radiation, the location in the body, and the age of the person getting it
(younger people are generally at greater risk later on).
If cancer does develop after
radiation therapy, it does not happen right away. For leukemias, most cases
develop within 5 to 9 years after exposure. In contrast, other cancers often
take much longer to develop. Most of these cancers are not seen for 10 years
after radiation therapy, and some are diagnosed even more than 15 years later.
When considering radiation exposure
from radiation therapy treatment, the benefits generally outweigh the risks.
However, some combinations of radiation therapy and chemotherapy are more risky
than others. Doctors do their best to ensure the treatment that is given
destroys the cancer while minimizing the risk that a secondary cancer will
develop later on.
Other
forms of man-made radiation
People may also be exposed to
ionizing radiation from other man-made sources.
Nuclear tests and facilities: The United States government conducted above-ground nuclear
tests in the South Pacific and in the state of Nevada between 1945 and 1962.
Many people in the military at the time were part of training exercises in the
area and were exposed to ionizing radiation from these tests. Others were
exposed to radiation while working at facilities making the bombs or at other
nuclear sites. More information on this topic is available in our document, Cancer Among Military Personnel
Exposed to Nuclear Weapons.
Non-military people living near or
downwind of nuclear test sites may have also been exposed to radioactive
byproducts. Levels of radiation are likely to be higher near these sites, but
some radioactive particles from the tests entered the atmosphere and traveled
great distances, landing thousands of miles away from the original site. While
exposure levels were likely to be higher at the time of testing, some radiation
in the soil today is the result of these tests.
People who work in nuclear power
plants may be exposed to higher levels of radiation than the general public,
although their exposure levels are monitored carefully. Emissions of radiation
from nuclear power plants are carefully monitored and controlled. According to
the Environmental Protection Agency (EPA), nuclear power plant operations
account for less than one-hundredth (1/100) of a percent of the average
American's total radiation exposure.
Consumer products: Some consumer products contain small amounts of ionizing
radiation.
For example, tobacco products
contain low levels of radiation, which may come from the soil it's grown in or
the fertilizer used to help it grow. Tobacco may account for a significant
portion of the yearly radiation that smokers are exposed to.
Some building materials used in the
home or other structures may contain low levels of naturally occurring
radiation. This can vary depending on what they're made of, but the levels are
unlikely to contribute much to a person's overall exposure to radiation,
according to the EPA.
Many smoke detectors contain a small
amount of a very low-level radioactive material that helps detect the smoke.
This material is sealed in a container and does not pose a significant risk of
radiation exposure.
Food irradiation: Ionizing radiation can be used to kill bacteria and other
germs on certain foods, which may make them safer to eat and help them last
longer. Some people may be concerned that irradiated food may itself contain
radiation.
It's important to understand that
the radiation does not stay in the food. According to the United States
Department of Agriculture (USDA), irradiating food does not cause it to
become radioactive and does not change nutritional value or flavor of the food.
Airport security scanners: In recent years, some airports have begun to use whole body
scanners as a way to detect objects hidden by clothing. These scanners are
different from the metal detectors most people are familiar with.
One type of body scanner, based on
backscatter technology, uses very weak x-rays aimed at the surface of the body
to capture a whole body image. The Transportation Security Administration (TSA)
says the radiation from such a scan is about the same amount a person gets from
cosmic rays when flying for 2 minutes in an airplane at 30,000 feet. A person
would need to get more than 1,000 backscatter scans in a year to reach the same
dose they'd get from a standard chest x-ray, according to the American College
of Radiology (ACR).
The other type of body scanner is
based on millimeter wave technology. Neither millimeter wave scanners nor metal
detectors use ionizing radiation.
