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Home>Water University>Water Contaminants>Alpha Particles

Alpha Particles


This is a fact sheet about a chemical that may be found in some public or private drinking water supplies. It may cause health problems if found in amounts greater than the health standard set by the United States Environmental Protection Agency (EPA).

What is an alpha particle?

Alpha particles (symbol a ) are a type of ionizing radiation ejected by the nuclei of some unstable atoms. They are large subatomic fragments consisting of 2 protons and 2 neutrons.

Who discovered alpha particles?

Ernest Rutherford, an English scientist, discovered alpha particles in 1899 while working with uranium. Rutherford's studies contributed to our understanding of the atom and its nucleus through the Rutherford-Bohr planetary model of the atom.

What are the properties of an alpha particle?

An alpha particle is identical to a helium nucleus having two protons and two neutrons. It is a relatively heavy, high-energy particle, with a positive charge of +2 from its two protons. Alpha particles have a velocity in air of approximately one-twentieth the speed of light, depending upon the individual particle's energy.

What are the conditions that lead to alpha particle emission?

When the ratio of neutrons to protons in the nucleus is too low, certain atoms restore the balance by emitting alpha particles. For example: Polonium-210 has 126 neutrons and 84 protons, a ratio of 1.50 to 1. Following radioactive decay by the emission of an alpha particle, the ratio becomes 124 neutrons to 82 protons, or 1.51 to 1.

Alpha emitting atoms tend to be very large atoms (that is, they have high atomic numbers). With some exceptions, naturally occurring alpha emitters have atomic numbers of at least 82 (the element lead).

Which radionuclides are alpha emitters?

There are many alpha emitting radioactive elements, both natural and manmade. You can find fact sheets for several key alpha emitters at the Radionuclides page:
Alpha Emitter Atomic Number
americium-241 95
plutonium-236 94
uranium-238 92
thorium-232 90
radium-226 88
radon-222 86
polonium-210 84


What happens to atoms during alpha emission?

The nucleus is initially in an unstable energy state. An internal change takes place in the unstable nucleus and an alpha particle is ejected leaving a decay product. The atom has then lost two protons along with two neutrons.

Since the number of protons in the nucleus of an atom determines the element, the loss of an alpha particle actually changes the atom to a different element. For example, polonium-210 is an alpha emitter. During radioactive decay, it loses two protons, and becomes a lead-206 atom, which is stable (i.e., nonradioactive).

What uses do alpha emitters have?

The positive charge of alpha particles is useful in some industrial processes. For example, radium-226 may be used to treat cancer, by inserting tiny amounts of radium into the tumorous mass. Polonium-210 serves as a static eliminator in paper mills and other industries. The alpha particles, due to their positive charge, attract loose electrons, thus reducing static charge.

Some smoke detectors take advantage of alpha emissions from americium-241 to help create an electrical current. The alpha particles strike air molecules within a chamber, knocking electrons loose. The resulting positively charged ions and negatively charged electrons, create a current as they flow between positively and negatively charged plates within the chamber. When smoke particles enter the device, the charged particles attract them, breaking the current and setting off the alarm.

Exposure to Alpha Emitters

How do alpha emitters get into the environment?

Most alpha emitters occur naturally in the environment. For example, alpha particles are given off by uranium-238, radium-226, and other members of the uranium decay series. These are present in varying amounts in nearly all rocks, soils, and water. However, human activity, create or worsen the potential for exposure of people and contamination of various environmental media. For example, uranium mining wastes, known as uranium mill tailings, have high concentrations of uranium and radium. Once brought to the surface, they can be become airborne or enter surface water as runoff. A second example is the mining and processing of phosphate for fertilizer. The currently used process generates large piles or "stacks" of phosphogypsum, in which naturally occurring radium is concentrated.

How do alpha particles change in the environment?

Alpha particles don't get very far in the environment. Once emitted, they travel relatively slowly (at approximately one-twentieth the speed of light) due to their electric charge and large mass. They lose energy rapidly in air, usually expending it within a few centimeters. Because alpha particles are not radioactive, once they have lost their energy, they pick up free electrons and become helium.

Alpha particles also cannot penetrate most matter they encounter. Even a piece of paper, or the dead outer layers of human skin is sufficient to stop alpha particles.

Health Effects

How can alpha particles affect peoples health?

The health effects of alpha particles depend heavily upon how exposure takes place. External exposure (external to the body) is of far less concern than internal exposure, because alpha particles lack the energy to penetrate the outer dead layer of skin.

However, if alpha emitters have been inhaled, ingested (swallowed) or absorbed into the blood stream, sensitive living tissue can be exposed to alpha radiation. The resulting biological damage increases the risk of cancer; in particular, alpha radiation is known to cause lung cancer in humans when alpha emitters are inhaled.

The greatest exposures to alpha radiation for average citizens comes from the inhalation of radon and its decay products, several of which also emit potent alpha radiation.

Is there a medical test to determine exposure to alpha particles?

There are tests that can detect the presence of alpha-emitting radionuclides in the body. However, they require special equipment and testing is generally done by specialized laboratories and facilities or hospitals.

Protecting People from Alpha Particles

How do I know I'm near alpha emitters and alpha particles?

You must have specialized equipment to detect alpha radiation. Generally, this equipment is expensive and requires an expert to operate it.

The one alpha-emitting radionuclide that you can easily measure yourself is radon. Inexpensive home test kits are available to test your home for radon.

What is the government doing to protect people from exposure to alpha emitters and alpha particles?

The U.S. Congress passes laws that authorize EPA and other federal agencies, to protect public health and the environment from radionuclides, including alpha emitters. EPA has issued a variety of regulations that limit the release of radionuclides to the environment. You can learn about limits set for individual radionuclides by selecting alpha emitters from the Radionuclides page.

How do I protect myself and my family from alpha particles?

Protecting yourself from external exposure to alpha radiation is easy, since alpha particles are unable to penetrate the outer dead layers of skin or clothing. However, tissue that is not protected by the outer layer of dead cells, such as eyes or open wounds, must be carefully protected.

The exposure pathways of concern are inhalation or ingestion of alpha emitters, which continue to emit alpha particles. Alpha emitting radionuclides taken into the body release alpha particles to sensitive tissues. Their high energy transfers directly to tissue, causing damage that may lead to cancer.

The most significant way people come in contact with alpha emitters is in their home, school, or place of business. Radon, is a heavy gas and tends to collect in low-lying areas such as basements. Testing for radon in your home and taking any corrective action necessary is the most effective way to protect you and your family from alpha emitters.

Note: This fact sheet is part of a larger publication adapted from U.S. EPA publication: EPA National Primary Drinking Water Regulations.