The picocurie (pCi) is a unit used to quantify the amount of radioactive material present in a substance. It measures the material’s activity, which describes the rate at which its atoms undergo radioactive decay. Because the unit is exceptionally small, it is appropriate for monitoring the extremely low levels of naturally occurring radiation found in the environment.
Understanding Radioactive Activity: The Curie
Radioactive activity is a measure of how frequently unstable atomic nuclei within a material spontaneously transform, or disintegrate, in a given time period. This process of nuclear decay releases energy in the form of radiation. The primary unit used in the United States to quantify this rate of decay is the Curie (Ci), named in honor of scientists Marie and Pierre Curie.
One Curie is defined as the quantity of any radionuclide that undergoes exactly 3.7 x 10^10 atomic disintegrations every second. This value was originally established to approximate the activity of one gram of radium-226. This high number—37 billion decays per second—highlights that a single Curie represents a very high level of radioactivity.
Although the International System of Units (SI) officially uses the Becquerel (Bq), defined as one disintegration per second, the Curie remains widely employed in U.S. government, industry, and medicine. The unit provides a standard reference point for measuring the intensity of radiation. However, when monitoring environmental samples, concentrations are often too low to be conveniently expressed using the full Curie unit.
The Scale of Measurement: Why “Pico” Matters
The need to measure extremely low levels of radiation led to the adoption of prefixes that scale the base unit. The prefix “pico” denotes a factor of one trillionth, or 10^-12. Therefore, a picocurie (pCi) is one trillionth of a Curie.
To grasp this minute scale, a picocurie is to a Curie as one second is to approximately 32,000 years. This scale is necessary because natural background radiation present in the air, soil, and human body exists at these trace concentrations. Measurement instruments used in environmental monitoring are sensitive enough to detect these minuscule decay rates.
The picocurie provides a practical unit for expressing measurable levels of radioactivity in environmental samples without resorting to cumbersome scientific notation. This unit makes it possible to set regulatory standards and assess low-level exposure risks. The use of this small unit reflects that even very low concentrations of radioactive material must be tracked for public health purposes.
Common Contexts for Picocurie Measurement
The picocurie is the standard unit for expressing the concentration of naturally occurring radioactive gas, such as radon, in indoor air. Radon is a colorless, odorless gas that results from the natural decay of uranium in soil and rock. Indoor radon levels are reported in picocuries per liter of air (pCi/L) in the United States.
The Environmental Protection Agency (EPA) established an action level for indoor radon at 4.0 pCi/L, the point at which mitigation measures are strongly recommended. This measurement assesses low-level background or environmental radiation exposure, distinct from the higher doses measured in medical procedures. The pCi/L unit allows homeowners and regulators to make informed decisions about intervention.
Picocurie measurements are also used to monitor radionuclides in public drinking water supplies. The EPA sets Maximum Contaminant Levels (MCLs) for various naturally occurring radionuclides, expressed in pCi/L.
Drinking Water Standards
The combined concentration of radium-226 and radium-228 must not exceed 5 pCi/L. The standard for gross alpha-particle activity, which measures a broad class of alpha-emitting radionuclides excluding radon and uranium, is set at 15 pCi/L.