Radon Decay, Half-Life, and What a Test Really Measures in Milwaukee, WI

Radon is a radioactive gas created naturally as uranium and radium break down in soil and rock. Because it has no smell, color, or taste, the only dependable way to know whether a home has elevated indoor levels is to measure the air.

Understanding the science behind radon helps explain why test results can vary, why a two-day test is only a snapshot, and why longer testing often provides a better picture of typical exposure.

What is radon, and where does it come from?

Radon is a naturally occurring radioactive gas. It forms during the decay of uranium, which is present in varying amounts in soil, rock, and building materials.

The radon most often discussed in homes is radon-222. It can move through soil gas and enter a building through openings that contact the ground, including:

• Cracks in concrete floors and foundation walls
• Gaps around plumbing, wiring, or sump pits
• Construction joints
• Floor drains and utility penetrations
• Exposed soil in crawl spaces
• Porous masonry or other foundation materials

Homes in Milwaukee may have different radon levels from nearby homes because soil conditions, foundation design, air pressure, drainage, and ventilation can vary from property to property. A neighbor’s test result cannot reliably predict another household’s result.

What does “radioactive decay” mean?

Radioactive decay is the spontaneous transformation of an unstable atom into a more stable form. When radon atoms decay, they release ionizing radiation, primarily in the form of alpha particles.

An alpha particle does not travel far through air and cannot pass through intact skin. The concern indoors arises when radon decay products attach to airborne dust particles. If inhaled, some of these particles can lodge in the lungs and release radiation close to sensitive tissue.

Radon itself is a gas, but its short-lived decay products are solid particles. This distinction explains why radon is measured as a gas concentration while health risk is connected to the radiation released by radon and its decay products inside the respiratory system.

What is radon’s half-life?

Radon-222 has a half-life of about 3.8 days. A half-life is the time required for half of a radioactive substance to decay.

That does not mean all radon disappears after 3.8 days. Instead:

• After one half-life, about 50 percent remains
• After two half-lives, about 25 percent remains
• After three half-lives, about 12.5 percent remains

New radon is continually entering a building from the soil. Indoor concentration therefore depends on the balance between radon entering, radon decaying, and indoor air being exchanged with outdoor air.

A home can maintain a fairly steady average level even though individual radon atoms are constantly appearing and decaying.

Why do radon decay products matter?

When radon decays, it produces a sequence of radioactive elements, including polonium, lead, and bismuth isotopes. Several of these decay products have much shorter half-lives than radon and can attach to dust or smoke particles.

The alpha radiation from these products is the principal reason long-term exposure to elevated radon increases lung cancer risk. The risk depends on the concentration in the air, how long people are exposed, and other factors such as tobacco smoke exposure.

There is no practical medical test that can determine a person’s exact lifetime radon exposure. Indoor air measurement is used instead.

How is radon measured?

Most household results are reported in picocuries per liter, written as pCi/L. This unit describes the amount of radioactive activity in a volume of air.

A pCi/L is a very small unit, but the number is not a percentage and does not describe how much of the air is radon. It represents radioactive disintegrations occurring within one liter of air.

Common measurement methods include:

• Charcoal tests: These generally measure radon or radon-related activity absorbed onto activated charcoal during a short testing period. A laboratory analyzes the device after it is returned.
• Alpha-track detectors: These contain a material that records microscopic damage caused by alpha particles. They are commonly used for longer testing periods.
• Continuous monitors: These electronically sample air over time and may record hourly or daily readings. Some also track temperature, humidity, pressure, or motion to help identify interference.
• Electret-based devices: These use a charged disk whose electrical voltage changes as radiation passes through the measurement chamber.

Each method has limits. A result is meaningful only when the device is correctly placed, operated for the required period, and analyzed according to the test instructions.

Why can two radon tests produce different results?

Radon levels naturally change over time. A short-term result reflects conditions during one limited period rather than a permanent characteristic of the home.

Levels can change because of:

• Wind and barometric pressure
• Frozen or saturated soil
• Heating and cooling patterns
• Open or closed windows and doors
• Exhaust fans, fireplaces, and combustion appliances
• Changes in soil moisture
• Foundation repairs or remodeling
• Differences between heating and non-heating seasons

Winter can be a useful testing season in Milwaukee because homes are typically closed more tightly and indoor-outdoor pressure differences may be stronger. Even so, a winter result should not automatically be treated as a full-year average.

What is the difference between short-term and long-term testing?

A short-term test usually runs for two to four days. It is useful for an initial screening or when a relatively quick answer is needed, but it can be influenced by unusual weather or household conditions.

A long-term test runs for at least 91 days and may continue for many months. Because it covers more changing conditions, it generally provides a better estimate of the home’s average annual radon concentration.

If a short-term result is near or above the action level, a second test is commonly recommended. A long-term test is often useful for understanding average exposure, while a second short-term test may be used when a faster confirmation is needed.

How should a result of 4 pCi/L be understood?

A result of 4 pCi/L is the commonly used level at which action to reduce radon is recommended. It is not a line between “safe” and “unsafe.” Radon risk decreases as the concentration decreases, so lower levels are preferable whenever they can reasonably be achieved.

A single test result should be interpreted with the test duration, device type, placement, weather, and building conditions in mind. Results at or above 4 pCi/L generally call for follow-up testing and consideration of mitigation. Results below 4 pCi/L may still justify additional testing, particularly after major renovations or changes that increase use of a basement or lower level.

What happens after a mitigation system is installed?

Mitigation changes the movement of soil gas beneath or around the building, usually by creating a controlled pathway that removes radon before it enters indoor air. Testing after the work verifies whether the system is reducing the concentration as intended.

Post-mitigation testing is not optional from a scientific standpoint. The only way to confirm performance is to measure the indoor air again under appropriate conditions. Continued periodic testing is also useful because foundation conditions, ventilation, and building use can change over time.

For area households, the practical lesson is straightforward: radon decay is predictable at the atomic level, but indoor concentrations are shaped by a constantly changing building and environment. A properly conducted long-term measurement is often the clearest way to understand the exposure pattern inside a particular home.