Radon: The Radioactive Indoor Air Pollutant Most Homes Never Test For

When people picture air pollution, they usually picture something visible: smog over a skyline, exhaust from a tailpipe, or smoke from a wildfire. Yet some environmental pollutants cannot be detected by sight or smell. Radon is a naturally occurring radioactive gas released as uranium in soil and rock gradually breaks down. It exists in outdoor air, but the U.S. Environmental Protection Agency (EPA) explains that radon can also accumulate inside buildings and affect indoor air quality.

The environmental story of radon is different from pollution caused by factories, vehicles, or fossil fuel combustion. Radon begins in the natural geological environment. The problem develops when gas moving through soil becomes concentrated inside enclosed structures. This creates a direct connection between geology, soil conditions, building design, ventilation, and the quality of the air people breathe indoors. The EPA states that testing is the only way to know a building’s radon level because the gas cannot be seen or smelled.

From Bedrock to Basement

Uranium decays into radium, and radium eventually produces radon. Because radon is a gas, it can migrate through soil, fractured rock, porous fill, and openings around foundations. In outdoor air, radon is usually present at much lower concentrations because it disperses into the atmosphere. EPA guidance estimates the average outdoor radon level at about 0.4 pCi/L, compared with an estimated average indoor level of about 1.3 pCi/L in the United States.

Under a building, the conditions change. Heated indoor air can rise and escape through upper parts of a structure, contributing to lower air pressure near the foundation. Soil gas may then enter through slab cracks, sump pits, construction joints, utility penetrations, and crawl spaces.

Local geology also plays an important role. The amount of uranium in soil and rock, the permeability of the ground, and pathways through fractured material can influence radon movement. This means neighboring buildings may record different radon levels even when they appear structurally similar.

Weather and seasonal conditions can cause further variation. Changes in temperature, air pressure, soil moisture, and building ventilation may affect how radon moves from the ground into indoor spaces. A reading taken during one short period may therefore differ from the average exposure measured over several months.

What the Numbers Mean

Radon in air is commonly measured in picocuries per liter, or pCi/L, in the United States. The EPA recommends fixing a home when radon reaches 4.0 pCi/L or higher. Because there is no known safe level of radon exposure, the agency also recommends considering action when concentrations fall between 2.0 and 4.0 pCi/L.

These numbers place radon within the wider issue of indoor environmental quality. Unlike outdoor pollution that may spread across cities or regions, indoor radon levels can change from one building to another because soil conditions, foundation pathways, and building characteristics all influence accumulation.

The health evidence also shows why this natural pollutant receives environmental attention. The EPA estimates that radon contributes to about 21,000 lung cancer deaths each year in the United States. The World Health Organization estimates that radon may account for between 3% and 14% of lung cancers in a country, depending on average radon concentrations and smoking prevalence.

These findings also support the need to treat radon as part of indoor air pollution monitoring. Linking radon data with geological conditions, building practices, and public environmental guidance can help communities identify areas where testing and prevention deserve greater attention.

Radon therefore shows that a pollutant does not need to come from an industrial source to create an environmental exposure problem. Naturally occurring substances can also become hazardous when buildings or other human-made environments allow them to concentrate.

Testing Is the Only Detection Method

Because radon cannot be seen or smelled, testing is the only reliable way to detect elevated indoor concentrations. Short-term tests provide measurements over a limited period, while long-term tests can provide a better picture of average exposure because radon levels may change with weather, seasons, and building conditions. The EPA recommends testing all homes for radon.

Testing should focus on the lowest level of a home that people regularly occupy. This matters because radon usually enters from the soil beneath or around a structure. Changes to how a building is used can also create a reason to test again. For example, the EPA recommends retesting when people begin occupying a lower level such as a basement.

Renovation can also change airflow and radon entry pathways. The CDC recommends retesting after remodeling and considering another test every two years to confirm that radon levels remain low. Regular measurement treats indoor air quality as an environmental condition that can change over time rather than a one-time building check.

Reducing Radon in the Indoor Environment

The encouraging part of the radon story is that established methods can reduce indoor concentrations without rebuilding an entire structure. Professional radon mitigation systems address the pathway between soil gas and indoor air. A suction pipe can be placed beneath a slab or below a sealed crawl space membrane, while a fan draws radon from beneath the building and vents it outdoors above the structure.

This approach changes the pressure beneath the foundation so radon is directed away from occupied indoor spaces. Sealing foundation cracks and other openings can make these systems more effective, but the CDC explains that the correct reduction system depends on the design of the home and whether it has features such as a basement or crawl space.

Targeting the pathway of radon entry allows the problem to be addressed without major changes to most of the existing building structure. Foundations, walls, and other building materials can remain largely in place while soil gas movement is controlled. This can limit unnecessary material use and renovation waste. After a system is installed, testing remains necessary to confirm that indoor radon levels have fallen.

A Manageable Piece of the Indoor Environment

Radon is a reminder that environmental quality includes the air inside buildings as well as the atmosphere outside them. Soil, bedrock, weather, building pressure, and foundation design can all influence how a naturally occurring radioactive gas moves through the environment and accumulates indoors.

The issue also highlights the value of prevention and measurement. Radon-resistant construction can limit entry pathways in new buildings, while testing can identify elevated concentrations in existing structures. When high levels are found, targeted mitigation can redirect soil gas before it enters occupied spaces.

Unlike many large-scale pollution problems, radon exposure can be assessed at the individual building level. Yet the wider environmental lesson is significant: the interaction between natural geology and the built environment can create pollution risks that remain invisible without monitoring. Recognizing that connection helps make indoor air quality a stronger part of environmental planning, sustainable building design, and long-term pollution prevention.