Understand Radon Risk in U.S. Homes and Workplaces

Radon is a naturally occurring radioactive gas that can enter buildings through cracks and openings in foundations. It has no smell or color, so testing is the only way to know your level. Long-term exposure is linked to lung cancer risk.

Visit EPA Radon Page

EPA action level: 4.0 pCi/L Lower levels can still carry risk Testing is the first step

At a glance (U.S.)

EPA identifies radon as the second leading cause of lung cancer overall and the leading cause among people who do not smoke.

~1 in 15 U.S. homes are estimated to have radon at or above the EPA action level.

4.0 pCi/L EPA recommends taking action to reduce indoor radon at or above this level.

~21,000 Estimated U.S. lung cancer deaths per year associated with radon exposure (EPA).

How Radon Enters and Breaks Down

Use the controls to simulate how soil gas infiltration can increase indoor radon concentration. Blue particles represent radon, purple particles represent short-lived decay products, and amber bursts represent emitted alpha radiation events.

Radon gas Decay products Alpha emission

How Weather and Seasons Affect Radon

Radon levels can vary day-to-day and season-to-season. Cold weather, tight building operation, wind effects, and pressure differences between indoors and soil can change how much radon is pulled into occupied spaces.

Common Seasonal Patterns

Winter: Closed windows and stronger stack effect often increase indoor radon.
Summer: Open windows may dilute radon in some homes, but air conditioning patterns can vary results.
Rain and snow cover: Temporary sealing of soil surface can redirect soil gas toward foundations.
Wind events: Wind pressure around buildings can raise or lower entry rates depending on structure.
Rapid pressure changes: Barometric shifts can produce short-term radon spikes.

Testing and Operations Implications

Use both short- and long-term tests: Long-term testing better captures seasonal variability.
Test in lowest occupied level: Basements and slab-on-grade levels are commonly most informative.
Retest after envelope/HVAC changes: New insulation, sealing, or airflow balancing can shift radon behavior.
Commercial schedules matter: Night setback and weekend ventilation changes can influence trends.
Track conditions: Log weather and HVAC mode when reviewing radon measurements.

Condition	Typical Indoor Effect	Planning Response
Cold outdoor temperatures	Often increases indoor concentration due to stack effect and reduced window opening.	Prioritize winter testing and verify mitigation fan operation.
Heavy rain / wet soil	Can transiently increase soil-gas pressure pathways into structures.	Review short-term spikes with weather logs before final interpretation.
High wind periods	Can create pressure-driven variability across building zones.	For large buildings, evaluate zone-by-zone pressure and monitoring points.

Mitigation Techniques

Mitigation depends on building type, foundation design, occupancy profile, and measured concentration. The methods below are commonly used in U.S. practice.

Homes (Residential)

Active sub-slab depressurization (ASD): Fan and pipe system draws soil gas from beneath slab and vents above roofline.
Sealing entry points: Seal cracks/joints to support ASD performance, not as a standalone fix.
Crawlspace depressurization or membrane systems: Common for homes with crawlspaces.
Post-mitigation testing: Verify levels after installation and re-test periodically.
Radon-resistant new construction: Gas-permeable layer, plastic sheeting, vent pipe, and fan-ready electrical setup.

Commercial and Large Buildings

Diagnostic mapping: Identify pressure zones and slab communication across larger footprints.
Multi-point depressurization: Multiple suction points, larger fans, and zoned monitoring.
HVAC pressure balancing: Reduce negative pressure that can pull radon indoors.
Continuous monitoring: Trending and alarms for facilities with high occupancy.
Operations integration: Include radon controls in facility management and preventive maintenance plans.

Measured Level	Typical Response	Notes
< 2.0 pCi/L	Maintain awareness, retest based on EPA guidance and after major renovations.	Risk is not zero; maintain ventilation and good building maintenance.
2.0 - 3.9 pCi/L	Consider mitigation, especially with vulnerable occupants or high occupancy time.	EPA notes there is no known safe radon level.
>= 4.0 pCi/L	Plan and implement mitigation promptly with qualified professionals.	EPA action level; verify effectiveness with follow-up tests.