San Mateo County, CA radon potential

EPA's own radon map rates San Mateo County zone 2 - moderate potential, predicting a county-wide average indoor level between 2 and 4 pCi/L. Nationally, San Mateo County ranks higher in radon potential than 65% of rated counties (the 65th percentile).

How San Mateo County compares

San Mateo County, California is home to about 742,893 people across 448.603 square miles. Averaged across its land area, San Mateo County runs about 1,656 people per square mile. Within its own EPA zone, San Mateo County is bigger by population than 97% of the counties that share it. San Mateo County falls in EPA Radon Region 9, one of 95 counties nationally EPA assigns to that same geologic province.

What this actually means for a house in San Mateo County

San Mateo County's Zone 2 prediction clears the 1.3 pCi/L national indoor average without reaching EPA's own 4 pCi/L point for recommending a fix. A county-wide moderate average, like San Mateo County's, can still hide a specific house well above or below it — testing is what actually answers the question.

Testing and mitigation in San Mateo County

A short-term test kit is the standard first step and typically costs far less than mitigation itself. If a test comes back at or above 4 pCi/L, a radon mitigation system for a typical home runs roughly $800–$2,500 (around $1,200 for a common single-family install), driven mainly by foundation type, not by county.

Find California's state radon program

For San Mateo County, an ongoing number beats a one-time test - a Airthings radon monitor tracks San Mateo County's levels over time. RadonZoneCheck earns a commission there, at no extra cost to you. Use code RADONZONE-10OFF at checkout for 10% off.

Where San Mateo County's figures come from

Every figure on this page traces to EPA's own Map of Radon Zones and Census data, documented in the methodology guide. If you're unsure why a low zone still deserves a test, the zone explainer covers that directly.