Is radon high in San Marcos, Texas?

Low. Averaging the model's ZIP-code predictions across San Marcos (weighted by how much of the city each ZIP covers) gives a long-term basement mean of 1.3 pCi/L - about 26% below the US average of 1.8 and close to the Texas average of 1.5. That is below the EPA's 2 pCi/L "consider fixing" threshold, though individual homes still vary. San Marcos ranks 68 of 83 Texas cities on this site.

Which San Marcos ZIP codes have the highest radon?

Predictions for the 2 modeled ZIP codes in San Marcos range from 1.3 to 1.4 pCi/L in the basement. 0% of them are predicted at or above 4 pCi/L and 0% at or above 2. The city is fairly uniform, so the city-wide number is a reasonable guide for any neighborhood.

Should I test for radon in San Marcos?

Yes, once. The EPA recommends testing every home regardless of area. A short-term kit costs little and settles the question for your house; San Marcos's low predicted average describes the typical home, not yours.

What does EPA Radon Zone 3 mean for San Marcos?

San Marcos sits mainly in Hays County, which the EPA classifies as Zone 3 - low potential (< 2 pCi/L). The zone map dates from 1993 and covers whole counties. The 2001-2021 model agrees with the zone for the city. Neither replaces a test of your own home.

Radon levels in San Marcos, Texas

LowEPA Zone 3

Predicted long-term basement average of 1.3 pCi/L (model estimate) across the 2 modeled ZIP codes in San Marcos - about 26% below the US average of 1.8, close to the Texas average of 1.5. Ranked #68 of 83 Texas cities on this site. Most of the city lies in Hays County (its ZIPs also reach into 1 other county).

Test San Marcos, TX once anyway - radon varies house to house

The predicted basement average here is 1.3 pCi/L, below the EPA's 2 pCi/L "consider fixing" threshold. That's good news for the area - but the EPA still recommends every home be tested, because foundation cracks, soil pockets and ventilation make one house very different from its neighbor. One inexpensive kit settles it.

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Predicted (model estimate). The pCi/L figures on this page are long-term area averages from a peer-reviewed national model built on millions of real radon tests (2001-2021). They describe the typical home in this area - not your home, which can be several times higher or lower. The EPA radon zone is a separate, 1993 county-level classification. How the numbers work.

1.3 ± 0.08

pCi/L predicted basement mean (model SD)

0.5 ± 0.18

pCi/L predicted upstairs (above-ground) mean

1.3-1.4

range of ZIP-level basement predictions in the city

of San Marcos ZIPs predicted ≥ 4 pCi/L (0% ≥ 2)

What this means for San Marcos

The typical San Marcos home is predicted below 2 pCi/L, the lowest band. That describes the average house; the ZIP-level predictions inside the city still span 1.3 to 1.4 pCi/L.

The city figure is not a separate estimate: it is the mean of the model's ZIP-code predictions, weighted by the share of the city's land in each ZIP (Census 2020 ZCTA-to-place relationship file). The 1993 EPA zone (3) for Hays and the 2001-2021 model agree here.

Comparable Texas cities: Spring (1.3), Baytown (1.3), Missouri City (1.4), Victoria (1.4).

ZIP codes in San Marcos

ZIPs that overlap the city, largest share of its land first. Predicted basement and upstairs means are model estimates; ZIPs the model does not cover show a dash.

ZIP	Share of city	Basement	Upstairs	Grade
78666	91%	1.3 ± 0.08	0.5	Low
78656	9%	1.4 ± 0.07	0.6	Low

Testing help in Texas

We link only state program pages we could verify, and Texas's did not respond when we checked, so start from the EPA state radon contacts page. If a test comes back at 4 pCi/L or higher, mitigation typically costs $800-$2,500 installed; use EPA find-a-professional and read the mitigation guide.

San Marcos radon questions

Is radon high in San Marcos, Texas?: Low. Averaging the model's ZIP-code predictions across San Marcos (weighted by how much of the city each ZIP covers) gives a long-term basement mean of 1.3 pCi/L - about 26% below the US average of 1.8 and close to the Texas average of 1.5. That is below the EPA's 2 pCi/L "consider fixing" threshold, though individual homes still vary. San Marcos ranks 68 of 83 Texas cities on this site.
Which San Marcos ZIP codes have the highest radon?: Predictions for the 2 modeled ZIP codes in San Marcos range from 1.3 to 1.4 pCi/L in the basement. 0% of them are predicted at or above 4 pCi/L and 0% at or above 2. The city is fairly uniform, so the city-wide number is a reasonable guide for any neighborhood.
Should I test for radon in San Marcos?: Yes, once. The EPA recommends testing every home regardless of area. A short-term kit costs little and settles the question for your house; San Marcos's low predicted average describes the typical home, not yours.
What does EPA Radon Zone 3 mean for San Marcos?: San Marcos sits mainly in Hays County, which the EPA classifies as Zone 3 - low potential (< 2 pCi/L). The zone map dates from 1993 and covers whole counties. The 2001-2021 model agrees with the zone for the city. Neither replaces a test of your own home.

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