Mold in Texas: what the numbers actually say

Texas is the only state where one border town lives in a swamp climate and another lives in a desert. Houston posts an annual average relative humidity near 75 percent by NOAA National Weather Service climatological normals for 1991 to 2020. El Paso, more than 700 miles west, sits near 40 percent. The line between them is the defining feature of Texas mold risk: east of Interstate 35, conditions look like Louisiana. West of the Pecos, mold needs a plumbing failure to survive.

Dew point tells the eastern story more honestly than RH. Gulf Coast metros run annual average dew points in the low 60s F against a national average near 50 F, and from late May through September coastal dew points sit at or above 70 F. At that load, any surface an air conditioner cools below the dew point sweats. Supply registers, closet walls on exterior corners, and uninsulated ductwork all become condensation farms in a Houston or Corpus Christi August.

Then come the storms. Texas trails only Florida for hurricane landfalls since 1851 in the NOAA National Hurricane Center record. Hurricane Harvey stalled over the upper coast in 2017, dropped more than 60 inches of rain near Nederland (a U.S. tropical cyclone record), and flooded more than 100,000 homes in Harris County alone. Inland, the Interstate 35 corridor from San Antonio through Austin is nicknamed flash flood alley, and the February 2021 winter storm burst pipes in homes across the state. Every one of those events starts the same clock: EPA guidance says wet porous materials are at risk of mold colonization within 24 to 48 hours.

The Texas gradient vs the U.S. average

Humidity values are approximate, from NOAA 1991-2020 normals, rounded to whole percent.

Why Houston tops the ranking. Three factors compound. First, humidity: the Houston airports report annual averages within a point or two of New Orleans. Second, geography: the metro drains through slow bayous across flat clay that barely absorbs water, which is why one stalled storm can put whole neighborhoods under water for days. Third, scale: millions of housing units sit in that flood plain, from slab-on-grade ranches to older pier-and-beam blocks where a damp crawl space feeds the floor above it.

Why Dallas-Fort Worth still makes the list. DFW humidity is close to the national average, so the risk shifts from climate to plumbing and weather damage. Spring supercells drop hail that opens roofs. Expansive clay soil moves foundations enough to crack supply lines inside slabs. The 2021 freeze showed how fast burst pipes turn a dry metro wet. The mold that follows hides in wall cavities rather than on window sills, which is why mold behind walls is the DFW pattern to learn.

EPA indoor air quality guidance lists the same core genera for warm humid climates. Texas homes east of Interstate 35 concentrate five of them.

Species information is general and informational. Health responses vary. See a licensed physician for symptom evaluation. If you suspect hidden growth after a flood or leak, an ERMI dust test can profile the spores that settled in your home. Species sources: EPA Mold Remediation in Schools and Commercial Buildings, EPA A Brief Guide to Mold Moisture and Your Home, CDC National Center for Environmental Health guidance.

Three moves cover most of the controllable risk, and one piece of state paperwork protects your resale value.

A. Run the storm checklist every season

Coastal metros, by June 1: roof inspection, gutters cleared, HVAC serviced, flood policy verified (NFIP has a 30-day waiting period). First 48 hours after any flood, surge, or burst pipe: remove standing water, open windows only when outdoor humidity is lower than indoor, run any working AC or dehumidifier, and photograph everything for insurance. First 7 days: cut out wet drywall a foot above the water line, pull saturated insulation and carpet pad, dry framing with air movers, and log moisture readings daily.

B. Hold indoor humidity below 50 percent

EPA generally advises keeping indoor relative humidity between 30 and 50 percent when possible. Use a reliable hygrometer and repeat readings in different locations and at different times. The Indoor Humidity Context Calculator reports measured RH plus calculated dew point and absolute humidity; it does not predict mold growth or prescribe equipment settings. Use our guide on how to prevent mold for general moisture-control context.

C. Know the licensing line before you hire or DIY

Texas regulates this work. Under the Texas Mold Assessment and Remediation Rules (TMARR), a job where contamination covers 25 contiguous square feet or more requires a TDLR-licensed assessor and remediator, and the state separates the two roles on the same project. For small spots under the threshold, our how to get rid of mold guide covers safe cleanup. For anything bigger, hire a qualified mold remediation professional (one who follows the IICRC S520 standard; ACAC or RIA credentials and state licenses count too) and confirm the Texas license on TDLR's site. When licensed work finishes and passes assessment, you get the Certificate of Mold Damage Remediation. Keep it: it documents the fix for insurers and transfers to a buyer if you sell within five years.

Monthly average relative humidity across the state, based on NOAA NWS climatological normals for 1991-2020. Values are approximate statewide averages, rounded to whole percent.

The statewide average hides the gradient. Houston runs roughly ten points above every bar on this chart, and El Paso runs roughly 25 below. The midsummer dip reflects hot dry afternoons inland, not safety: coastal dew points peak in July and August, and May and September bring the state's heaviest rain. Type your ZIP into the mold risk by ZIP tool for your local read.

Texas runs one of the strictest state mold programs in the country. These are the authoritative starting points.

Every source chip on this page points here. Agency and statute pages first, then the federal technical references.