Air Quality in San Marcos, TX: Moderate (66 AQI)
AQI 66 · Moderate · PM2.5 · Updated 2026-05-03
San Marcos's air quality is moderate. Unusually sensitive people should consider reducing prolonged outdoor exertion.
Data: EPA AirNow, FEMA National Risk Index Last verified: 2026-05-03
What is AQI?
The Air Quality Index (AQI) is a standardized scale from 0 to 500 that measures air pollution levels. 0-50 is "Good," 51-100 is "Moderate," 101-150 is "Unhealthy for Sensitive Groups," and above 150 is "Unhealthy" for everyone. AQI tracks five major pollutants: ground-level ozone, particulate matter (PM2.5 and PM10), carbon monoxide, sulfur dioxide, and nitrogen dioxide. San Marcos's average AQI of 66 falls in the "Moderate" range.
How do wildfires affect air quality?
Wildfire smoke contains fine particulate matter (PM2.5) that can travel hundreds of miles and dramatically spike AQI readings. During fire season, even cities far from active fires can experience unhealthy air. HEPA air purifiers (rated for PM2.5) are the most effective indoor protection. Check AirNow.gov for real-time conditions during fire season.
Air Quality by ZIP Code in San Marcos
AQI varies across San Marcos's 2 ZIP codes based on proximity to pollution sources and local geography.
| ZIP Code | AQI | Category | Wildfire | Mold | |
|---|---|---|---|---|---|
| 78666 | 66 | Moderate | 40 | 47 | View report → |
| 78667 | 66 | Moderate | 40 | 42 | View report → |
Air Quality Overview
San Marcos, Texas has moderate air quality with an average AQI of 66. Air quality is acceptable, though some pollutants may be a concern for a small number of people who are unusually sensitive to air pollution. The dominant pollutant is PM2.5.
The Air Quality Index (AQI) is an EPA-standardized scale from 0 to 500. Values below 50 are "Good," 51-100 "Moderate," 101-150 "Unhealthy for Sensitive Groups," 151-200 "Unhealthy," 201-300 "Very Unhealthy," and above 300 "Hazardous."
Dominant Pollutant: PM2.5
PM2.5 (fine particulate matter) consists of particles smaller than 2.5 micrometers — about 30 times thinner than a human hair. These particles can penetrate deep into the lungs and enter the bloodstream.
Health effects of PM2.5 exposure:
- Short-term: coughing, shortness of breath, aggravated asthma
- Long-term: reduced lung function, cardiovascular disease, premature death
- Most vulnerable: children, elderly, people with heart or lung disease
Common sources in urban areas: vehicle exhaust, construction dust, industrial emissions, wildfire smoke, residential wood burning.
Air Quality by ZIP Code in San Marcos
AQI varies across San Marcos's 2 ZIP codes based on proximity to pollution sources, traffic patterns, and local geography:
Wildfire Smoke Risk
San Marcos has an elevated wildfire smoke risk with an average smoke risk score of 40 (scale 0-100). There have been 73 fires recorded within 100 km of city ZIP codes.
| Metric | Value |
|---|---|
| Avg. smoke risk score | 40 |
| Max smoke risk score | 40 |
| Fires within 100 km | 73 |
| High-risk ZIP codes | 0 |
Wildfire smoke contains PM2.5, carbon monoxide, and volatile organic compounds. Even short-term exposure can trigger asthma attacks and respiratory distress.
Indoor Mold Risk
San Marcos has an average mold risk score of 45 out of 100.
| Factor | Value |
|---|---|
| Mold risk score | 45/100 |
| Average humidity | 65% RH |
| High-risk ZIPs | 0 |
| Peak season | winter high |
Mold risk is influenced by humidity levels, housing age (older homes have more moisture issues), flood history, and water infrastructure quality. Indoor humidity above 60% creates favorable conditions for mold growth.
Signs of mold problems: musty odors, visible growth on walls or ceilings, worsening allergies indoors, condensation on windows. Professional mold testing ($300-$600) can identify hidden issues.
Health Recommendations
With moderate air quality (AQI 66), most people can be active outdoors. Sensitive groups should take precautions:
- Sensitive groups (asthma, COPD, heart disease): consider reducing prolonged outdoor exertion
- Keep windows closed on high-pollution days
- Use a HEPA air purifier in main living areas
- Check AirNow.gov before outdoor exercise
San Marcos vs. Nearby Texas Cities
How San Marcos's air quality compares to nearby cities:
| City | AQI | Category | Population |
|---|---|---|---|
| San Marcos | 66 | Moderate | 137,288 |
| Abilene | N/A | N/A | 137,705 |
| Carrollton | 54 | Moderate | 136,170 |
| Beaumont | 31 | Good | 135,882 |
ZIP Codes in San Marcos
San Marcos covers 2 ZIP codes. Air quality can vary by neighborhood based on proximity to highways, industrial areas, and green spaces. Check your specific ZIP code for localized data.
Data Sources
- Air quality: EPA AirNow — real-time and historical AQI data
- Wildfire risk: FEMA National Risk Index
- Mold risk: Derived from EPA, FEMA, Census ACS (humidity, housing age, flood history)
- Monthly AQI: EPA Air Quality Statistics
Updated daily.