Understanding Mold Exposure: The Most Impacted Cities and States

Mold isn't just an unsightly problem—it's a health concern that affects millions of Americans. While mold can grow virtually anywhere with sufficient moisture, certain US cities face significantly higher mold exposure risks due to climate, humidity, and other environmental factors.

Understanding which regions are most affected by mold can help homeowners and renters take proactive steps to protect their health and property. This guide explores the geographic distribution of mold across the United States, the factors contributing to mold growth, and evidence-based prevention strategies.

States with the Highest Mold Exposure Risk

Research from Quest Diagnostics' Allergies Across America study—the largest analysis of allergy testing in the United States—identified California, Ohio, and Texas as states with the highest concentration of cities affected by mold allergies. These states had the greatest number of cities ranked among the top 20 worst for allergy sufferers, indicating widespread mold exposure challenges.

California: A Mold Hotspot

California's diverse climate creates multiple breeding grounds for mold growth. From humid coastal regions to inland areas with temperature fluctuations, the Golden State contends with varied mold challenges. Cities such as Los Angeles and Riverside-San Bernardino experience significant mold sensitization rates among allergy patients, according to the Quest Diagnostics study.

The coastal humidity combined with California's dense population means millions of residents may be exposed to elevated mold levels in their homes and workplaces.

Ohio: Climate-Driven Mold Growth

Ohio's position in the Midwest exposes residents to a climate characterized by high humidity levels and considerable seasonal variation. According to the CDC, mold grows where there is moisture, and Ohio's humid conditions provide an ideal environment for mold proliferation.

"Mold will grow where there is moisture, such as around leaks in roofs, windows, or pipes, or where there has been a flood," notes the Centers for Disease Control and Prevention.

The combination of aging housing stock and moisture-prone weather patterns contributes to Ohio's status as a high-risk state for mold exposure.

Texas: Size and Diversity Create Challenges

As the second-largest state by both area and population, Texas faces diverse mold challenges across its varied climate zones. The Quest Diagnostics study found that cities like Dallas and San Antonio experience some of the highest mold sensitization rates in the nation, with Dallas leading at 21% of allergy patients showing sensitization to mold.

The Most Mold-Affected US Cities

The Allergies Across America study identified specific metropolitan areas with the highest percentages of mold sensitization among allergy patients:

Top 10 Cities for Mold Exposure

1. Dallas, Texas - 21%
2. Riverside-San Bernardino, California - 20%
3. Phoenix, Arizona - 19%
4. Los Angeles, California - 17%
5. Chicago, Illinois - 17%
6. Minneapolis-St. Paul, Minnesota - 16%
7. Saint Louis, Missouri - 16%
8. Denver, Colorado - 16%
9. Kansas City, Missouri - 16%
10. San Antonio, Texas - 15%

According to research published in the Journal of Environmental and Public Health, these cities demonstrate elevated levels of water damage-associated molds, influenced by factors including humidity, precipitation, and water damage history.

Additional High-Risk States

Beyond California, Ohio, and Texas, several other states face significant mold challenges. FDP Mold Remediation's comprehensive study analyzed climate data, housing age, and environmental factors to identify states with the highest mold risk:

Southern states dominate the highest-risk categories:

• Florida (Mold Index: 38.76) - High temperatures averaging 73°F year-round combined with 54.6 inches of annual rainfall
• Mississippi (Mold Index: 38.66) - Highest rainfall in the US at 59.4 inches annually with 72.5% average humidity
• Louisiana (Mold Index: 38.52) - 74% average humidity creates optimal mold conditions
• Alabama (Mold Index: 37.69) - 57.9 inches of annual rainfall
• Tennessee (Mold Index: 36.51) - Humid subtropical climate with aging housing stock

Additional states with notable mold prevalence include Florida, Oklahoma, South Carolina, Nevada, Arizona, South Dakota, Tennessee, and Kansas. These states experience mold challenges due to factors such as high humidity, significant rainfall, and water damage from weather events.

Understanding the Geography of Mold

Climate Factors That Promote Mold Growth

According to the Environmental Protection Agency, the key to mold control is moisture control. Geographic regions with certain climate characteristics face elevated mold risk:

High-Risk Climate Conditions:

• Average humidity levels above 60%
• Annual rainfall exceeding 40 inches
• Temperature ranges between 40°F and 100°F
• Seasonal flooding or water intrusion events

"It is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth," states the EPA's guidance on mold prevention.

The Southwestern US: An Unexpected Mold Region

While intuition might suggest that dry regions have less mold, the southwestern United States presents a complex picture. The Quest Diagnostics study identified this region as housing 4 of the top 5 worst cities for mold sensitization, despite generally arid conditions.

Cities like Phoenix and Denver show high mold allergy rates, which researchers attribute to factors beyond outdoor humidity. Indoor moisture from plumbing issues, HVAC condensation, and poor ventilation can create mold problems even in dry climates.

How Mold Impacts Health

Mold exposure can trigger a range of health effects, particularly in sensitive individuals. Exposure to mold and damp environments may cause various health responses:

Common Health Effects

• Respiratory symptoms: Stuffy nose, sore throat, coughing, or wheezing
• Eye and skin irritation: Burning eyes, skin rash, or irritation
• Asthma complications: Severe reactions in people with asthma
• Allergic reactions: Hay fever-type symptoms including sneezing and runny nose

People with asthma, allergies, or compromised immune systems are particularly vulnerable to mold's health effects. The Institute of Medicine found sufficient evidence linking indoor mold exposure with upper respiratory tract symptoms, cough, and wheeze in otherwise healthy individuals.

For those experiencing persistent symptoms that may be related to mold exposure, MoldCo's lab testing services can help identify potential mold-related health impacts through biomarker analysis.

Prevention Strategies for High-Risk Areas

Regardless of where you live, effective mold prevention focuses on moisture management. The EPA and CDC recommend these evidence-based strategies:

Control Indoor Humidity

• Maintain humidity below 50% - Ideally between 30-50% relative humidity
• Use dehumidifiers in high-moisture areas like basements
• Run exhaust fans in bathrooms and kitchens that vent to the outside
• Ensure proper ventilation throughout your home

Address Water Issues Promptly

• Fix leaks immediately in roofs, walls, windows, and plumbing
• Dry water-damaged areas within 24-48 hours to prevent mold growth
• Clean and repair roof gutters regularly
• Ensure proper drainage around your home's foundation

Proactive Home Maintenance

• Inspect for moisture problems regularly, especially in high-risk areas
• Use mold-resistant products when building or remodeling
• Remove or replace water-damaged materials that cannot be thoroughly dried
• Avoid carpeting in bathrooms, basements, or other moisture-prone areas

For residents in high-risk cities and states, these prevention measures are particularly critical. Regular home inspections and prompt attention to moisture issues can significantly reduce mold growth risk.

When to Seek Professional Help

While prevention is ideal, existing mold problems require professional assessment and remediation. Consider professional help when:

• Mold covers an area larger than 10 square feet
• Mold growth appears after flooding or major water damage
• Mold returns despite repeated cleaning attempts
• You experience persistent health symptoms that may be mold-related

MoldCo's comprehensive testing can help identify mold in your environment using the HERTSMI-2 standard, while health biomarker panels can assess whether mold exposure may be affecting your wellbeing.

The Connection Between Geography and Mold Illness

Research shows that while mold spores exist everywhere, concentrated exposure in high-risk geographic areas can increase health impacts. Studies using DNA-based mold analysis found that water damage-associated molds show distinct geographic patterns across the United States, though individual homes' mold burdens depend more on moisture control than location alone.

Understanding your region's mold risk helps inform prevention strategies, but remember: the best defense against mold is controlling moisture in your specific home environment, regardless of geographic location.

Take Control of Your Indoor Environment

Living in a high-risk city or state doesn't mean mold problems are inevitable. Awareness of your area's mold challenges empowers you to take proactive steps:

1. Assess your risk - Consider your local climate and home conditions
2. Implement prevention strategies - Focus on moisture and humidity control
3. Monitor for signs - Watch for musty odors, visible growth, or health symptoms
4. Test when needed - Use professional testing to identify hidden problems
5. Act quickly on moisture issues - Address water problems within 24-48 hours

For more information about identifying and addressing mold in your home, explore MoldCo's comprehensive guide or take the signs questionnaire to assess your potential mold exposure.

Conclusion

Mold exposure represents a significant concern across the United States, with certain cities and states experiencing disproportionately high rates of mold-related health issues. Dallas, Riverside-San Bernardino, Phoenix, Los Angeles, and Chicago lead the nation in mold sensitization rates, while southern states like Florida, Mississippi, and Louisiana face the highest overall mold risk due to climate factors.

Understanding the environmental factors that contribute to mold growth—particularly humidity, rainfall, and moisture management—empowers residents in high-risk areas to implement effective prevention strategies. Whether you live in a known mold hotspot or a lower-risk region, controlling indoor moisture remains the single most important factor in preventing mold growth and protecting your health.

This information is for educational purposes only and is not medical advice. If you suspect mold exposure or experience persistent health symptoms, consult with qualified healthcare providers for personalized advice.