Mold Exposure Across the US: States and Cities at Highest Risk

Mold is a pervasive issue that can affect any indoor environment, leading to a range of health problems, particularly for individuals with allergies or respiratory conditions. While mold can grow virtually anywhere, certain cities and states in the United States are more prone to mold issues due to specific environmental factors and building practices.

Understanding Mold Sensitization in the United States

Research published in the Journal of Allergy and Clinical Immunology analyzed over 7.9 million serum-specific IgE test results from more than 1.6 million patients across the United States. The findings reveal that 22% of patients tested positive for at least one fungal allergen, with 13.7% showing sensitivity to multiple mold species. This groundbreaking study provides the most comprehensive picture of fungal sensitization patterns across different geographic regions.

"Independent risk factors for fungal sensitization include male sex, teen ages, atopic dermatitis, asthma, and ecoregion. Fungal sensitization was generally higher in urban areas and ecoregions composed predominantly of grasslands and prairies compared to woodlands and forest," according to the study published in the Journal of Allergy and Clinical Immunology.

High-Risk States for Mold Exposure

Studies have identified California, Ohio, and Texas as states with a significant number of cities that rank among the worst for mold allergies. This is based on the prevalence of mold sensitization among allergy sufferers within these states.

Southern States Face the Highest Mold Risk

According to comprehensive research by FDP Mold Remediation, southern states consistently rank highest for mold-related challenges. The study created a "Mold Index" based on four key factors: average humidity, annual rainfall, temperature, and building age. Florida topped the list with a Mold Index of 38.76, followed by Mississippi (38.66) and Louisiana (38.52).

The southeastern region's combination of high temperatures, substantial rainfall, and elevated humidity creates optimal conditions for mold growth. Florida maintains year-round temperatures averaging 73°F with approximately 54.6 inches of annual rainfall, while Mississippi experiences the highest rainfall among all states at 59.44 inches annually with 72.5% average humidity.

California's Mold Challenges

In California, four cities have been identified within the top 20 worst cities for mold allergies. The state's diverse climate, ranging from coastal humidity to inland dryness, creates varied environments conducive to mold growth.

Ohio and Texas: Mold Sensitization Concerns

Ohio and Texas each have two cities listed in the top 20, with Dallas, Texas, having the highest percentage of allergy patients showing sensitization to mold at 21%. These states experience high humidity levels and significant rainfall, which contribute to the proliferation of mold.

The study's findings align with broader research on fungal allergen prevalence. In Dallas specifically, the combination of warm temperatures (averaging 68°F) and moderate humidity creates conditions where mold spores can thrive both outdoors and indoors.

Southwestern United States: A Mold Hotspot

The southwestern region of the United States is particularly problematic for mold allergy sufferers, with four of the top five worst cities for mold sensitization located in this area. Cities like Riverside-San Bernardino and Phoenix are notable for their high percentages of mold sensitization among allergy patients.

Interestingly, even in drier southwestern climates, mold sensitization rates remain surprisingly high. Riverside-San Bernardino shows a 20% mold sensitization rate, while Phoenix reports 19%—rates that rival more humid regions. This demonstrates that indoor factors, particularly building practices and moisture control, can be more important than outdoor climate alone.

Cities with High Mold Sensitization Rates

The cities with the highest percentages of allergy patients showing sensitization to mold include:

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%

These rankings reveal a surprising pattern: mold sensitization is not limited to humid, rainy regions. Denver and Phoenix, both located in relatively dry climates, show comparable sensitization rates to much wetter regions like Minneapolis-St. Paul and Kansas City. This suggests that indoor environmental factors—including building construction, ventilation systems, and moisture management—may play a more critical role than outdoor climate alone.

Other States with Notable Mold Prevalence

Beyond the top-ranked states, several others experience significant mold challenges. States such as Florida, Oklahoma, South Carolina, Nevada, Arizona, South Dakota, Tennessee, and Kansas show elevated mold prevalence. Factors like high humidity, annual precipitation, and water damage from natural disasters make these states more susceptible to mold growth.

According to the Institute of Medicine's comprehensive review of building dampness and mold, environmental factors such as climate and the likelihood of natural disasters can contribute significantly to the prevalence of mold allergens in a given area. Mold levels are typically higher during warmer months when humidity is elevated, though mold spores can be found year-round in any climate.

Climate Change and Increasing Mold Duration

Research indicates that climate change is extending mold exposure seasons across the United States. Increased temperatures and altered precipitation patterns are creating longer periods of favorable conditions for mold growth. This trend suggests that regions previously considered low-risk may face increasing mold challenges in coming years.

The Critical Role of Indoor Environmental Factors

While outdoor climate patterns matter, indoor conditions often have a greater impact on mold exposure. As noted by building science experts at Lawrence Berkeley National Laboratory, "Homes and other buildings should be designed, operated, and maintained to prevent water intrusion and excessive moisture accumulation when possible."

Mold Growth Conditions

Mold thrives in environments with high humidity, moderate temperatures, and organic matter to consume. Homes with poor ventilation, such as basements and attics, are common sites for mold growth. Additionally, building materials and maintenance practices play a significant role in indoor mold proliferation, regardless of the external humidity levels.

The four essential factors for mold growth include:

• Mold spores (present everywhere in microscopic quantities)
• Moisture (from leaks, condensation, or high humidity above 50%)
• Food sources (any organic material containing carbon, including wood, paper, and dust)
• Comfortable temperatures (typically between 50-70°F, though mold can survive in a wider range)

Building Age and Construction Practices

An often-overlooked factor in mold risk is the age and construction method of buildings. The FDP Mold Remediation study found that buildings constructed before central air conditioning and energy-efficient building methods became standard are more susceptible to moisture infiltration from the outdoor environment.

Older buildings often lack:

• Proper vapor barriers
• Adequate insulation
• Modern HVAC dehumidification systems
• Effective drainage systems around foundations

These deficiencies can lead to significant indoor air quality concerns from mold growth, even in drier climates. This explains why Nevada, despite being the driest state, still has mold issues in older structures.

Preventing and Controlling Mold Growth

To prevent mold growth, it is essential to control indoor humidity levels, promptly address water leaks, and ensure proper ventilation in high-risk areas. The World Health Organization emphasizes that "well-designed, well-constructed, well-maintained building envelopes are critical to the prevention and control of excess moisture and microbial growth."

Recommended Prevention Strategies

• Maintain indoor humidity between 30-60% using dehumidifiers or air conditioning
• Fix water leaks immediately to prevent moisture accumulation
• Ensure proper ventilation in bathrooms, kitchens, and high-moisture areas
• Dry wet areas within 24-48 hours to prevent mold colonization
• Use exhaust fans to remove moisture during cooking and showering
• Direct water away from foundations through proper grading and gutters
• Inspect regularly for signs of water damage or musty odors

For a comprehensive approach to identifying and addressing mold in your home, consult the MoldCo Home & Health Guide, which provides detailed home inspection checklists and evidence-based mold prevention strategies.

When Professional Testing and Remediation Are Necessary

If you suspect mold exposure based on your location or symptoms, professional testing can help identify the extent of the problem. MoldCo offers specialized testing services including HERTSMI-2 home testing, which analyzes dust samples for the five most problematic mold species associated with health impacts.

Take the MoldCo Signs questionnaire to assess whether you may be experiencing symptoms related to mold exposure. If you're showing signs of mold-related illness, consider the MoldCo Starter Panel to evaluate biomarkers associated with mold exposure.

Conclusion: Location Matters, But Control Is Key

While mold can grow in any state, certain regions face higher risks due to environmental factors and building practices. Understanding the conditions that favor mold growth and taking proactive measures can help mitigate the risks associated with mold exposure.

The research is clear: 22% of Americans show sensitivity to at least one fungal allergen, and geographic location plays a significant role in exposure risk. However, proper building design, maintenance, and moisture control can dramatically reduce mold growth regardless of your location.

Whether you live in high-risk Florida or dry Nevada, vigilant moisture management and prompt attention to water intrusion remain your best defenses against problematic mold growth.

Disclaimer: This information is intended for general educational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a healthcare professional for personalized medical advice.