Mold Allergies Across America: What One of the Largest Allergy Studies Ever Conducted Revealed

Why Mold Isn’t Just a Florida Problem

When most people think about mold, they often picture humid climates, coastal regions, or homes that have experienced flooding.

Florida, Louisiana, and other moisture-prone areas are commonly associated with mold growth and indoor air quality concerns. But what if mold-related health effects were far more widespread than many people realize?

In 2011, Quest Diagnostics released one of the largest allergy studies ever conducted in the United States, analyzing millions of allergy blood tests from patients across the country. The findings revealed something surprising:

Mold sensitization wasn’t limited to humid regions.

In fact, some of the highest rates of mold sensitization were found in cities located in dry, desert, and semi-arid climates.

The study serves as an important reminder that mold is not simply a weather problem.

It’s a building problem.

One of the Largest Allergy Analyses Ever Performed

Quest Diagnostics’ Health Trends™ Report, Allergies Across America™, analyzed nearly 14 million allergen-specific IgE blood tests collected from more than 2 million patient encounters over a four-year period.

These results were drawn from a national laboratory database containing more than 1.5 billion patient encounters dating back to 2000.

The study evaluated reactions to eleven common allergens, including:

• Mold
• Ragweed
• Dust mites
• Cat dander
• Dog dander
• Several common food allergens

The size of the dataset provided researchers with an unprecedented look at allergy sensitization patterns throughout the United States.

Mold Sensitization Increased Nationwide

One of the most notable findings was that mold sensitization increased by approximately 11.6% during the study period.

Overall allergen sensitization increased by 5.8%, but mold was among the fastest-growing environmental allergens identified in the report.

Researchers also found that mold sensitization occurred across all regions of the country rather than being concentrated in one geographic area.

Unlike allergens such as ragweed, which tend to have predictable regional patterns, mold-related sensitization appeared to be a truly nationwide issue.

This finding challenges the common belief that mold allergies are primarily a problem in humid climates.

The Cities with the Highest Mold Sensitization Rates

Quest evaluated allergy testing data from thirty major metropolitan areas and calculated the percentage of tested patients who demonstrated sensitivity to mold.

Some of the highest rates included:

City	Mold Sensitization Rate
Dallas, Texas	21%
Riverside-San Bernardino, California	20%
Phoenix, Arizona	19%
Los Angeles, California	17%
Chicago, Illinois	17%
Denver, Colorado	16%
Minneapolis-St. Paul, Minnesota	16%
Kansas City, Missouri	16%
St. Louis, Missouri	16%

These results are particularly interesting because several of the highest-ranking cities are located in areas not typically associated with mold concerns.

The Desert Myth

Perhaps the most surprising finding was the number of dry-climate cities appearing near the top of the list.

Consider these examples:

• Phoenix, Arizona: 19%
• Riverside-San Bernardino, California: 20%
• Los Angeles, California: 17%
• Denver, Colorado: 16%
• Las Vegas, Nevada: 10%

Most people would not consider these cities to be “mold climates.”

Yet significant numbers of allergy-tested individuals showed sensitivity to mold.

Why?

Because mold growth is driven by moisture inside buildings, not simply outdoor weather conditions.

Even in dry climates, homes and buildings can experience:

• Plumbing leaks
• Roof leaks
• Window intrusion
• HVAC condensate problems
• Poor bathroom ventilation
• Crawlspace moisture
• Flooding events
• Hidden water damage inside walls

When moisture becomes trapped inside a building, mold can grow regardless of what the weather looks like outside.

What About Florida?

Given Florida’s humidity, many people expect the state to dominate mold allergy rankings.

Interestingly, Florida cities in the Quest study showed moderate mold sensitization rates compared to some western cities.

Among allergy-tested patients:

• Tampa, Florida: 10%
• Orlando, Florida: 9%
• Miami, Florida: 8%

This does not necessarily mean Florida has less mold.

Rather, it highlights an important distinction:

The study measured mold sensitization in patients who underwent allergy testing. It did not measure mold contamination in buildings.

Many factors can influence sensitization rates, including genetics, environmental exposures, healthcare access, and testing patterns.

The key takeaway is that mold sensitivity exists nationwide and should not be dismissed based solely on geography.

Why Indoor Environments Matter More Than Outdoor Climate

One of the most valuable lessons from the Quest study is that buildings create their own ecosystems.

People often focus on outdoor humidity and rainfall when discussing mold, but most mold exposures occur indoors.

The conditions that drive indoor mold growth include:

• Water intrusion
• Elevated humidity
• Building defects
• Poor ventilation
• HVAC issues
• Chronic condensation
• Inadequate drying following leaks or flooding

A home in Arizona with a hidden plumbing leak may have a greater mold problem than a well-maintained home in Florida.

The condition of the building matters more than the ZIP code.

What This Means for Homeowners

If you’re experiencing persistent allergy symptoms, respiratory issues, sinus problems, headaches, or unexplained indoor air quality concerns, it may be worth considering the condition of your home environment.

Many homeowners assume mold cannot be the issue because:

• They live in a dry climate
• They don’t see visible mold
• They haven’t experienced major flooding

However, water damage often remains hidden behind walls, beneath flooring, inside attics, crawlspaces, and HVAC systems.

The Quest study reinforces an important message:

Mold exposure is not limited to one region, one climate, or one type of home.

Where moisture exists, mold can follow.

The Bigger Picture

The Quest Diagnostics study represents one of the largest analyses of allergy sensitization ever conducted in the United States.

Its findings challenge long-held assumptions about mold and demonstrate that mold sensitivity exists throughout the country, including regions many people consider too dry for mold problems.

For homeowners, building professionals, healthcare providers, and indoor environmental specialists, the lesson is clear:

Mold should not be viewed as a regional issue.

It should be viewed as a building health issue.

Because when moisture enters a structure and remains unresolved, mold doesn’t care whether the home is located in Florida, Arizona, Colorado, or California.

Healthy buildings begin with moisture control, proper maintenance, and an understanding that indoor environments often tell a very different story than outdoor climates.