Types of Mold Found in the St. Louis Region: A Complete Guide
The St. Louis metropolitan area experiences some of the highest mold spore counts in the United States, particularly during fall months when conditions are ideal for mold growth. Understanding the types of mold found in the St. Louis region can help residents manage allergies, protect their health, and maintain safer indoor environments.
This comprehensive guide explores the most common mold species in St. Louis County and surrounding areas, their health impacts, and seasonal patterns you need to know.
Why St. Louis Has High Mold Counts
St. Louis's humid continental climate creates perfect conditions for mold growth year-round. The combination of hot, humid summers and mild, wet autumns provides the moisture and temperature conditions that mold spores need to thrive. In September 2025, St. Louis recorded an alarming 139,869 mold spores per cubic meter of air—among the highest counts recorded in the region's history.
Recent rainfall followed by warm temperatures creates the ideal environment for mold to flourish on decaying leaves, grass, and plant matter throughout the metro area. These conditions make understanding local mold types essential for St. Louis residents.
Most Common Mold Types in St. Louis
1. Cladosporium
Most CommonCharacteristics: The most prevalent mold species in outdoor air throughout the St. Louis region, Cladosporium thrives on fabrics, carpets, and wood surfaces both indoors and outdoors.
Health Effects:
- Skin irritation and rashes
- Respiratory issues and asthma exacerbation
- Eye irritation and watery eyes
- Chronic sinus infections
Where Found: Commonly appears on shower curtains, window sills, carpets, and outdoor vegetation. Particularly abundant during fall months when leaves begin to decay.
2. Alternaria
Most AllergenicCharacteristics: Alternaria is the most common allergenic mold found in St. Louis homes, especially in bathrooms and areas with water damage. This dark-colored mold grows in chain-like formations.
Health Effects:
- Severe allergic reactions
- Asthma attacks in sensitive individuals
- Upper respiratory symptoms
- Skin infections in immunocompromised individuals
Where Found: Bathrooms, showers, under sinks, basements, and areas with water leaks. Outdoor concentrations spike in fall when plant material decays.
3. Aspergillus
Potentially DangerousCharacteristics: A diverse genus with multiple species found throughout St. Louis County. Aspergillus fumigatus is particularly common and forms long chains of spores that easily become airborne.
Health Effects:
- Aspergillosis (serious lung infection)
- Allergic bronchopulmonary aspergillosis (ABPA)
- Respiratory distress
- Invasive infections in immunocompromised individuals
Where Found: Soil, compost piles, HVAC systems, stored grains, and areas with water damage. Indoor concentrations can be dangerous for those with weakened immune systems.
4. Penicillium
Characteristics: Penicillium notatum and related species are commonly identified in St. Louis area air samples. This blue-green mold spreads quickly and produces a musty odor.
Health Effects:
- Chronic sinus infections
- Pulmonary inflammation
- Allergic reactions
- Respiratory infections
Where Found: Water-damaged buildings, carpets, wallpaper, insulation, and mattresses. Thrives in areas with high humidity.
5. Aureobasidium
Characteristics: A yeast-like fungus that starts pink or black and turns darker with age. Common on wooden surfaces and painted walls in the St. Louis area.
Health Effects:
- Skin infections and dermatitis
- Eye and nail infections
- Allergic reactions
- Respiratory symptoms
Where Found: Behind wallpaper, on painted surfaces, wooden decks, window frames, and caulking in bathrooms.
6. Chaetomium
Characteristics: Often found in water-damaged homes after flooding events common in the St. Louis region. Has a musty smell and cotton-like texture.
Health Effects:
- Skin and nail infections
- Neurological damage (in severe cases)
- Chronic sinus infections
- Allergic reactions
Where Found: Drywall, wallpaper, baseboards, and carpets in water-damaged buildings.
7. Fusarium
Characteristics: Grows in both cold and warm temperatures, making it a year-round concern in St. Louis. Often appears pink, white, or reddish in color.
Health Effects:
- Eye, skin, and nail infections
- Respiratory allergic reactions
- Potentially life-threatening in immunocompromised individuals
- Bone infections in rare cases
Where Found: Carpets, wallpaper, water-damaged fabrics, humidifiers, and air conditioning drip pans.
8. Mucor
Potentially DangerousCharacteristics: Fast-growing mold that appears white or grayish. Part of the group that causes mucormycosis, a serious infection.
Health Effects:
- Mucormycosis (rare but serious)
- Sinus infections
- Respiratory distress
- Potentially fatal in immunocompromised individuals
Where Found: HVAC systems, old carpets, areas with water damage, and organic compost.
9. Acremonium
Potentially DangerousCharacteristics: Starts as a small moist mold that evolves into a fine powdery substance. Often confused with other mold types initially.
Health Effects:
- Bone marrow disease
- Immune system compromise
- Respiratory infections
- Organ damage (in severe cases)
Where Found: Condensation areas, humidifiers, cooling coils, drain pans, and window sealants.
Seasonal Mold Patterns in St. Louis
🍂 Fall (Peak Season)
September through November
This is when St. Louis experiences its highest mold counts, with September typically seeing peak levels. The combination of decaying leaves, warm days, and cool nights creates explosive mold growth.
- Average spore count: Can exceed 100,000 spores per cubic meter
- Dominant species: Cladosporium, Alternaria, and Aspergillus
- Contributing factors: Leaf decay, autumn rains, temperature fluctuations
☀️ Summer
June through August
Warm temperatures and high humidity create elevated mold counts, particularly following thunderstorms. St. Louis County has measured up to 26 days of high mold counts in a single summer season.
- Average spore count: Moderate to high (10,000-50,000+ spores)
- Dominant species: Cladosporium, Penicillium, Aspergillus
- Contributing factors: Humidity, storms, high temperatures
🌸 Spring
March through May
Mold levels begin rising as temperatures warm and plant growth accelerates. Early spring rains contribute to increased outdoor mold.
- Average spore count: Low to moderate (5,000-20,000 spores)
- Dominant species: Cladosporium, Alternaria
- Contributing factors: Spring rains, thawing ground, plant growth
❄️ Winter
December through February
The lowest mold counts of the year occur during winter months, though indoor mold can still be problematic in heated buildings with poor ventilation.
- Average spore count: Low (under 5,000 spores)
- Dominant species: Indoor species like Penicillium and Aspergillus
- Contributing factors: Indoor heating, poor ventilation, condensation
Health Impacts of St. Louis Mold Exposure
Allergic Reactions
Approximately 10-20% of St. Louis residents are sensitive to mold allergens, experiencing:
- Sneezing and runny nose
- Itchy, watery eyes
- Coughing and wheezing
- Skin rashes and hives
- Throat irritation
- Sinus congestion
Asthma Exacerbation
For the estimated 300,000+ asthma sufferers in the St. Louis metropolitan area, high mold counts can trigger:
- Severe asthma attacks
- Increased need for rescue inhalers
- Emergency room visits
- Decreased lung function
- Chronic respiratory inflammation
Serious Infections
Immunocompromised individuals face higher risks of invasive mold infections, particularly from Aspergillus and Mucor species prevalent in the St. Louis region.
St. Louis County Mold Monitoring System
St. Louis County operates one of the most comprehensive pollen and mold monitoring programs in the United States through the St. Louis County Pollen and Mold Center. Understanding this system helps residents protect their health.
How Monitoring Works
- Daily sampling: Air samples collected on weekdays
- Spore counting: Trained technicians count spores under microscopes
- Species identification: Major mold types are identified and reported
- Public reporting: Data published daily at pollenandmold.stlouisco.com
Count Level Categories
Where to Check Daily Counts
- St. Louis County Official Site: pollenandmold.stlouisco.com
- Missouri DHSS Dashboard: ephtn.dhss.mo.gov
- Local News: FOX 2 and KSDK provide daily allergy forecasts
- Charts and Historical Data: pollenandmoldcharts.stlouisco.com
Protecting Yourself from St. Louis Mold
Outdoor Protection
- Monitor daily counts before spending time outdoors
- Limit outdoor activities when counts exceed 50,000 spores
- Keep windows closed during high count days, especially fall mornings
- Shower after outdoor activities to remove spores from skin and hair
- Use HEPA air filters in your home and car
Indoor Prevention
- Control humidity - Keep levels below 50% with dehumidifiers
- Fix water leaks immediately - Don't let moisture accumulate
- Use exhaust fans in bathrooms and kitchens
- Clean regularly - Focus on bathrooms, basements, and HVAC systems
- Replace HVAC filters monthly during high mold seasons
- Remove wet materials quickly - Especially after flooding events
Medical Management
- Allergy testing at St. Louis area allergists for mold sensitivities
- Antihistamines for symptom relief during peak seasons
- Nasal corticosteroids to reduce inflammation
- Immunotherapy (allergy shots) for long-term management
- Emergency action plans for asthma sufferers
When to Seek Professional Mold Remediation
Contact a certified mold remediation specialist if you:
- See visible mold growth covering more than 10 square feet
- Smell persistent musty odors without visible mold
- Have experienced flooding or significant water damage
- Notice health symptoms that improve when away from home
- Find mold in HVAC systems or ductwork
- Discover mold after storm damage or plumbing leaks
Need Professional Mold Testing in St. Louis?
We provide comprehensive mold testing services throughout the St. Louis metropolitan area, including Clayton, Webster Groves, The Hill, and Central West End.
Key Takeaways for St. Louis Residents
Fall is peak season - September through November brings the highest mold counts, often exceeding 100,000 spores per cubic meter
Cladosporium dominates - This mold type is most prevalent in outdoor St. Louis air year-round
Monitor daily counts - Use the St. Louis County Pollen and Mold Center website before planning outdoor activities
Indoor control is crucial - Managing humidity and fixing water leaks prevents dangerous indoor mold growth
Know your risks - Those with asthma, allergies, or compromised immune systems should take extra precautions during high-count days
Year-round awareness - While fall is worst, summer humidity and indoor winter conditions create year-round mold concerns
Conclusion
Understanding the types of mold found in the St. Louis region empowers residents to protect their health and homes. With some of the nation's highest fall mold counts, St. Louis County's comprehensive monitoring system provides essential data for managing exposure. By knowing which mold species are most common—Cladosporium, Alternaria, Aspergillus, and Penicillium—and when they peak, you can take proactive steps to reduce exposure and minimize health impacts.
Stay informed by checking daily mold counts at the St. Louis County Pollen and Mold Center, maintain proper indoor humidity levels, and consult with local allergists if you experience persistent symptoms. With proper awareness and precautions, St. Louis residents can successfully navigate even the highest mold count days.
Additional Resources
- St. Louis County Pollen and Mold Center: pollenandmold.stlouisco.com
- Missouri Department of Health and Senior Services: ephtn.dhss.mo.gov
- St. Louis County Services: stlouiscountymo.gov/services/pollen-and-mold
This article is for informational purposes only and should not replace medical advice from qualified healthcare professionals.