Overview
Aspergillus is a genus of mold that includes more than 300 identified species. It ranks among the most common mold genera found in both indoor and outdoor environments worldwide. Research consistently identifies Aspergillus and Cladosporium as the most diverse and prevalent mold genera in indoor air samples.
While many Aspergillus species are harmless, certain species can cause allergic reactions, respiratory symptoms, and—in immunocompromised individuals—serious infections. Some species also produce mycotoxins, toxic compounds that may present health concerns under specific conditions.
Key Point: There are no federal regulations or Threshold Limit Values (TLVs) for airborne mold concentrations in the United States. The EPA and CDC do not establish “safe” levels of indoor mold (EPA, 2024).
Common Aspergillus Species
Four species account for the majority of health-related concerns in indoor environments:
Aspergillus fumigatus
The primary species causing human aspergillosis infections. Commonly found in decaying organic matter, compost, and HVAC systems. Presents the greatest infection risk to immunocompromised individuals.
Aspergillus niger
Also known as “black mold” (though this term more commonly refers to Stachybotrys). Produces the mycotoxin gliotoxin. Commonly found on fruits, vegetables, and damp building materials.
Aspergillus flavus
Produces aflatoxin, a known carcinogen. More common in agricultural settings but can occur indoors on improperly stored grains or nuts. Less common in typical residential settings.
Aspergillus versicolor
Common in damp buildings. Produces the mycotoxin sterigmatocystin when water activity exceeds 0.95. Research indicates this species has a higher Fragment-to-Spore ratio, potentially increasing exposure through fragmented particles.
Where Aspergillus Is Found
Indoor Locations
- HVAC systems (condensate pans, evaporator coils, ductwork)
- Damp or water-damaged building materials
- Areas with poor ventilation
- Houseplant soil and decaying organic matter
- Stored food products (especially grains and nuts)
- Basements, crawl spaces, and areas with persistent moisture
Outdoor Sources
- Soil and compost piles
- Decaying leaves and vegetation
- Agricultural areas
- Construction sites with disturbed soil
Because Aspergillus spores are small and lightweight, they easily become airborne and can enter indoor environments through windows, doors, and HVAC systems. Some level of Aspergillus presence is normal in most indoor environments.
Growth Conditions
| Factor | Optimal Range | Notes |
|---|---|---|
| Temperature | 12-50°C (54-122°F) | Optimal growth near 37°C (98.6°F) |
| Relative Humidity | >60% | CDC recommends maintaining 30-50% indoor RH |
| Water Activity (aw) | 0.75-0.95+ | Mycotoxin production increases at aw >0.95 |
| Substrates | Cellulose, soil, organic matter | Drywall paper, dust, decaying plants |
Aspergillus species can grow on a wide variety of substrates and tolerate a broad range of conditions. This adaptability contributes to their prevalence in indoor environments worldwide.
Health Effects
Health effects from Aspergillus exposure vary significantly based on species, exposure level, and individual susceptibility. The CDC categorizes effects into three primary types:
Allergic Reactions
The most common health effect. Sensitive individuals may experience:
- Stuffy or runny nose
- Eye irritation (itching, watering, redness)
- Wheezing and coughing
- Skin irritation or rash
- Asthma exacerbation
- Hypersensitivity pneumonitis (in severe cases)
Aspergillosis (Infection)
Primarily affects immunocompromised individuals. Aspergillus fumigatus is the primary causative species. Risk factors include:
- Weakened immune system (HIV/AIDS, organ transplant, chemotherapy)
- Chronic lung conditions (COPD, cystic fibrosis, asthma)
- Very young or elderly age
- Prolonged corticosteroid use
Mycotoxin Exposure
Certain Aspergillus species produce mycotoxins under specific conditions:
- Aflatoxin (A. flavus) — carcinogenic, primarily a food contamination concern
- Gliotoxin (A. niger) — immunosuppressive properties
- Sterigmatocystin (A. versicolor) — carcinogenic, produced at high water activity
Important: If you are immunocompromised or experience persistent symptoms, consult a healthcare provider. Mold inspectors do not provide medical advice.
Testing & Identification
Aspergillus is identified through laboratory analysis of air or surface samples. Common methods include:
Air Sampling Methods
- Spore trap sampling — Captures spores on a sticky medium for microscopic analysis
- Culturable sampling — Grows viable molds on growth media for species identification
- PCR/DNA analysis — Identifies species through genetic material (most precise but costlier)
What Testing Reveals
Laboratory reports typically identify Aspergillus at the genus level, sometimes with species-level identification. Spore counts are compared to outdoor baseline samples to assess whether indoor levels exceed normal background concentrations.
Aspergillus in Oklahoma Homes
Oklahoma’s humid subtropical climate creates conditions favorable for Aspergillus growth, particularly during the high-humidity months from May through September.
Regional Factors
- Seasonal humidity patterns — Peak growth aligns with Oklahoma’s humid summer months
- HVAC dependency — Year-round AC use creates condensation opportunities in ductwork and coils
- Storm and flood damage — Tornado and flooding aftermath creates moisture intrusion pathways
- Older housing stock — Some Oklahoma homes lack modern moisture control features
Common Problem Areas
- HVAC systems (condensate pans, evaporator coils, supply ducts)
- Attics with inadequate ventilation
- Crawl spaces with moisture barriers issues
- Basements and storm shelters
- Areas around windows with condensation
Professional Assessment: TrueSight Environmental provides objective mold testing in Oklahoma with no conflict of interest—we don’t sell remediation services. Learn about our inspection process →
Research & Citations
- Centers for Disease Control and Prevention. “Mold: Basic Facts.” CDC.gov, 2024. https://www.cdc.gov/mold/
- Environmental Protection Agency. “Mold Resources.” EPA.gov, 2024. https://www.epa.gov/mold/
- National Institutes of Health. “Aspergillus and Health Effects.” NIH.gov.
- IICRC S520 Standard for Professional Mold Remediation. Institute of Inspection, Cleaning and Restoration Certification.
- EPA. “Mold Remediation in Schools and Commercial Buildings.” EPA 402-K-01-001.