Types of Mold: Complete Identification Guide to Indoor Mold Species

Introduction

"The color of the mold does not indicate its toxicity." That finding from the EPA contradicts what most people assume about mold—and it's just one of many misconceptions that can lead to either unnecessary panic or dangerous dismissal.

If you've discovered mold in your home, your first question was probably "what type is it?" That instinct is correct. Different mold species behave differently, produce different toxins, and require different responses. The challenge is that visual identification alone can't give you the answer.

Types of mold range from harmless background species that exist in every building to mycotoxin-producing organisms that can trigger serious illness in susceptible individuals. Research from Lawrence Berkeley National Laboratory found that approximately 47% of U.S. homes have dampness or mold—meaning understanding mold species isn't just academic, it's practical knowledge for nearly half of American households.

This guide covers:

• The major toxic mold species and their characteristics
• Which molds produce mycotoxins (and which don't)
• Why visual identification fails—and what works instead
• The critical difference between mold allergies and mold illness
• How to test for species identification
• When to take action and what kind

Medical Disclaimer: This guide provides educational information about mold species and their potential health effects. Always consult with a healthcare provider for medical concerns related to mold exposure.

Table of Contents

• Major Toxic Mold Species
• Understanding Mycotoxin Production
• Common Household Molds
• Water Damage Indicator Molds
• Why Color Doesn't Determine Toxicity
• Health Impact Comparison
• Testing Methods for Species Identification
• The Bigger Picture: Beyond Just Mold
• Key Takeaways
• Take Action

Major Toxic Mold Species

Not all molds are created equal. These species produce significant mycotoxins and warrant the most attention when found indoors.

Stachybotrys chartarum (Black Mold)

Stachybotrys—commonly called "black mold"—has earned its reputation. This species produces trichothecenes and satratoxins, among the most potent mycotoxins known.

Identification characteristics:

• Dark green to black coloration
• Slimy or wet appearance when actively growing
• Requires chronic moisture (not just humidity)
• Grows on cellulose materials: drywall, ceiling tiles, paper

Stachybotrys is classified as an ERMI Group 1 mold—the highest concern category for water damage indicators. Its presence signals ongoing or severe past water intrusion.

However, context matters. Research from the National Toxicology Program notes that the scientific community continues to debate whether Stachybotrys health effects come primarily from direct pathogenicity or from mycotoxin exposure. What's not debated: this species requires professional attention.

For complete coverage, see our Stachybotrys (black mold) guide.

Aspergillus Species

Aspergillus represents one of the most diverse mold genera, with species ranging from relatively harmless to seriously dangerous. The major species of concern include:

• A. fumigatus - Highest infection risk, produces gliotoxin
• A. flavus - Produces aflatoxins (carcinogenic)
• A. niger - Common black mold, moderate concern
• A. versicolor - ERMI Group 1 water damage indicator

This diversity illustrates why color-based identification fails. Aspergillus species can appear green, yellow, brown, or black depending on the specific species and growth conditions.

Key mycotoxins from Aspergillus:

• Aflatoxins - Carcinogenic, cause liver damage
• Ochratoxin A - Kidney toxicity, immune suppression
• Gliotoxin - Immune suppression, enables opportunistic infections

A. fumigatus poses the highest infection risk of any common indoor mold. Immunocompromised individuals are particularly vulnerable to invasive aspergillosis—an infection that requires immediate medical attention.

Penicillium

Penicillium species appear blue-green and powdery—the classic "moldy bread" appearance. Yes, this is the same genus that gave us penicillin antibiotics, but that doesn't make indoor Penicillium growth safe.

Characteristics:

• Blue-green, powdery texture
• Thrives in cool, damp conditions
• Common in carpets, wallpaper, insulation, behind refrigerators
• Produces Ochratoxin A and Citrinin

Penicillium species are strong allergens and can trigger respiratory symptoms even without significant mycotoxin production. Their ability to grow in cooler temperatures means they persist in environments where other molds might not thrive.

Chaetomium

Chaetomium grows in chronically wet materials and produces a distinctive musty odor that can serve as a detection clue before visual growth becomes apparent.

Identification:

• White to gray initially, darkens with age
• Distinctive strong musty smell
• Requires chronically wet materials (weeks of moisture)
• Produces chaetoglobosins (mycotoxins)

Chaetomium is classified as ERMI Group 1 and indicates severe, long-term water damage. Its presence suggests the moisture problem has existed for an extended period—typically longer than the two weeks required for Stachybotrys colonization.

Fusarium

Fusarium species stand out for their unusual characteristics: pink, white, or red coloration, and the ability to grow at near-freezing temperatures.

Key concerns:

• Produces trichothecenes and fumonisins
• Major agricultural contaminant (affects food supply)
• Can cause eye infections and respiratory issues
• Geographic range expanding with climate change

The European Environment Agency's 2024 report documents that climate change is expanding the geographic range of Fusarium and other mycotoxin-producing species—a trend expected to continue through 2100.

Understanding Mycotoxin Production

What Are Mycotoxins?

Mycotoxins are secondary metabolites produced by certain mold species. They're not essential for mold survival, but they serve various ecological functions—and happen to be toxic to humans.

Critical points:

• Not all molds produce mycotoxins
• Production depends on species, environmental conditions, and substrate
• Mycotoxins persist after mold dies
• Fragments carry mycotoxins (not just intact spores)

Research published in Critical Reviews in Food Science and Nutrition validates the FAO estimate that approximately 25% of the world's crops are contaminated with mycotoxins—and notes the actual figure above detectable levels may reach 60-80%.

The 500:1 Fragment Ratio

Here's a finding that changes how you should think about mold exposure: for each intact spore, there are approximately 500 fragments. These fragments carry mycotoxins and inflammatory compounds—but aren't detected by standard spore trap testing.

This explains why air quality tests often show "normal" results in buildings where occupants are clearly getting sick. The fragments causing the problem are too small to capture with conventional methods.

DNA-based testing methods like ERMI detect these fragments by identifying mold DNA regardless of particle size.

Major Mycotoxin Categories

Aflatoxins (from Aspergillus flavus, A. parasiticus)

• Among the most carcinogenic natural substances known
• Cause liver damage and immune suppression
• Major food supply contaminant

Ochratoxin A (from Aspergillus, Penicillium species)

• Kidney toxicity
• Immune suppression
• Long half-life in the body—exposure effects persist

Trichothecenes (from Stachybotrys, Fusarium)

• Severe toxicity at low concentrations
• Immune system damage
• Include satratoxins from Stachybotrys

Gliotoxin (from Aspergillus fumigatus)

• Suppresses immune function
• Enables secondary infections
• Particularly dangerous for immunocompromised individuals

Common Household Molds

These species are found in most buildings and primarily cause allergic reactions rather than toxic effects. They're common—but "common" doesn't mean harmless for sensitive individuals.

Alternaria

Alternaria is one of the most potent allergenic molds. Dark green to brown with a velvety texture, it thrives in damp areas and peaks during late summer and fall.

Common locations:

• Showers and bathrooms
• Window frames and sills
• Areas with condensation
• Outdoor sources (decaying plants)

For the estimated 21% of U.S. asthma cases attributable to dampness and mold, Alternaria is often a primary trigger.

Cladosporium

The most common indoor and outdoor mold, Cladosporium appears olive-green to brown or black. It grows in both cool and warm conditions, making it nearly ubiquitous.

Found on:

• Fabrics and carpets
• Wood surfaces
• HVAC systems
• Exterior siding

While primarily allergenic rather than toxic, elevated Cladosporium levels indicate moisture problems that may allow more dangerous species to establish.

Aureobasidium

This mold transitions from pink to black as it ages and develops a slimy texture. It indicates chronic moisture and commonly grows on painted surfaces and wood.

Health effects: Primarily dermatitis and allergic reactions.

Mucor

Mucor grows extremely fast—white to gray and fuzzy—on decaying organic matter. While usually just an allergen, it can cause mucormycosis in immunocompromised individuals, a rare but serious fungal infection.

Common sources: Old food, HVAC systems, areas with poor air circulation.

Water Damage Indicator Molds (ERMI Classification)

The Environmental Relative Moldiness Index (ERMI) categorizes molds into two groups based on their association with water-damaged buildings.

Group 1: Water Damage Indicators

These 26 species signal current or past water intrusion:

• Stachybotrys chartarum
• Chaetomium globosum
• Aspergillus fumigatus
• Aspergillus versicolor
• Wallemia sebi

Finding Group 1 molds means your building has (or had) a significant moisture problem. The source may be hidden—behind walls, in crawl spaces, or in HVAC systems.

Group 2: Common Indoor Molds

These 10 species represent normal indoor background levels:

• Cladosporium species
• Aspergillus niger
• Various Penicillium species

Their presence isn't alarming, but elevated levels warrant investigation.

HERTSMI-2: The Clinical Standard

HERTSMI-2 (Health Effects Roster of Type Specific Formers of Mycotoxins and Inflammagens) tests only five key species—the ones most associated with health effects in sensitive individuals.

Scoring interpretation:

• Greater than 15: Too dangerous for previously ill patients
• 11-15: Borderline, additional assessment needed
• Less than 11: Generally safe for re-entry

This scoring has clinical validation: research shows a HERTSMI-2 score below 11 was associated with less than 2% CIRS recurrence rate in a study of 807 patients. It's the only peer-reviewed test directly linking environmental measurements to patient outcomes.

For complete testing guidance, see our mold illness testing guide.

EPA Limitations on ERMI

Transparency matters: the EPA has explicitly warned about ERMI limitations.

"As the approach has not been validated through a multi-lab study, we cannot comment on the accuracy of information obtained from others using ERMI." — Kevin Oshima, Ph.D., EPA ERMI project lead

ERMI is valuable but not definitive alone. Additionally, research published in NIH/PMC found that Swiffer-collected samples showed 7x higher Stachybotrys levels than vacuum samples from identical environments—illustrating how sampling method affects results.

Best practice combines multiple testing methods with clinical correlation.

Why Color Doesn't Determine Toxicity

The Black Mold Myth

When people say "black mold," they usually mean Stachybotrys—but that assumption creates two problems:

1. Many toxic molds aren't black. Fusarium is often pink. Aspergillus flavus is yellow-green. Penicillium is blue-green.
2. Many black molds aren't particularly dangerous. Cladosporium appears black and is primarily allergenic. Aspergillus niger is black but less concerning than yellow A. flavus.

Why Visual Identification Fails

The same mold species can appear different colors based on:

• Age: Many molds darken as they mature
• Substrate: Growth surface affects pigmentation
• Moisture: Wet vs. dry appearance varies dramatically
• Lighting: Apparent color shifts with light conditions

A green mold could be relatively harmless Cladosporium or mycotoxin-producing Aspergillus versicolor. A black mold could be dangerous Stachybotrys or common Cladosporium. Without laboratory analysis, you simply can't know.

What This Means for You

Don't panic at black mold—and don't dismiss other colors. Instead:

• Focus on moisture sources (all mold needs moisture)
• Get professional testing if health symptoms are present
• Document what you find (photos, locations, extent)
• Consider professional mold inspection for hidden problems

Health Impact Comparison

Allergic Response vs. Chronic Inflammatory Response

This distinction explains why some people recover quickly after leaving a moldy environment while others remain ill for months or years.

Mold Allergy (IgE-mediated)

• Involves the adaptive immune system
• Responds to antihistamines (Claritin, Zyrtec)
• Typical allergic symptoms: sneezing, itchy eyes, congestion
• Affects approximately 20% of the population
• High allergens: Alternaria, Cladosporium, Penicillium

Chronic Inflammatory Response Syndrome (CIRS)

• Involves the innate immune system
• Does NOT respond to antihistamines
• Multi-system symptoms: fatigue, brain fog, pain, respiratory issues
• Affects approximately 24% with HLA genetic susceptibility
• Triggered by biotoxins from water-damaged buildings

The critical insight from clinical research published in Annals of Medicine and Surgery:

"CIRS is NOT an allergy to mold. It is NOT mold growing in your body."

This is why allergy medications fail CIRS patients. Different mechanism, different treatment. For more on this distinction, see our mold allergy vs illness guide.

Neurological Effects: The Research

A 2025 review in MDPI Toxins documented that mycotoxins cross the blood-brain barrier, causing:

• Neurochemical imbalances affecting dopamine, serotonin, and GABA
• Pro-inflammatory cytokines in brain tissue
• Measurable changes in quantitative EEG

A study of 277 Polish children found that duration of mycotoxin exposure was negatively correlated with IQ over a 6-year period.

Perhaps most significantly, NeuroQuant MRI studies show actual brain volume changes in CIRS patients—changes that reverse with proper treatment. This is physical evidence countering the "it's all in your head" dismissal that many mold-affected patients face.

For mold exposure symptoms guidance, see our complete symptom reference.

Common Misdiagnoses

CIRS patients are frequently misdiagnosed with:

• Fibromyalgia
• Chronic Fatigue Syndrome
• Depression or anxiety
• "Stress"
• Allergies
• IBS

If standard treatments for these conditions haven't worked, environmental illness may be the actual cause. The path forward starts with proper testing—both environmental and biomarker-based.

Testing Methods for Species Identification

Visual Inspection

Best for: Locating moisture sources, assessing visible growth extent

Limitations: Cannot determine species, toxicity, or quantity

Visual inspection identifies that you have mold. It cannot tell you what kind or how dangerous it is.

Spore Trap Air Sampling

Air sampling captures what's airborne at the moment of testing.

Limitations identified by practitioners:

• Snapshot in time only
• Stachybotrys spores are heavy—don't aerosolize easily
• Misses the 500:1 fragment ratio
• Cannot differentiate similar-looking spores
• Results vary with HVAC operation, humidity, recent activity

Culturable Testing

Laboratory growth allows species identification of viable organisms.

Limitations:

• Not all species culture equally (underestimates diversity)
• Only captures living organisms
• Slower turnaround than molecular methods

DNA Testing (ERMI/HERTSMI-2)

DNA-based methods identify species regardless of viability.

Advantages:

• Species-specific identification
• Quantitative results
• Detects fragments and dead organisms
• Captures historical contamination

When to use which:

SituationRecommended TestingVisible mold IDSurface sampling + cultureGeneral air qualityAir sampling + ERMICIRS patient re-entryHERTSMI-2Post-remediation verificationERMI + air samplingInsurance/legal documentationMultiple methods

For comprehensive testing guidance, see our complete mold testing guide.

The Bigger Picture: Beyond Just Mold

The Biotoxin Stew

Water-damaged buildings release more than just mold spores. The full "biotoxin stew" includes:

• Mold spores and fragments
• Mycotoxins
• Bacterial endotoxins
• Beta-glucans (inflammatory compounds)
• VOCs (volatile organic compounds)
• Actinobacteria byproducts

Emerging research suggests that Actinobacteria—not just mold—may be primary biotoxins in water-damaged buildings. These bacteria produce compounds with antibiotic activity that disrupt gut and respiratory microbiomes.

This explains why some patients don't improve after mold-only remediation. The bacterial component persists.

Food Supply Exposure

Indoor air isn't the only exposure route. Mycotoxin-contaminated foods include:

• Grains and cereals
• Nuts (especially peanuts)
• Wine and beer
• Spices
• Coffee

Complete exposure assessment considers both environmental and dietary sources.

Geographic Considerations

Mold prevalence varies by region:

• Southeast U.S.: High diversity, year-round humidity
• Southwest: Lower indoor mold, but Coccidioides (Valley Fever) concern
• Northwest: Wood-decay fungi predominant
• Northeast: Strong seasonal patterns

Climate affects which species thrive, which testing is most relevant, and which prevention strategies work best.

Key Takeaways

1. Color doesn't indicate toxicity. Many toxic molds aren't black; many black molds aren't particularly dangerous.
2. Not all molds produce mycotoxins. Species identification matters for accurate health risk assessment.
3. Visual identification cannot determine danger. Professional testing is required to identify species and quantity.
4. CIRS is not a mold allergy. Different immune mechanism means antihistamines won't help—proper treatment requires a different approach.
5. Water damage indicators matter most. ERMI Group 1 molds signal moisture problems requiring attention.
6. HERTSMI-2 less than 11 is the re-entry threshold. This peer-reviewed benchmark indicates safe environments for previously ill patients.
7. It's not just mold. Water-damaged buildings contain multiple biotoxins including bacteria—explaining why mold-focused remediation sometimes fails.

Medical Disclaimer

This guide provides educational information about mold species and their potential health effects. The content is not intended as medical advice, diagnosis, or treatment. Mold exposure affects individuals differently based on genetic susceptibility, exposure duration, and overall health status.

If you're experiencing symptoms potentially related to mold exposure, consult with a healthcare provider experienced in environmental illness. Testing and treatment decisions should be made in partnership with qualified medical professionals who can evaluate your individual situation.

MoldCo connects patients with CIRS-specialized providers and testing resources. All treatment protocols are determined by your provider based on your specific needs, not by MoldCo directly.

Related Resources

Species-Specific Guides

• Complete Black Mold Guide - Stachybotrys identification and risks
• Mold Locations Guide - Where different species grow

Health and Symptoms

• Mold Exposure Symptoms - Complete symptom reference
• Mold Allergy vs Mold Illness - Understanding the difference
• CIRS Treatment Guide - Treatment pathway for mold illness

Testing and Detection

• Complete Mold Testing Guide - All testing options explained
• Mold Illness Testing - ERMI, HERTSMI-2, and biomarkers
• Mold Inspection Guide - Finding hidden mold

Taking Action

• Mold Remediation Guide - Professional removal process
• Mold Prevention Guide - Stop mold before it starts
• Indoor Air Quality Guide - Maintaining healthy air

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Last updated: December 2025