Understanding Mold: Health Risks, Common Species, and What Your Lab Results Mean

Mold is everywhere. It is in the air you breathe right now, outdoors and indoors. In most cases, that is perfectly normal and not a health concern. But when mold grows unchecked inside a home, certain species can reach concentrations that affect your health, damage your property, and lower your indoor air quality.

Understanding which molds matter, what the actual health risks are, and how to interpret a lab report puts you in control. This guide covers what you need to know in plain language.

Common Indoor Mold Species

Thousands of mold species exist, but a handful show up repeatedly in indoor environments. These are the ones you are most likely to see on a lab testing report.

Cladosporium

Cladosporium is one of the most common molds found both indoors and outdoors. It typically appears as olive-green to brown or black colonies and thrives on a wide range of surfaces including drywall, wood, fabrics, and HVAC systems.

Health impact: Generally considered an allergen. It can trigger hay fever symptoms, asthma attacks, and skin irritation in sensitive individuals. Cladosporium is not typically associated with serious infections in healthy people, but elevated indoor levels above outdoor baselines indicate a moisture problem that needs attention.

Penicillium

Penicillium species are extremely common indoors and are often found on water-damaged building materials, wallpaper, carpet, and food. Colonies are typically blue-green and spread quickly.

Health impact: A significant allergen and a common trigger for asthma symptoms. Some Penicillium species produce mycotoxins. Elevated indoor counts relative to outdoor counts suggest active growth inside the home that warrants investigation.

Aspergillus

Aspergillus is a large genus with over 180 species. Several are commonly found indoors, particularly in dust, HVAC systems, and water-damaged materials. Colony colors range from yellow-green to black depending on the species.

Health impact: Aspergillus is where health risks become more serious. Certain species, particularly Aspergillus fumigatus and Aspergillus flavus, can cause aspergillosis, a range of lung infections that are especially dangerous for people with weakened immune systems, chronic lung disease, or cystic fibrosis. Some species produce aflatoxins, among the most potent naturally occurring carcinogens known.

Stachybotrys (Black Mold)

Stachybotrys chartarum, commonly called “black mold” or “toxic black mold,” grows on materials with high cellulose content such as drywall, ceiling tiles, and wood that have been wet for an extended period. It requires sustained moisture to establish itself.

Health impact: Stachybotrys produces satratoxins, a class of trichothecene mycotoxins. Exposure has been associated with respiratory symptoms, headaches, fatigue, and in severe cases, pulmonary hemorrhage in infants. However, it is important to note that Stachybotrys is not the only mold that produces mycotoxins, and the color of mold does not determine its toxicity. Many dark-colored molds are harmless, and some light-colored molds produce dangerous toxins.

Chaetomium and Ulocladium

These species deserve special mention. When Chaetomium or Ulocladium are identified in lab results, they are strong indicators of prolonged moisture intrusion — not just a one-time leak, but an ongoing or long-standing water problem. Their presence often justifies more thorough remediation protocols and a more aggressive investigation of the moisture source, compared to situations involving only lighter water damage indicators.

What Are Mycotoxins?

Mycotoxins are toxic compounds produced by certain mold species as secondary metabolites. Think of them as chemical byproducts of mold growth. Not all molds produce mycotoxins, and molds that can produce them do not always do so. Mycotoxin production depends on the species, the material the mold is growing on, moisture levels, temperature, and competition with other microorganisms.

The key mycotoxin groups relevant to indoor environments include:

• Trichothecenes (produced by Stachybotrys and Fusarium) — can cause skin irritation, respiratory distress, and immune suppression
• Aflatoxins (produced by some Aspergillus species) — potent carcinogens primarily associated with contaminated food, but can be present in indoor dust
• Ochratoxin A (produced by some Aspergillus and Penicillium species) — associated with kidney damage and is a possible carcinogen

Standard mold air and surface testing identifies mold species and spore counts. Mycotoxin testing is a separate analysis that specifically looks for these toxic compounds.

When Mold Is and Is Not a Health Concern

Mold is generally not a concern when:

• Indoor mold levels are at or below outdoor levels (outdoor mold naturally enters through doors, windows, and HVAC systems)
• The species found indoors match the species found outdoors at similar concentrations
• There is no visible mold growth, no water damage, and no musty odors
• Occupants report no unexplained respiratory or allergic symptoms

Mold becomes a concern when:

• Indoor spore counts significantly exceed outdoor counts for any species
• Species found indoors are not present outdoors, indicating an indoor growth source
• Mycotoxin-producing species are identified at elevated levels
• Water damage or sustained moisture issues are present
• Occupants experience symptoms that improve when they leave the building

The Regulatory Landscape: Why Lab Data Matters Even More

One of the most important things homeowners should understand is that California does not regulate mold, and there are no widely agreed-upon, established “safe levels” of indoor mold published by any authority.

The EPA has stated that if there is visible mold, there is generally no reason for lab testing. But that position raises dozens of questions in practice: What about mold you can see but cannot identify? What about insurance documentation? What about species that indicate prolonged water damage versus minor condensation? What about legal disputes between tenants and landlords, or buyers and sellers?

The reality is that varying and sometimes contradictory positions on mold exist among so-called authorities — the EPA, CDC, state health departments, and private certification organizations all have different opinions, approaches, and recommendations. This lack of consensus is precisely why independent, science-based lab data from an AIHA-accredited laboratory is so valuable. Lab results provide objective evidence that stands on its own regardless of which authority’s position you follow.

How to Read Your Mold Lab Report

A standard mold lab report from an accredited laboratory contains several key sections. Here is what to look for.

Sample details. Each sample is identified by location (e.g., “master bedroom,” “outdoor control”) and type (air cassette, surface swab, bulk material). Always verify that the locations listed match where samples were actually taken.

Spore counts. For air samples, results are reported as spores per cubic meter of air (spores/m3). These numbers have meaning only in comparison. An indoor count of 500 spores/m3 of Cladosporium means nothing by itself. Compared to an outdoor count of 2,000 spores/m3, it is normal. Compared to an outdoor count of 50 spores/m3, it indicates an indoor source.

Species identification. The lab identifies which mold genera or species are present. Pay attention to which species are found indoors that are not found outdoors, or that are found at much higher concentrations indoors than outdoors.

The outdoor baseline. This is arguably the most important sample. Without an outdoor control sample, indoor results cannot be properly interpreted. Any inspector who does not collect at least one outdoor sample is not following best practices.

Analytical method. Most air samples are analyzed via direct microscopy (non-viable analysis), which identifies spore types and counts. Culturable analysis grows the mold on media to identify living organisms. Each method has strengths and limitations, and your inspector should explain which was used and why.

If you have questions about a lab report you have received, an independent inspection company can review it with you and explain what the findings mean for your specific situation. Check out our FAQ for more common questions about mold testing.

Get Answers About Your Home’s Air Quality

At Tanjun Mold Inspections, we provide independent mold testing with clear, honest explanations of your results. No upselling. No remediation sales pitch. Just the facts about what is in your air and what it means.

Call (818) 964-1533 or schedule your inspection online to get your home tested by a conflict-free mold inspector.