What it is:

Aspergillus and Penicillium are among the most commonly identified mold groups in air samples. They are often grouped together in non-viable air testing because many spores in these groups can appear similar under laboratory analysis.

Where it commonly comes from:

These molds can be found both indoors and outdoors. Indoors, they are often associated with dust, carpeting, drywall, insulation, wood, stored contents, and HVAC systems, especially in areas with elevated moisture or recurring humidity.

How it can get into the home:

These spores may enter from normal outdoor air, but they may also develop indoors where moisture and organic material are present. Indoor buildup can occur in damp wall cavities, beneath flooring, around supply or return ducts, near condensate issues, or in dust-heavy areas with poor airflow.

What it may indicate:

Small amounts may be typical in indoor air. However, when indoor levels are notably higher than the outdoor control sample, it may suggest indoor conditions are contributing, such as elevated moisture, poor ventilation, dust accumulation, or hidden contamination within the home.

What this could mean for your home:

Elevated findings may point toward hidden moisture, stale air conditions, poor filtration, HVAC-related distribution, or areas where dust and particles are accumulating over time. In some homes, this group may be one of the stronger indicators that something indoors is contributing to the air sample results.

Potential air quality and comfort considerations:

This group has been associated with indoor air quality concerns and may contribute to musty odors or air that feels stale, dusty, or uncomfortable. Some individuals may report irritation, allergy-like symptoms, or feeling worse indoors when levels are elevated, although sensitivity can vary widely from person to person.

Why people usually test for this:

Homeowners often test when there is a persistent odor, ongoing allergy-like symptoms, concern about hidden mold, or suspicion that the HVAC system or a past moisture event may be affecting indoor air quality.

Learn more:

EPA Mold Basics

CDC Mold Information

Aspergillus (Wikipedia)

Penicillium (Wikipedia)

What it is:

Cladosporium is one of the most common molds found in both indoor and outdoor air. It frequently appears in air samples and is often part of the normal outdoor spore mix.

Where it commonly comes from:

It is commonly associated with outdoor air, vegetation, soil, and decaying plant material. Indoors, it may also be found on wood, fabrics, carpeting, window frames, and areas exposed to periodic condensation or elevated humidity.

How it can get into the home:

Cladosporium often enters through ordinary air exchange, open doors and windows, ventilation pathways, and movement of outdoor air into the home. It may also develop on indoor surfaces that stay damp enough to support growth.

What it may indicate:

Its presence indoors is not automatically unusual because it is so common outdoors. The more important question is whether indoor levels are similar to the outdoor sample or whether they appear notably elevated. Higher indoor levels may suggest excess humidity, poor airflow, or localized growth conditions on indoor materials.

What this could mean for your home:

If elevated indoors, it may point toward window condensation, bathroom moisture, humid rooms, poor ventilation, or surfaces where dampness is recurring. In some cases, it may also suggest that outdoor spores are being retained indoors because of filtration or airflow issues.

Potential air quality and comfort considerations:

Cladosporium has been associated with general indoor air quality concerns when present at higher levels. Some individuals may report irritation or allergy-like symptoms in environments where airborne mold and humidity are elevated, especially if the home already feels damp or musty.

Why people usually test for this:

People often test when the home feels humid, smells slightly musty, has window condensation, or there is concern that indoor air does not feel as fresh as the air outside.

Learn more:

EPA Mold Basics

CDC Mold Information

Cladosporium (Wikipedia)

What it is:

Cladosporium is one of the most common molds found in both indoor and outdoor air. It frequently appears in air samples and is often part of the normal outdoor spore mix.

Where it commonly comes from:

It is commonly associated with outdoor air, vegetation, soil, and decaying plant material. Indoors, it may also be found on wood, fabrics, carpeting, window frames, and areas exposed to periodic condensation or elevated humidity.

How it can get into the home:

Cladosporium often enters through ordinary air exchange, open doors and windows, ventilation pathways, and movement of outdoor air into the home. It may also develop on indoor surfaces that stay damp enough to support growth.

What it may indicate:

Its presence indoors is not automatically unusual because it is so common outdoors. The more important question is whether indoor levels are similar to the outdoor sample or whether they appear notably elevated. Higher indoor levels may suggest excess humidity, poor airflow, or localized growth conditions on indoor materials.

What this could mean for your home:

If elevated indoors, it may point toward window condensation, bathroom moisture, humid rooms, poor ventilation, or surfaces where dampness is recurring. In some cases, it may also suggest that outdoor spores are being retained indoors because of filtration or airflow issues.

Potential air quality and comfort considerations:

Cladosporium has been associated with general indoor air quality concerns when present at higher levels. Some individuals may report irritation or allergy-like symptoms in environments where airborne mold and humidity are elevated, especially if the home already feels damp or musty.

Why people usually test for this:

People often test when the home feels humid, smells slightly musty, has window condensation, or there is concern that indoor air does not feel as fresh as the air outside.

Learn more:

EPA Mold Basics

CDC Mold Information

Cladosporium (Wikipedia)

What it is:

Ascospores are another very common category of spores identified in air samples and are frequently associated with outdoor fungi tied to plants, soil, and organic debris.

Where it commonly comes from:

They often originate outdoors from leaves, plants, mulch, soil, and other decaying organic material.

How it can get into the home:

Ascospores may enter through ordinary airflow, windows and doors, ventilation, and the movement of outdoor air into indoor spaces. In some homes they may also linger indoors longer when ventilation is limited.

What it may indicate:

Indoor findings are usually interpreted in relation to the outdoor sample. Similar indoor and outdoor levels often reflect normal outdoor transfer. Elevated indoor levels may suggest that spores are being retained indoors, that air movement is poor, or that outdoor particulates are being concentrated inside.

What this could mean for your home:

If elevated indoors, results may support looking at filtration, ventilation, humidity, and general air movement within the home. This can be especially relevant in homes that feel stuffy, hold odors, or do not seem to exchange air well.

Potential air quality and comfort considerations:

Ascospores can add to the overall airborne particulate load. Some individuals may report that indoor air feels less comfortable, especially if there are already concerns about dust, odor, or sensitivity to airborne irritants.

Why people usually test for this:

These are commonly reviewed when someone wants to know whether indoor air concerns reflect outdoor influence or whether the home may be retaining airborne spores more than expected.

Learn more:

EPA Mold Basics

CDC Mold Information

Ascospore (Wikipedia)

What it is:

Chaetomium is a mold commonly associated with water-damaged building materials and prolonged moisture exposure.

Where it commonly comes from:

It is often found on drywall, wood, insulation, paper-backed materials, and other building components that have remained damp for an extended time.

How it can get into the home:

Unlike many outdoor-driven spore types, Chaetomium is often more closely tied to interior materials that have been affected by leaks, water intrusion, condensation, or long-term dampness. It is often associated with hidden moisture rather than ordinary outdoor air transfer.

What it may indicate:

When identified, it may suggest past or ongoing water damage, long-term dampness, or concealed moisture conditions affecting building materials. It is often considered more significant than a simple outdoor-type spore because of its stronger connection to wet materials.

What this could mean for your home:

This type is often linked to materials that have absorbed moisture and may be deteriorating behind the scenes, such as inside walls, ceilings, cabinetry, or beneath flooring. It may support a closer look at areas that have leaked before or have remained chronically damp.

Potential air quality and comfort considerations:

Chaetomium has been associated with musty indoor environments and may contribute to the kind of damp, unpleasant air conditions that prompt people to investigate mold in the first place. Some occupants may report discomfort or sensitivity in moisture-affected environments, especially if other mold types are also elevated.

Why people usually test for this:

Testing is commonly done after leaks, water damage, staining, odor complaints, or when there is concern that hidden building materials may have been affected even though the damage is not fully visible.

Learn more:

EPA Mold Basics

CDC Mold Information

Chaetomium (Wikipedia)

What it is:

Stachybotrys is a mold commonly associated with prolonged moisture exposure and water-damaged cellulose-based materials. It is often informally referred to as “black mold,” although color alone is never enough to identify mold type.

Where it commonly comes from:

It is often associated with wet drywall, wood, insulation facing, paper products, and other materials that have remained damp over time.

How it can get into the home:

It is not typically thought of as entering the home from ordinary outdoor air in the same way many other spores do. Instead, it is more often associated with indoor materials that have experienced significant or prolonged moisture damage.

What it may indicate:

When identified, it may suggest long-term moisture-related conditions, chronic dampness, or past water damage that was not fully corrected. Because its spores are heavier and less easily airborne than some other molds, it is less commonly detected in routine air samples, so its presence can be more notable.

What this could mean for your home:

Its presence may point toward materials that stayed wet for an extended period and may now be deteriorating, damaged, or in need of repair. It may support a closer evaluation of hidden moisture sources and water-damaged building components.

Potential air quality and comfort considerations:

Stachybotrys has been associated with poor indoor conditions in homes with significant moisture damage. Some individuals may report strong musty odors, environmental discomfort, or feeling worse indoors when extensive moisture-related issues are present. Sensitivity varies, and indoor air results alone do not determine health effects.

Why people usually test for this:

People often request testing after known leaks, flooding, visible staining, strong odors, or concern that past water damage may have resulted in concealed mold growth.

Learn more:

EPA Mold Basics

CDC Mold Information

Stachybotrys (Wikipedia)

What it is:

Alternaria is a common mold found in both indoor and outdoor environments and is regularly identified in air testing.

Where it commonly comes from:

It is often associated with soil, plants, outdoor debris, and decaying vegetation. Indoors, it may also appear in areas with recurring moisture, such as bathrooms, around windows, or on damp surfaces.

How it can get into the home:

Alternaria may enter through ordinary outdoor airflow, doors, windows, and ventilation. It may also become established indoors where local moisture conditions allow it to grow on surfaces or organic debris.

What it may indicate:

Indoor findings may simply reflect outdoor influence, but higher indoor levels could suggest localized moisture conditions, elevated humidity, or an interior source that should be evaluated more closely.

What this could mean for your home:

Results may support looking at bathrooms, window areas, damp trim, condensation-prone rooms, or any area where surfaces stay wet long enough to support mold growth.

Potential air quality and comfort considerations:

Alternaria has been associated with indoor air quality concerns and may contribute to discomfort in sensitive individuals when levels are elevated. Homes with noticeable humidity, odor, or recurring dampness may be more likely to show this type indoors.

Why people usually test for this:

Clients often test when there is musty odor, concern about visible staining, window condensation, bathroom moisture, or ongoing allergy-like complaints indoors.

Learn more:

EPA Mold Basics

CDC Mold Information

Alternaria (Wikipedia)

What it is:

Fusarium is a mold that can be found in both indoor and outdoor environments and is often associated with damp or water-affected materials.

Where it commonly comes from:

It may originate outdoors, but indoor findings are often associated with wet materials, hidden leaks, water intrusion, or areas that have remained damp or slow to dry.

How it can get into the home:

Fusarium may enter from outside air, but it may also develop indoors where moisture conditions support growth on building components, contents, or other organic materials.

What it may indicate:

Elevated indoor findings may point toward moisture intrusion, damp materials, concealed leaks, or finishes that have been affected by water over time. It can be a meaningful clue when trying to determine whether a home has a moisture-related issue.

What this could mean for your home:

Its presence may support a closer look at areas affected by leaks, chronic dampness, plumbing issues, exterior water intrusion, or materials that may not have dried properly after a past event.

Potential air quality and comfort considerations:

When elevated indoors, Fusarium may contribute to poor indoor air conditions and may be associated with damp, stale, or musty environments. Some individuals may report discomfort in spaces where moisture-related mold is present.

Why people usually test for this:

People often test after water damage, plumbing leaks, recurring dampness, or when there is concern that building materials may have been affected even if there is not obvious visible growth.

Learn more:

EPA Mold Basics

CDC Mold Information

Fusarium (Wikipedia)

What it is:

Curvularia is a mold typically associated with outdoor environments, especially soil and plant material.

Where it commonly comes from:

It is usually found outdoors in vegetation, mulch, soil, and organic debris.

How it can get into the home:

It may enter through normal air movement, ventilation, open windows and doors, or through the transfer of outdoor particles into the home.

What it may indicate:

Its appearance in indoor air samples is often interpreted in relation to the outdoor control sample. Similar levels may reflect ordinary environmental transfer, while elevated indoor levels may suggest retention of particulates indoors or localized conditions contributing to buildup.

What this could mean for your home:

If elevated indoors, results may support looking at air filtration, dust accumulation, ventilation patterns, and whether outdoor air is being drawn in and concentrated inside the home.

Potential air quality and comfort considerations:

Curvularia can add to the overall airborne spore load and may contribute to general indoor air concerns, especially in homes where occupants feel the air is not as fresh or comfortable as it should be.

Why people usually test for this:

These results are often reviewed when clients want to understand whether indoor findings are simply outdoor-related or whether the home may be retaining spores more than expected.

Learn more:

EPA Mold Basics

CDC Mold Information

Curvularia (Wikipedia)

What it is:

Epicoccum is a mold commonly associated with outdoor environments and is occasionally identified in indoor air testing.

Where it commonly comes from:

It is often found on plants, soil, and outdoor organic materials.

How it can get into the home:

It may enter through natural airflow, ventilation, open doors and windows, or on items and materials brought in from outdoor areas.

What it may indicate:

Indoor detection often reflects outdoor influence, though elevated indoor findings may suggest spores are being retained indoors or that localized conditions are allowing buildup on materials or surfaces.

What this could mean for your home:

If elevated indoors, it may support looking at airflow, filtration, housekeeping-related dust accumulation, and areas where outdoor-associated particles may be collecting inside the home.

Potential air quality and comfort considerations:

Epicoccum may contribute to the total airborne spore load indoors. Some people may notice that indoor air feels more irritating, dusty, or uncomfortable when outdoor-related spores are entering and remaining inside in larger amounts.

Why people usually test for this:

Clients often review this type when they are trying to understand why indoor air feels stale or why symptoms seem worse inside than outside, even when there is no obvious visible mold source.

Learn more:

EPA Mold Basics

CDC Mold Information

Epicoccum (Wikipedia)

What it is:

Nigrospora is a mold commonly associated with plants and outdoor organic material.

Where it commonly comes from:

It usually originates outdoors on plant surfaces, grasses, and organic debris.

How it can get into the home:

It may enter through ordinary outdoor air exchange, open doors and windows, ventilation, or materials brought in from exterior areas.

What it may indicate:

Similar indoor and outdoor findings often reflect normal transfer from the exterior environment. Elevated indoor findings may suggest retention of particulates, unusual airflow patterns, or environmental conditions allowing spores to accumulate inside.

What this could mean for your home:

Higher indoor levels may support looking at filtration, dust control, air movement, and whether outdoor air is entering more freely than expected.

Potential air quality and comfort considerations:

Nigrospora may contribute to general indoor air concerns by increasing the total spore load. Some sensitive individuals may notice irritation or discomfort when airborne particles are elevated indoors.

Why people usually test for this:

This is often reviewed when homeowners want better clarity on whether the air issue is coming from inside the home or from outdoor conditions that are affecting the indoor environment.

Learn more:

EPA Mold Basics

CDC Mold Information

Nigrospora (Wikipedia)

What it is:

Memnoniella is a mold often associated with water-damaged materials and is structurally related in some ways to Stachybotrys.

Where it commonly comes from:

It is commonly associated with drywall, wood, paper-based materials, and other surfaces that have experienced prolonged moisture exposure.

How it can get into the home:

It is generally tied more to water-damaged indoor materials than to ordinary outdoor air transfer. It often shows up in the context of chronic dampness or prior water damage.

What it may indicate:

When identified, it may suggest moisture-related conditions, past water intrusion, or damp materials that have remained affected over time.

What this could mean for your home:

Its presence may support concern for hidden damp materials, deterioration of finishes, or areas where water damage was not fully corrected. It can be a clue that there is more going on behind the surfaces than what is visible during a normal walkthrough.

Potential air quality and comfort considerations:

Memnoniella may contribute to damp, musty indoor environments and may be part of the reason occupants describe the home as smelling off or feeling uncomfortable. As with other moisture-indicator molds, sensitivity can vary by person and situation.

Why people usually test for this:

People often test when there has been a history of leaks, staining, long-term dampness, or odors that suggest moisture may be affecting building materials out of sight.

Learn more:

EPA Mold Basics

CDC Mold Information

Memnoniella (Wikipedia)

What it is:

Smuts and Myxomycetes are spore types commonly associated with outdoor biological materials, especially vegetation, grasses, soil, and organic debris.

Where it commonly comes from:

These are primarily outdoor-related and are often associated with plant material, lawns, landscaping, wooded areas, and soil-rich environments.

How it can get into the home:

They usually enter through open doors and windows, ventilation, or normal air exchange with the outdoors. In some cases they may also be tracked in from exterior surfaces and yard areas.

What it may indicate:

Their presence in indoor samples often reflects outdoor influence rather than an indoor growth source. The outdoor control sample is especially important when interpreting these results. Elevated indoor levels may suggest that outdoor spores are entering and lingering inside more than expected.

What this could mean for your home:

Unexpectedly high indoor levels may support looking at windows, doors, ventilation, pressure relationships, or filtration. This type of finding may be more about how outside air is interacting with the home than about hidden indoor mold growth.

Potential air quality and comfort considerations:

These spores can add to the overall airborne particulate load and may affect how clean or comfortable indoor air feels, especially for people who are sensitive to outdoor allergens or organic particulates.

Why people usually test for this:

These often come up when clients are trying to understand whether their indoor air concerns are being driven by outdoor conditions, especially during certain seasons or in heavily vegetated settings.

Learn more:

EPA Mold Basics

CDC Mold Information

Smuts (Wikipedia)

Myxomycetes / Myxogastria (Wikipedia)

What it is:

Rust spores are commonly associated with outdoor plants and vegetation rather than typical indoor mold growth reservoirs.

Where it commonly comes from:

They are usually generated outdoors from grasses, leaves, landscaping, crops, and other plant material.

How it can get into the home:

Rust spores may enter through open doors and windows, on pets or clothing, through ventilation, or through normal outdoor-to-indoor air transfer.

What it may indicate:

Their appearance indoors often reflects ordinary outdoor influence. Indoor significance is best understood by comparing the results to the outdoor sample and considering whether the home is concentrating outdoor material more than expected.

What this could mean for your home:

Elevated indoor levels may point more toward air exchange, filtration, or pressure issues than toward an indoor mold growth source. They can be useful in understanding how outdoor conditions are affecting indoor air.

Potential air quality and comfort considerations:

Rust spores can still add to the indoor particulate burden and may contribute to discomfort for individuals who are sensitive to outdoor allergens or airborne organic particles.

Why people usually test for this:

These are commonly reviewed when the goal is to compare indoor and outdoor air and determine whether the home is shielding occupants from outdoor spore loads or allowing them to build up inside.

Learn more:

EPA Mold Basics

CDC Mold Information

Rust fungus (Wikipedia)

What it is:

Some laboratory reports include spore categories that are grouped by similar visual characteristics rather than exact species-level identification. Uredo or Cladosporium-like types may fall into these broader categories depending on the analysis method.

Where it commonly comes from:

These may be associated with outdoor vegetation, plant pathogens, soil, and other natural environmental sources.

How it can get into the home:

They may enter through normal outdoor air exchange, ventilation, open windows and doors, or from nearby vegetation and exterior surfaces.

What it may indicate:

These categories are often interpreted more broadly and in relation to the outdoor control sample. By themselves, they do not always point to a specific indoor mold reservoir. Elevated indoor levels may suggest that outdoor-related spores are entering and collecting inside more than expected.

What this could mean for your home:

If found at elevated indoor levels, evaluation may focus on airflow, filtration, ventilation, and environmental conditions rather than assuming a hidden indoor source without other supporting evidence.

Potential air quality and comfort considerations:

These types may still contribute to the total airborne spore load indoors and may matter to people who are sensitive to airborne particulates, seasonal allergens, or poor indoor air exchange.

Why people usually test for this:

These are most helpful when people want a fuller picture of all airborne spores present and how the indoor environment compares to the outdoor baseline.

Learn more:

EPA Mold Basics

CDC Mold Information

Urediniomycetes / Uredo-related group (Wikipedia)