Is That Mould Dangerous? How to Identify Common Types at Home

Mould is one of those problems that often hides in plain sight. A small patch on the bathroom ceiling, a musty smell in the wardrobe, or a dark stain near a window can seem harmless at first — but different mould species behave differently, spread differently, and affect your health in different ways.

Over the years, research has shown that mould growing in damp indoor environments can trigger a wide range of health issues — from sneezing, runny nose, sinus irritation, and itchy eyes to asthma flare-ups and even infections in some cases. In other words, persistent indoor dampness allows mould to release particles and allergens that may impact sensitive individuals, children, and those with existing respiratory concerns.

Understanding which mould you’re dealing with can make a big difference in how you treat it and how quickly the problem returns.

What causes mould growth indoors?

Mould spores are always present in the environment — outdoors and indoors — but they only become a problem inside the home when the conditions are right for them to grow. The single biggest factor behind indoor mould is moisture. When a space stays damp, humid, or poorly ventilated, spores can settle onto surfaces and begin to multiply.

Research examining airborne fungi in Australian homes and outdoor environments found that indoor mould species often mirror what’s outdoors, including common genera like Cladosporium, Aspergillus, and Penicillium.

Mould exposure varies depending on many factors:

1. Excess humidity and poor airflow

High indoor humidity, lack of ventilation and limited air circulation create stagnant, moisture-rich conditions that mould loves. This can happen in:

• bathrooms
  
• laundries
  
• kitchens
  
• bedrooms with closed windows
  
• basements or lower-ground areas

Humidity can rise from daily activities like showering, cooking, or drying clothes indoors.

2. Water leaks or structural moisture

Persistent moisture from:

• roof leaks
  
• plumbing leaks
  
• cracked tiles
  
• leaking shower recesses
• rising damp

…creates ideal conditions for mould colonisation. Dampness indoors can also lead to the breakdown of building materials like plasterboard, insulation and especially drywall (Gyprock), which becomes more susceptible to fungal colonisation. Once mould establishes itself and the material becomes saturated, growth accelerates and becomes harder to remove.

3. Condensation on cold surfaces

Windows, walls, and ceilings may accumulate condensation, especially in winter or in poorly insulated homes. This repeated wetting allows mould to appear on sills, frames and corners.

Read related article: The Surprising Reason You Have Mould Around Your Windows (It’s Not Just Condensation)

4. Dust, organic matter and building debris

Mould indoors is influenced not only by moisture but also by the presence of organic particles like dust, dirt, or debris. These materials act as nutrients for mould when combined with moisture.

Areas that are dusty and damp — such as unused rooms, storerooms, ceiling cavities or around HVAC systems — are particularly prone to mould proliferation.

5. Heating, cooling and ventilation systems

Heating and air-conditioning systems can influence mould levels by spreading spores or creating temperature/humidity differences inside the home. Poorly maintained filters or ductwork may also support microbial growth.

6. Outdoor environmental conditions

Australian climate factors — high humidity, rain, shade, and warm weather — can raise indoor mould levels. When outdoor spore counts are high (for example, after rain), spores can enter through windows, doors and on clothing, settling on damp surfaces indoors.

Common types of mould found in homes

1. Stachybotrys (black mould)

Often referred to as “black mould,” Stachybotrys chartarum is one of the most well-known mould species found in water-damaged buildings. Unlike many other household moulds that can grow quickly on damp surfaces, Stachybotrys needs long-term, persistent moisture to thrive. This means it typically appears only when there has been a serious leak, flooding, or chronic dampness that has soaked into building materials.

Appearance:

• Usually dark black, sometimes deep green
  
• Has a slimy, wet, or gelatinous surface when actively growing
  
• As it dries, it may look powdery or soot-like
  
• Grows in irregular, dense patches
  
• Prefers surfaces rich in cellulose, such as:
  
  • plasterboard and drywall
    
  • timber and particle board
    
  • insulation
    
  • carpet underlay
    
  • wallpaper backing

Stachybotrys is most commonly found in:

• areas affected by leaks behind walls
  
• under flooring after flooding
  
• inside ceilings with chronic roof leaks
  
• basements or subfloors with poor drainage
  
• any place where moisture has remained for weeks or months

Unlike many other moulds, it rarely appears on tiles, grout, or bathroom surfaces because these areas typically dry between uses.

Why it matters:

Stachybotrys is often associated with more significant indoor air quality concerns because:

• it thrives in deep water damage, which can indicate structural problems
  
• it can produce mycotoxins under certain conditions
  
• disturbed colonies can release spores that irritate the respiratory system

People exposed to environments contaminated with Stachybotrys may experience:

• persistent coughing
  
• sinus irritation
  
• worsening asthma symptoms
  
• fatigue or headaches
  
• a strong musty odour in affected rooms

It’s important to note that not all black mould is Stachybotrys, and not all Stachybotrys releases toxins. Colour alone is not enough to identify this species without testing.

2. Cladosporium

Cladosporium is one of the most widespread indoor and outdoor moulds. It appears as black, olive, or dark green patches and grows on:

• walls
  
• ceilings
  
• textiles
  
• carpets
  
• air-conditioning systems

Appearance:

• Usually dark olive-green, brown, or black, sometimes appearing almost soot-like
  
• Forms powdery, velvety, or suede-like patches
  
• Often shows as clusters of small dots that gradually merge into larger colonies
  
• Edges may appear feathery or irregular, especially on porous surfaces

Why it matters:

This mould is strongly linked to allergy symptoms. It can trigger asthma flare-ups, wheezing, coughing and hay-fever-like reactions. People with mould sensitivity often react to it quickly because it produces spores in large quantities.

3. Aspergillus

Aspergillus includes many species, some harmless and others more problematic. Indoors, it commonly grows in:

• damp bathrooms
  
• near air vents
  
• around window frames
  
• dust-heavy areas
  
• water-damaged materials

Appearance:

• Colours vary widely: yellow, green, black, grey, or white, depending on the species
  
• Has a powdery, granular, or dusty texture
  
• Grows in dense, circular colonies that can look dry or slightly elevated
  
• Often forms sharp-edged patches, unlike softer moulds such as Cladosporium

Why it matters:

It can cause respiratory irritation, sinus problems and allergic reactions. In people with weakened immune systems or chronic lung conditions, certain Aspergillus strains can lead to more serious infections (rare but medically significant).

4. Penicillium

Penicillium is a fast-spreading mould that often grows on:

• water-damaged walls and plasterboard
  
• carpets
  
• insulation
  
• furniture
  
• stored food

Appearance:

• Commonly blue-green, grey-green, or turquoise, though some species appear white
  
• Grows rapidly as soft, powdery or fuzzy patches
  
• Usually forms brush-like structures that give it a velvety finish
  
• Typically spreads outward in a circular, dense colony

Why it matters:

Penicillium spreads quickly and releases large numbers of spores into the air, contributing to poor indoor air quality. It is known to worsen allergies, sinus infections, asthma symptoms and overall respiratory irritation.

5. Alternaria

Often found outdoors, Alternaria easily enters homes through open windows or on clothing. Indoors, it thrives in damp areas such as:

• bathrooms
  
• laundries
  
• kitchens
  
• leaky window frames

Appearance:

• Usually dark green, brown, or black with a distinctive “velvety” surface
  
• Appears as small, round spots that expand into large, overlapping patches
  
• Texture is fuzzy or wool-like, especially around the edges
  
• Often shows a concentric ring pattern, giving it a “target-like” look

Why it matters:

This mould is strongly associated with allergic rhinitis, conjunctivitis and asthma. It’s particularly problematic for individuals who already have inhalant allergies.

6. Epicoccum

Although less frequently discussed, Epicoccum species are noted in studies as common indoors — especially in humid or dusty spaces. You may find it in:

• HVAC systems
  
• damp walls
  
• fabric materials
  
• stored paper products

Appearance:

• Colour ranges from bright orange, yellow, or reddish-brown, often very pigmented
  
• Grows in patchy, irregular colonies rather than uniform spots
  
• Often has a powdery or suede-like texture
  
• Can look mottled with darker and lighter zones within the same patch

Why it matters:

It can trigger hay fever–like symptoms and skin irritation in mould-sensitive individuals.

7. Trichoderma

Trichoderma is a fast-growing mould commonly found in persistently damp areas. While often overlooked, it can be one of the more destructive indoor moulds because it breaks down the materials it grows on, causing gradual structural damage if left untreated. Trichoderma thrives in environments where moisture lingers, such as:

• bathrooms and shower areas
  
• damp or unpainted walls
  
• behind wallpaper
  
• around leaking pipes
  
• inside HVAC systems, where condensation occurs
  
• any surface that has remained wet for long periods

Its ability to colonise paper, wood, and fabric-based materials makes it especially problematic after leaks.

Appearance:

• Usually bright to dark green
  
• Often with distinct white edges
  
• Dense, compact, and sometimes slimy in early growth stages
  
• Spreads quickly across damp surfaces

Because Trichoderma multiplies rapidly, it can cover large areas in a short time when conditions are right.

Why it matters:

• Several Trichoderma species can produce mycotoxins, which may irritate the respiratory system when spores become airborne.
  
• It may trigger allergic reactions, including sneezing, coughing, or itchy eyes.
  
• This mould can digest building materials, particularly paper-based products such as plasterboard and wallpaper, leading to structural damage.

Prompt removal and moisture control are essential to prevent long-term issues.

8. Chaetomium

Chaetomium is a mould frequently associated with water-damaged buildings. It thrives on materials that have absorbed water deeply, and like Stachybotrys, it is an indicator of long-term moisture problems rather than short-term humidity.

Chaetomium commonly appears on:

• water-damaged drywall or plasterboard
  
• wet carpet underlay and carpet backing
  
• insulation materials that have absorbed moisture
  
• timber framing or subflooring after leaks
  
• basements, crawl spaces, and areas with poor drainage

It often appears after flooding or prolonged leaks, especially when materials were not dried promptly.

Appearance:

• Starts as grey or brown fuzzy colonies
  
• Over time, it darkens to black
  
• Has a cotton-like texture, often woolly or suede-like
  
• Sometimes grows in clustered, irregular patches

Because it shares similar colour changes with black mould species, Chaetomium is often mistaken for Stachybotrys.

Why it matters:

• Chaetomium can cause skin or nail irritation, especially if touched directly.
  
• Spores may aggravate respiratory symptoms, particularly in people with asthma, allergies, or mould sensitivity.
• It is a strong sign of structural water damage, often requiring professional remediation.

9. Candida and Yeasts

Image source: Scientific Reports, Nature

While yeasts like Candida are more often associated with health and food surfaces, they can also appear in damp, poorly ventilated homes. They thrive on:

• wet window sills
  
• constantly damp bathroom surfaces
  
• areas with chronic condensation

Appearance:

• Cream-coloured, white, pale yellow or light pink patches
  
• Often shiny, slimy, or moist-looking
  
• Can appear as small circular spots that merge into irregular patches
  
• Texture is smoother than typical moulds (not fuzzy like Penicillium or Aspergillus)

Why it matters:

Yeasts can contribute to local irritation and may worsen respiratory symptoms when aerosolised.

10. Botrytis

Common in gardens and plants, Botrytis spores can enter the home and colonise humid indoor spaces.

Why it matters:

It is known to trigger allergic reactions, especially for people with seasonal allergies.

11. Streptomyces (bacteria, not mould — but still relevant)

Image source: EurekAlert! / American Association for the Advancement of Science (AAAS).

The research article Black WD (2020) also notes that damp homes often contain Streptomyces bacteria, which thrive in moist building materials and can coexist with mould.

Why it matters:

Although not a fungus, these bacteria can release compounds that irritate the respiratory system and contribute to the “musty” odour often associated with mould contamination.

When you should test for mould

Not every bit of mould needs lab testing — a small, obvious patch in the shower that you clean and keep dry again is usually straightforward to deal with. But in many situations, testing can be very helpful, especially when:

• you can smell mould but can’t see it,
  
• the problem keeps coming back, or
  
• people in the home are experiencing ongoing symptoms that might be mould-related.

Testing isn’t just about putting a name to the mould. It helps you answer three important questions:

1. Is mould actually present (and how much)?
   
2. Is the contamination localised or more widespread?
   
3. Do I need simple cleaning and prevention, or full remediation?

To get those answers, there are generally two levels of testing:

• At-home screening kits
• Professional mould assessment and sampling

Both have a role — they’re just used for different situations.

Read related article: DIY Mould Testing vs Hiring a Pro – What’s the Best Choice for Your Home?

When at-home mould test kits are useful

At-home test kits are handy as a first step when you want to check whether mould (or other microbes) are present in your environment. They’re especially useful when:

• You suspect mould but can’t see obvious growth
  
• There has been minor water damage and you want to check lingering contamination
  
• You want to screen specific rooms (bedroom, kids’ room, home office, etc.)
  
• You’ve cleaned mould before, but want to confirm if the environment is still affected

The Airogenix™ Mould & Bacteria / Yeast DIY Test Kit is a good example of this type of solution.

 It allows you to:

• Test air or surfaces for mould, bacteria, and yeast
  
• Use multiple plates to check different rooms or problem areas
  
• Get a visual indication of growth on the plates over time
  
• Optionally send samples to a lab (at your own cost) if you want expert identification

Because this kit also checks for bacteria and yeast, it gives a more complete picture of overall microbial load in the environment — not just visible mould patches.

👉 Get the Airogenix mould, bacteria and yeast test kit at Aussie Pharma Direct. 

When a DIY kit is a good choice:

• You want a screening tool to confirm if mould/microbial growth is present
  
• You’re renting and want some evidence before approaching your landlord
  
• You’ve noticed odours, condensation or minor staining and want to check further
  
• You’re planning a clean-up and want before-and-after checks on air or surfaces

What at-home kits don’t do:

• They won’t tell you exactly where hidden mould is growing inside walls, subfloors or cavities
  
• They don’t replace a full building moisture assessment
  
• They are not a medical diagnostic tool — they can’t tell you if symptoms are definitely caused by mould

They’re best thought of as a screening and monitoring tool: helpful for detecting problems early and deciding whether you need to escalate.

When professional mould testing is recommended

Professional mould testing and inspection is usually recommended when the situation is more serious, complex, or long-standing. This typically involves:

• A detailed visual inspection of the property
  
• Moisture readings in walls, floors and ceilings
  
• Targeted air and/or surface sampling for lab analysis
  
• A written report with findings and remediation recommendations

You should strongly consider professional assessment when:

• There has been major water damage, flooding, or long-term leaks
  
• Visible mould covers more than about 1 m² or appears in multiple rooms
  
• Mould keeps coming back despite cleaning and ventilation
  
• There is clear damage to plasterboard, flooring, insulation or framing
  
• People in the home have ongoing or severe symptoms (e.g. asthma flare-ups, chronic sinusitis, unexplained fatigue, headaches) that improve when they leave the property
  
• You need documentation for insurance, landlords, or body corporate

Read related article: What to Do After a Flood: Health Risks, Cleanup Precautions & Recovery Tips

A qualified mould assessor can help identify:

• how extensive the contamination is
  
• which building materials are affected
  
• whether there is hidden mould in walls or subfloors
  
• what level of remediation is actually required (DIY clean vs. professional removal)

Professional testing is especially important if Stachybotrys or other moisture-loving species are suspected, as these usually indicate deeper water damage.

Using both approaches together

In many cases, the most practical approach looks like this:

1. Start with a DIY check

Use something like the Airogenix™ Mould & Bacteria / Yeast DIY Test Kit to:

• Screen specific rooms
• Confirm whether microbial growth is present
• Help prioritise which areas need attention

2. Improve the environment and clean safely

If contamination appears mild and localised, you can:

• Fix obvious moisture sources,
• Clean using suitable mould removal products
• Improve ventilation and humidity control

Read related article: Don’t Use Bleach to Kill Mould (Here’s Why It Comes Back)

3. Escalate to professional help when needed If:

• mould is widespread,
• returns quickly,
• has caused structural damage, or
• health symptoms are significant,

... then professional inspection and remediation are the safer option.

👉 Get our FREE E-Book: When to DIY and When to Call a Pro: A Homeowner’s Guide to Mould Remediation

Wrapping up

Different mould species grow for different reasons, behave differently, and can affect your health in different ways. By understanding the types of mould commonly found indoors — and knowing what they look like, where they hide, and why they develop — you’re far better equipped to respond early and prevent a small issue from becoming a much bigger problem.

Whether you’re dealing with visible patches, musty smells, recurring dampness, or ongoing symptoms that don’t seem to add up, identifying the mould type can help you choose the right next step. Sometimes a simple clean and humidity control are enough; other times, testing or professional remediation is the safer path. Tools like the mould test kits make it easier to check your environment, while targeted mould-control products and proper ventilation support long-term prevention.

If your situation feels more complicated — or you’re unsure who to contact for testing, building inspections, or remediation — we’ve created a helpful guide to walk you through it.

You can read it here: Who to Call for Mould Problems: The Experts You Need for Every Type of Mould Issue.

Dealing with Mould? Start with SAN-AIR

👉 Explore the full range of SAN-AIR Mould Cleaning and Prevention Solutions at Aussie Pharma Direct.