Written by our inspectors to help you understand what mold is, why it grows, what it does to your health, and exactly how professionals find and remove it.
What is mold?
Mold (also spelled mold) is a type of fungus that grows in multicellular filaments called hyphae. It reproduces by releasing microscopic spores into the air, which are present almost everywhere — indoors and out — at low levels.
Spores are harmless at normal background levels. The problem starts when they land on a surface that provides the three things mold needs to grow: moisture, a food source (any organic material — timber, paper, plasterboard, dust, soap scum, fabric) and time (as little as 24–48 hours on wet materials). Temperature between 5°C and 40°C suits most species, which means almost every home is within range year-round.
Once established, mold digests the material it grows on and releases more spores, allergens, irritants and in some species, mycotoxins. Growth can be visible (spots, patches, fuzz) or hidden inside wall cavities, under floors, above ceilings or inside ducting — which is why a musty smell without visible growth should always be investigated.
Mold is distinct from mildew (a surface-level, usually powdery white or grey mold common on fabric and bathroom surfaces) and from efflorescence (white mineral salts on masonry that look similar but aren't biological). An inspector can tell the difference quickly; lab testing confirms it.
Health effects of mold exposure
Mold exposure affects people differently. Reactions depend on the species present, the concentration, the length of exposure and the individual's sensitivity. The World Health Organization and national health bodies recognise damp and mold as a significant indoor health risk.
Common symptoms in healthy adults
- Nasal congestion, sneezing and runny nose
- Coughing, wheezing and throat irritation
- Itchy, red or watery eyes
- Skin rashes or hives
- Headaches and fatigue
- Worsening of existing allergies
Higher-risk groups
- Infants and children — developing lungs and higher breathing rates
- Elderly people — reduced immune response
- People with asthma or COPD — mold is a known trigger for attacks
- People with allergies or allergic rhinitis
- Immunocompromised people — chemotherapy, transplant recipients, HIV
- Pregnant women
Serious conditions (less common)
- Allergic bronchopulmonary aspergillosis (ABPA)
- Hypersensitivity pneumonitis
- Invasive fungal infections in immunocompromised people
- Chronic sinusitis
- Mycotoxin-related illness with prolonged heavy exposure
This information is general and not medical advice. If you or someone in your household has persistent respiratory symptoms, see a doctor and let them know you suspect mold exposure. Removing the mold and fixing the moisture source is the only reliable way to end exposure.
Common types of household mold
There are thousands of mold species, but a handful account for most indoor growth. Colour and appearance give clues, but only laboratory analysis can confirm the genus and species.
Cladosporium
- Appearance
- Olive-green to brown or black, often powdery or velvety.
- Typically found
- Window sills, bathroom tiles, fabrics, timber, air-conditioning ducts. The most common indoor and outdoor mold.
- Health note
- Common allergen; triggers hay-fever-like symptoms and asthma.
Aspergillus
- Appearance
- Varies by species — yellow-green, grey, brown or black; often fluffy.
- Typically found
- Damp walls, insulation, stored food, dust, HVAC systems.
- Health note
- Allergic reactions and respiratory irritation. Some species cause aspergillosis in immunocompromised people and can produce mycotoxins (aflatoxin).
Penicillium
- Appearance
- Blue-green with a velvety texture and a strong musty odour.
- Typically found
- Water-damaged carpets, wallpaper, mattresses, insulation and stored food.
- Health note
- Allergies and asthma; some species produce mycotoxins. Spreads quickly indoors.
Stachybotrys chartarum
“Toxic black mold”
- Appearance
- Greenish-black and slimy when wet, powdery when dry.
- Typically found
- Very wet cellulose materials — plasterboard, paper, cardboard, timber — after prolonged leaks or flooding.
- Health note
- Produces mycotoxins. Associated with chronic coughing, fatigue, headaches and respiratory issues. Requires professional removal.
Alternaria
- Appearance
- Dark green or brown with a velvety, hair-like texture.
- Typically found
- Showers, baths, under sinks, around windows; common after water damage.
- Health note
- Strong allergen; a major trigger for asthma attacks, especially in children.
Chaetomium
- Appearance
- White to grey early, darkening to brown/black; cotton-like.
- Typically found
- Chronically wet plasterboard, wallpaper, carpets and skirting after leaks.
- Health note
- Musty odour; allergies, skin and nail infections; some species produce mycotoxins. Often indicates serious long-term water damage.
Aureobasidium
- Appearance
- Pink, brown or black; becomes darker with age.
- Typically found
- Behind wallpaper, on painted surfaces, wooden window frames, caulking.
- Health note
- Can cause eye, skin and nail infections; avoid direct contact.
Ulocladium
- Appearance
- Black, often mistaken for Stachybotrys.
- Typically found
- Bathrooms, kitchens, basements and areas with significant water damage.
- Health note
- Allergen that can worsen hay fever and asthma.
What causes mold in homes?
Mold is always a moisture problem first. Identify and fix the moisture, and the mold stops. Here are the usual suspects.
Leaks
Roof tiles, flashing, plumbing, shower trays and window seals. Even slow leaks saturate materials over time.
Condensation
Warm, humid indoor air meets cold surfaces — windows, external walls, wardrobes against outside walls — and water forms.
Poor ventilation
Bathrooms and kitchens without working extractor fans trap moisture. Sealed, energy-efficient homes can make this worse.
Flooding & water damage
Burst pipes, storms and overflowing appliances. Mold can start growing within 24–48 hours if materials aren't dried.
Rising & lateral damp
Groundwater drawn up through brickwork or penetrating through walls in contact with soil.
High humidity
Indoor relative humidity consistently above 60% supports mold growth even without a leak. Common in coastal and tropical areas.
Indoor drying & unflued heaters
Drying clothes indoors and unflued gas heaters release litres of water vapour into the air every day.
Blocked gutters & poor drainage
Overflowing gutters and ground sloping toward the house push water into walls and subfloors.
Signs you may have a mold problem
Visible growth
Spots or patches that are black, green, white, grey, orange or pink. Fuzzy, slimy or powdery textures.
Musty, earthy odour
Caused by microbial volatile organic compounds (mVOCs). Often the first sign of hidden growth behind walls or under floors.
Water stains & discolouration
Yellow-brown rings on ceilings, bubbling paint, peeling wallpaper or warped skirting boards.
Condensation
Regular water droplets on windows, cold walls or metal surfaces, especially in the morning.
Health symptoms that improve away from home
Allergy-like symptoms that ease on holiday or at work and return at home.
Warped or soft materials
Plasterboard that crumbles, timber that feels spongy, or floors that lift.
Where mold hides
Our inspectors check these locations on every visit — several are routinely missed by occupants.
- Bathroom ceilings, grout and silicone
- Behind and under washing machines, dishwashers and fridges
- Window frames and sills
- Inside wardrobes and cupboards on external walls
- Behind furniture pushed against cold walls
- Roof cavities and under insulation
- Subfloors and basements
- Air-conditioning units and ducting
- Around leaking skylights, chimneys and vents
- Under carpets and underlay after water damage
- Behind wallpaper and in wall cavities
- Mattresses and soft furnishings in humid rooms
How a professional inspection works
A mold inspection is a moisture investigation. We combine a trained eye with instruments that see what you can't.
Moisture meter
Measures moisture content in timber, plasterboard and masonry. Readings above ~16–18% in plasterboard indicate conditions for growth.
Thermo-hygrometer
Records temperature, relative humidity and dew point to show whether condensation is likely on cold surfaces.
Thermal imaging camera
Detects temperature differences that reveal hidden moisture behind walls, ceilings and floors without destructive testing.
Borescope
A small camera inserted through a tiny hole to view inside wall cavities and under floors.
Spore-trap air sampler
Pulls a measured volume of air through a cassette; the lab counts and identifies spores per cubic metre.
Surface swabs & tape lifts
Identify the genus of visible growth and distinguish mold from dirt, soot or efflorescence.
Particle counter
Measures airborne particulates (PM2.5/PM10) as an indicator of overall air quality.
Data loggers
Left in place for days or weeks to show how humidity changes with weather, occupancy and ventilation.
Sampling methods explained
- Spore-trap air sampling — a pump draws a known volume of air (typically 75 L) through a sticky slide. The lab counts spores per cubic metre by genus and compares indoor results to an outdoor control taken the same day. Indoor counts significantly above outdoor, or genera present indoors but not outdoors, indicate an indoor source.
- Surface sampling (swab / tape lift) — confirms whether a stain is mold and identifies the genus. Useful for distinguishing mold from soot, dirt or efflorescence.
- Bulk sampling — a piece of material (carpet, plasterboard, insulation) sent for analysis when deeper contamination is suspected.
- ERMI dust testing — DNA analysis of settled dust to score a home's overall mold burden; sometimes used by physicians investigating chronic illness.
Standards we follow
Our inspection and remediation processes follow the IICRC S520 Standard for Professional Mold Remediation and S500 Standard for Water Damage Restoration, alongside guidance from the WHO, US EPA and national health departments.
Remediation levels
The approach scales with the size of the contaminated area and whether it has reached HVAC systems. These thresholds are consistent with industry guidance.
| Level | Affected area | Approach |
|---|---|---|
| Level 1 — Small | Under 1 m² | Can often be handled by a careful occupant using PPE (gloves, N95/P2 mask, eye protection), detergent and water, and drying. Discard porous materials. |
| Level 2 — Medium | 1–3 m² | Professional remediation recommended. Localised containment, HEPA vacuuming, removal of affected porous materials, antimicrobial treatment and drying. |
| Level 3 — Large | 3–10 m² | Full containment with negative air pressure, HEPA air scrubbers, decontamination chamber, removal and bagging of materials, and post-remediation verification. |
| Level 4 — Extensive | Over 10 m² or HVAC contamination | As Level 3 with full PPE, possible occupant relocation, staged works, and independent clearance testing. HVAC systems cleaned or replaced. |
Cleaning a small patch yourself
- Fix the moisture source first — otherwise it will return.
- Wear gloves, eye protection and a P2/N95 mask. Open windows and keep children and pets away.
- Use a solution of detergent and warm water, or 80% white vinegar / 20% water. Avoid bleach on porous surfaces.
- Wipe (don't scrub dry — it releases spores) using a microfibre cloth. Discard the cloth afterwards.
- Dry the area thoroughly with a fan or dehumidifier.
- Discard porous items with growth (cardboard, paper, soft furnishings) in sealed bags.
If the area is larger than about 1 m², keeps returning, or anyone in the home is sensitive, call a professional.
Preventing mold
Control moisture and you control mold. These habits make the biggest difference.
Ventilation
- Run bathroom extractor fans during and 20 minutes after showering
- Use the rangehood when cooking and vent it outdoors
- Open windows for 10–15 minutes daily, even in winter
- Keep wardrobe and cupboard doors ajar occasionally
Humidity
- Keep indoor relative humidity between 40% and 60%
- Use a dehumidifier in damp rooms or humid seasons
- Avoid drying clothes indoors; if unavoidable, run a dehumidifier
- Never use unflued gas heaters indoors
Moisture control
- Fix leaks within 24–48 hours
- Dry wet carpets and materials immediately after flooding
- Clean gutters and check downpipes twice a year
- Re-seal shower screens, grout and window seals as they age
Layout & habits
- Leave a gap between furniture and external walls
- Wipe condensation from windows each morning
- Use mold-resistant paint in bathrooms and laundries
- Clean bathroom surfaces weekly with a mild detergent
Mold myths, debunked
Bleach kills mold.
On porous surfaces bleach only removes colour; the hyphae survive and regrow. Use detergent, vinegar or professional products.
A small patch is nothing to worry about.
Visible mold is often the tip of the iceberg. It indicates a moisture problem that may be feeding hidden growth.
Only black mold is dangerous.
Many colours of mold cause allergic reactions. Colour doesn't indicate toxicity — only lab testing can identify species.
Painting over mold fixes it.
Paint traps moisture and mold keeps growing underneath, eventually bubbling and peeling the paint.
Mold is only a winter problem.
Winter condensation is common, but summer humidity, leaks and floods cause mold year-round.
New homes don't get mold.
Tightly sealed modern homes trap moisture, and construction materials can hold moisture if not dried before being enclosed.
Tenants, landlords and insurance
If you rent
Landlords are generally required to provide and maintain premises in a reasonably clean and habitable condition, which includes addressing mold caused by structural issues such as leaks, rising damp or inadequate ventilation. Tenants are usually responsible for mold caused by their own actions, such as never ventilating or drying clothes indoors without extraction.
- Report mold to your landlord or agent in writing as soon as you notice it, with photos and dates.
- Keep a record of all communication and any health symptoms.
- An independent inspection report provides objective evidence of cause and severity for tribunal or negotiation.
If you own or manage property
- Respond to reports promptly — mold spreads fast and early action is cheaper.
- Have the moisture source diagnosed professionally rather than repeatedly cleaning.
- Keep inspection reports and clearance certificates on file for future tenancies and insurance.
Insurance
Many home insurance policies cover mold that results from a sudden, insured event (burst pipe, storm damage) but exclude mold from gradual causes (long-term leaks, condensation, lack of maintenance). Our reports document the cause clearly and we can liaise with your insurer or assessor directly.
Glossary
- Hyphae
- The thread-like filaments that form the body of a mold; they penetrate porous materials, which is why surface cleaning often fails.
- Spores
- Microscopic reproductive cells released by mold into the air. They are present everywhere and germinate when they land on a damp surface.
- Mycotoxins
- Toxic compounds produced by some mold species under certain conditions. Exposure can occur via inhalation, ingestion or skin contact.
- mVOCs
- Microbial volatile organic compounds — the gases responsible for the characteristic musty smell of mold.
- Relative humidity (RH)
- The amount of moisture in the air as a percentage of the maximum it can hold at that temperature. Above 60% supports mold growth.
- Dew point
- The temperature at which air becomes saturated and water condenses. Surfaces cooler than the dew point will become wet.
- HEPA
- High-Efficiency Particulate Air filter that captures 99.97% of particles at 0.3 microns, including mold spores.
- Containment
- Sealing off the work area with plastic sheeting and negative air pressure so spores don't spread during remediation.
- Negative air pressure
- Using air scrubbers to exhaust air out of the containment area so that air flows in, not out, keeping spores inside.
- Post-remediation verification (PRV)
- Inspection and testing after remediation to confirm the area is visually clean, dry and has normal airborne spore levels.
- IICRC S520
- The industry Standard and Reference Guide for Professional Mold Remediation, which our processes follow.
- ERMI
- Environmental Relative Moldiness Index — a DNA-based dust test that scores a home's mold burden against a national reference.
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