At first glance, mould looks like a fuzzy, chalky, or sometimes colourful patch appearing on damp surfaces. But mould is actually a living organism — a type of fungus — and one of the oldest life forms on Earth.
Mould reproduces by releasing microscopic spores into the air. When those spores land on a damp surface, they begin to grow. This often leads people to ask: what causes mould, and why does it grow inside homes? Let’s break it down.
Mould is present virtually everywhere — both indoors and outdoors — but it only becomes a problem when moisture allows it to multiply uncontrollably.
What Is Mould?
Mould belongs to the fungi kingdom, a separate branch of life from plants and animals. Unlike plants, mould does not produce its own food. Instead, it feeds by breaking down organic material.
In nature, mould plays an important role in decomposing leaves, bark, and other organic matter. Indoors, however, mould growth can occur on:
- Timber
- Plasterboard
- Carpets
- Paper
- Insulation
- Fabrics and upholstery
- Bathroom grout
- Furniture
For example, mould often appears on bathroom ceilings, behind kitchen appliances, or inside wardrobes where airflow is limited.
If left untreated, mould can spread quickly across surfaces and into porous materials.
What Causes Mould to Grow?
Mould spores are always present in the air. However, they only grow when certain conditions are met.
Mould Needs:
- Moisture or dampness
- Poor ventilation
- Warm temperatures
- Organic material to feed on
Common household triggers include:
- Leaking pipes
- Rising damp
- Condensation on windows
- Poor bathroom ventilation
- Wet carpets
- Water damage
- High indoor humidity from cooking or showering
Activities such as cooking and drying clothes indoors can increase indoor humidity, creating ideal conditions for mould growth.
Different Types of Mould
Mould species are typically grouped into three broad categories:
1. Allergenic Mould
These are the most common indoor moulds. They may trigger sneezing, a runny nose, eye irritation, or mild asthma symptoms. They are generally a nuisance rather than a serious threat.
2. Pathogenic Mould
These moulds can cause infections in people with weakened immune systems. Symptoms may resemble respiratory infections.
3. Toxigenic Mould
Some mould species produce mycotoxins. In high concentrations and prolonged exposure, these may pose more serious health concerns. Risk depends on exposure level and individual sensitivity.
What Health Risks Can Mould Cause?
Most homes contain small amounts of mould spores that do not cause health problems. However, mould exposure may lead to:
- Nasal congestion
- Coughing
- Wheezing
- Skin irritation
- Headaches
- Worsening asthma symptoms
Individuals with asthma, allergies, chronic lung conditions, or weakened immune systems may be more sensitive.
If you are concerned about mould-related health risks, seek advice from your healthcare provider or local public health authority.
Aspergillosis
Aspergillosis is linked to Aspergillus moulds, which are common indoors. Most people inhale Aspergillus spores every day without issues, but some people may experience symptoms such as a stuffy nose, headache, coughing, chest pain, and fever.
Blastomycosis
Blastomycosis is associated with Blastomyces, a fungus found near waterways. Spores can enter the lungs and cause symptoms such as fatigue, chest pain, and weight loss.
C. neoformans infection
C. neoformans is found worldwide. Most people never contract it, but those with weakened immune systems can develop serious infections affecting the lungs or nervous system, with symptoms such as neck pain, nausea, confusion, and fever.
Sporotrichosis
Sporotrichosis (rose gardener's disease) is linked to the fungus Sporothrix. It often infects through cuts in the skin, causing bumps, or if inhaled, cough and chest pain.
Stachybotrys chartarum infection
Stachybotrys chartarum is sometimes called “black mould” and can appear on water-damaged building materials. Effects vary, ranging from allergies to breathing difficulties, depending on the individual.
Where to Look for Mould in Your Home
The most common places for mould to grow in your home include bathrooms and kitchens without extraction fans, as well as any location with moisture and poor airflow:
- Wet carpets
- Areas around sinks or bathtubs
- Shower cubicles (especially hinges)
- Behind walls (where pipes might leak)
- Around windows (especially with condensation)
- Along water-damaged walls
- Bathroom grout between tiles
It's essential to maintain these areas in good repair to prevent mould growth.
What’s the Difference Between Mould and Mildew?
People often use the terms "mould" and "mildew" interchangeably, but they are distinct.
- Mould tends to be fuzzy or slimy and can be black, brown, grey, green, or blue.
- Mildew usually appears flatter and powdery, often white, grey, or yellow.
If you want to learn more, see our guide on mildew vs mould.
Need help removing mildew from outdoor settings? Read: how to remove mildew from outdoor furniture.
How to Prevent Mould Growth
Because mould requires moisture, prevention focuses on moisture control and ventilation:
- Fix leaks and dampness: Repair leaking pipes and clean up spills quickly.
- Improve ventilation: Use extractor fans, open windows, and increase airflow.
- Reduce condensation: Wipe wet surfaces and use dehumidifiers if needed.
- Reduce dust: Mould spores use dust to settle and grow.
How to Test for Mould
If you notice stains, odours, or discolouration in your home, you might wonder if it’s mould. Mould can resemble a stain or smudge and may have a musty smell.
Professional mould testing typically involves collecting a sample and analysing it in a lab. After a few days, you’ll receive a report identifying the mould species present.
For further information on mould testing, health advice, or tenant rights, consult your local health authority or relevant government resources.
How to Remove Mould
Removing mould requires a careful approach to help prevent spores from spreading. First, identify and fix the moisture source causing mould growth (such as leaks, humidity, or poor ventilation).
Wear rubber gloves and a mask to reduce exposure to mould spores, especially if you have allergies, asthma, chronic lung conditions, or a weakened immune system.
Use a cleaning solution and a microfibre cloth to wipe away mould from surfaces. Avoid dry brushing because it can release spores into the air. Work in a well-ventilated area by opening windows and doors. If available, consider using extractor fans.
After removing mould, dry the area thoroughly to reduce the chance of regrowth.
For fabric-specific steps, see: how to remove mould from fabric.
Mould Remover: Products and Solutions
There are many mould remover products available, from DIY solutions to commercial cleaners. Always follow the manufacturer’s instructions and safety guidance.
Some mould removers contain strong chemicals, so choose a product suitable for the surface you are cleaning and test on a small hidden area first.
Remember: mould remover products can clean visible mould, but they won’t solve the underlying cause (such as water leaks or poor ventilation). If mould keeps returning or you’re dealing with widespread contamination, seek professional help.
What Causes Mould? The Bottom Line
Mould is a natural fungus found virtually everywhere. You can't stop spores entering your home, but you can prevent mould growth by controlling moisture, improving ventilation, and reducing condensation.
