What is mould, exactly? On the surface, it looks like a furry, chalky, or sometimes colorful substance on damp surfaces. But mould is actually a living organism—part of the fungi kingdom—and is one of the oldest life forms on Earth. Its presence often raises the question: what causes mould, and why does it grow where it does? Let’s explore further.
What is mould?
Mould is a member of the fungi family—a separate branch of life, different from plants and animals. Scientists once thought it was a type of plant, but now know it’s more like animals because of how it digests food. You can find mould almost everywhere, both indoors and outdoors. In nature, most moulds grow on rocks or in damp, moist areas where other species can't survive. In modern homes, mould can grow in plumbing, timber, paper, walls, fabrics, carpets, and furniture.
What causes mould?
Mould is a natural part of the environment and helps break down dead organic matter, like tree bark or leaves. It reproduces by releasing tiny spores into the air. When these land on a surface, they can grow and divide into new mould cells. Mould is an extremophile, meaning it can survive in tough conditions. It will grow on almost any surface where there is moisture, so it often appears in homes.
Different Types of Mould
The types of mould and mildew you might find fall into three main categories:
1. Toxigenic
Toxigenic moulds can cause serious health issues, from eye irritation to immune problems. They produce mycotoxins—chemicals on the surface of the mould spores. Inhaling these can cause inflammation, cell damage, and sometimes organ failure.
2. Pathogenic
Pathogenic moulds are infectious, like germs. If they get into your body, they can multiply and cause infections, sometimes similar to bacterial pneumonia.
3. Allergenic
Allergenic moulds are the least harmful. They can still cause problems for people with allergies or asthma, but are usually more of a nuisance than a serious threat.
What diseases can mould cause?
Most homes have low levels of mould that don’t cause problems. But sometimes, mould can lead to infections and other diseases.
Aspergillosis
Aspergillosis is caused by Aspergillus moulds, which are common indoors. Most people breathe in Aspergillus spores every day without getting sick, but sometimes these spores can cause health problems like stuffy nose, headache, coughing, chest pain, and fever.
Blastomycosis
Blastomycosis is caused by Blastomyces, a fungus found near waterways. Spores can get into the lungs and cause symptoms like fatigue, chest pain, and weight loss.
C. neoformans infection
C. neoformans is found worldwide. Most people never get sick from it, but those with weak immune systems can get serious infections of the lungs or nervous system, with symptoms like neck pain, nausea, confusion, and fever.
Sporotrichosis
Sporotrichosis, or rose gardener's disease, is caused by Sporothrix. It often infects people through cuts in the skin, causing bumps or, if inhaled, cough and chest pain.
Stachybotrys chartarum infection
Stachybotrys chartarum is a classic black mould found in homes, often on drywall. Its effects can range from allergies to trouble breathing, and symptoms can vary from person to person.
How can you stop mould from growing?
Mould needs water to grow. To reduce mould in your home, focus on removing sources of moisture. The main causes of household mould are leaking pipes and rising damp. Mould can also grow in bathrooms from water vapor. You can’t control mould completely, as it’s a natural part of the environment, but you can stop it from growing out of control. Here are some tips:
- Fix leaks and dampness. Repair leaking pipes and clean up spills quickly.
- Reduce dust. Mould spores use dust to settle and grow.
- Improve ventilation. Extra airflow helps reduce humidity and stops mould from settling.
What’s the difference between mould and mildew?
People often use the terms mould and mildew interchangeably, but they are different. Mould is a microscopic fungus that grows in filaments called hyphae and usually thrives on materials with organic compounds. Mildew grows best in very damp places, like windowsills and shower walls. Common types of mildew include powdery mildew (grey or white blotches on plants) and downy mildew (starts yellow, then turns brown). Both mould and mildew produce a musty smell, especially in places with poor airflow. Mildew usually grows in a flat, powdery pattern, while mould is fuzzier or slimier and can be black, brown, grey, green, or blue.
Want to know how to remove mildew from outdoor furniture? Read our guide here.
Where to look for mould in your home
The most common places for mould to grow in your home are:
- Wet carpets
- Areas around sinks or bathtubs
- Shower cubicles, especially in the hinges
- Behind walls (where pipes might leak)
- Around windows (especially those left open)
- Along water-damaged walls
- Bathroom grout, between tiles
How to test for mould
If you notice stains, odors, or discoloration in your home, you might wonder if it’s mould. Stains often appear in bathroom grout or on damp walls. Odors are common in bathrooms, basements, and attics. If you think you have mould, you can get a professional to test it. They’ll take a sample and see if mould grows in a petri dish. After a few days, you’ll get a report on what species are in your home.
What causes mould? The bottom line
In summary, mould is a type of fungus and a natural part of the environment. You can’t stop it from entering your home, but you can prevent it from growing by reducing moisture and dust and increasing airflow.