Don’t Use Bleach to Kill Mould (Here’s Why It Comes Back)
, by Tatianna Gerard, 10 min reading time
It starts the same way for most of us. You notice a patch of mould — maybe along the bathroom ceiling, in the corner of a window frame, or creeping across the grout in your shower. It looks unsightly. Maybe even smells musty. So you do what most people do: reach for the bleach.
You spray, scrub, and wipe it away. The dark stain fades. The surface looks brighter. Maybe there’s even that sharp chemical scent in the air that makes it feel clean.
But fast forward a few days or weeks — and there it is again. The mould is back. Sometimes in the same spot, sometimes spreading further. And no matter how many times you bleach it, the cycle continues.
Sound familiar?
The truth is, bleach gives the illusion of control, but rarely solves the root of the problem. It can temporarily remove the visible signs of mould, but it doesn’t stop what’s happening beneath the surface — or in the air you’re breathing. And in some cases, it might even make things worse by leaving behind moisture and encouraging regrowth.
We’ll explore why bleach isn’t the long-term mould solution it’s often believed to be, explore the science behind mould regrowth, and introduce a safer, more effective approach that targets the problem at its source.
Why bleach doesn’t really kill mould
1. Bleach only works on non-porous surfaces
Bleach is a surface-level disinfectant. It doesn’t penetrate porous materials like:
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Gyprock or plasterboard
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Timber and particleboard
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Grout and silicone
- Unsealed tiles or cement
And unfortunately, these are some of the most common places where mould grows. While bleach may remove the visible mould on the surface, the roots (or hyphae) of the mould often grow deep into the material — far beyond bleach’s reach. So when you wipe it down, you’re really just bleaching the stain, not killing the source.
2. Bleach contains water — which feeds mould
Most household bleach solutions are about 90–95% water. And mould, being a living organism, thrives on moisture. So when you apply bleach to porous materials, the chlorine may evaporate quickly, but the water soaks in — giving the mould a new damp environment to regrow.
Ironically, in trying to kill mould with bleach, you might actually be giving it the perfect conditions to return.
3. Mould regrows — Especially when you don’t remove the spores
Bleach may help fade the black or green stains, but it doesn’t neutralise the microscopic spores that mould releases into the air. These spores can settle elsewhere in your home — in your carpet, behind walls, in your air conditioning — and start a new colony when conditions are right.
That’s why even after cleaning, mould often returns. Unless the airborne spores and environmental conditions are addressed, the cycle continues.
4. Repeated bleach use can create resistance
Just like bacteria can become resistant to antibiotics, some types of mould can become more resilient to repeated chemical exposure. There’s been increasing concern about antimicrobial resistance (AMR) — the process by which microorganisms like bacteria adapt over time and become less responsive to disinfectants, antibiotics, and other treatments. This phenomenon is well-documented in bacteria and has been highlighted as a major public health issue by bodies like the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC).
While mould is a fungus rather than a bacterium, researchers have observed that repeated exposure to sub-lethal levels of biocides — like bleach — can sometimes lead to adaptive responses in other microorganisms. For example, studies in environmental microbiology have shown that fungi and other microbes can develop stress tolerance mechanisms, such as changes in their cell wall structure, metabolic activity, or biofilm formation, which may make them more difficult to eliminate over time.
That doesn't mean mould becomes “immune” to bleach in the same way bacteria resist antibiotics — but the underlying biological principle is similar. So while there is currently limited direct evidence of mould developing resistance to cleaning agents, it’s reasonable to consider the possibility, especially given what we know about microbial adaptation under chemical stress. Instead of continuing the bleach-and-scrub cycle, the better approach is to target the mould at its source and prevent it from growing back.
5. It doesn’t improve air quality — and can make it worse
Beyond its ineffectiveness on deep mould, bleach also releases harsh fumes and volatile organic compounds (VOCs) that can irritate the lungs, eyes, and skin. This is especially problematic in enclosed areas like bathrooms or wardrobes.
For people with asthma, allergies, or chemical sensitivities, the side effects of using bleach may actually be worse than the mould itself — and without long-term results to show for it.
Biostat vs biocide
When it comes to fighting mould, not all antimicrobial agents work the same way. In fact, the type of agent you use can make a big difference in whether you’re solving the problem — or just pushing it down the road.
There are two main categories of antimicrobial action:
1. Biocides
A biocide is a substance that kills living microorganisms like bacteria, mould, and fungi. Bleach, vinegar, hydrogen peroxide, and other strong chemical agents fall into this category. They’re designed to wipe out pathogens quickly and are often marketed as heavy-duty solutions.
Why vinegar, bleach and other chemicals fail to tackle mould is a topic that’s been widely discussed in the mould remediation space — especially as more people look for safer, longer-lasting solutions. But here’s the catch: biocides often kill only what’s on the surface — and they can come with significant downsides.
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They struggle to penetrate porous materials where mould hides
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They don’t address airborne spores, which means the mould often returns
- They can be toxic or irritating to people and pets, especially in enclosed spaces
2. Biostats
A biostat, on the other hand, doesn’t kill mould aggressively — it stops it from growing and spreading in the first place. Biostats work by:
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Disrupting the biological processes mould relies on to grow and reproduce
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Creating an environment where mould can’t thrive
- Preventing regrowth over time, rather than just cleaning what’s visible
This approach is not only gentler and safer, but it’s also more effective for long-term control — especially in homes, caravans, boats, wardrobes, and other high-humidity areas.
Meet SAN-AIR: The long-term solution for mould removal & prevention
If you’re looking for a safer, more effective way to deal with mould — not just for today, but long-term — SAN-AIR offers a science-backed, biostatic solution that works differently from harsh chemical cleaners. Unlike bleach, which only targets what you can see, SAN-AIR products are designed to treat both surface and airborne mould, and help prevent it from coming back by inhibiting its ability to grow and spread.
Shop SAN-AIR Mould Treatment Starter Pack
Formulated using plant-based actives and supported by independent testing, SAN-AIR is proudly Australian-made and trusted in homes, schools, workplaces, and even boats and caravans.
SAN-AIR offers a comprehensive range of mould removal and prevention solutions — but at the core of its system are three key products that address each stage of mould management: testing, cleaning, and prevention.
1. SAN-AIR Mould & Bacteria Test Kit
Before you treat, you need to know what you're dealing with — and this easy-to-use test kit helps you do just that. It allows you to detect the presence of mould and bacteria in the air, water, or on surfaces using simple petri dishes and swabs.
Ideal for renters, homeowners, or even workplace safety checks, this kit is a great first step in identifying hidden mould you can’t always see. For a full walkthrough, check out this step-by-step guide on how to use the SAN-AIR Mould & Bacteria Test Kit.
2. SAN-AIR Surface Mould Remover Spray
This ready-to-use spray is perfect for targeting visible mould growth on surfaces like walls, ceilings, tiles, skirting boards, and even air vents. It works by breaking down the mould’s biological structure and inhibiting regrowth — without the use of bleach, alcohol, or synthetic fragrances.
It’s safe for use around children, pets, and sensitive individuals, and leaves no toxic residue behind.
3. SAN-AIR Mould Gone Gel
This is SAN-AIR’s most popular long-term prevention product — designed to purify the air and stop mould spores from spreading in enclosed environments. It works through vapour-phase action, slowly releasing plant-based actives into the air to inhibit the growth and reproduction of mould and bacteria.
Place it in wardrobes, bathrooms, storage spaces, boats, caravans, or anywhere prone to high humidity. The Mould Gone Gel works quietly in the background, creating conditions that mould simply can’t thrive in — no wiping, no scrubbing, just consistent protection.
Together, these products provide a complete system for testing, treating, and preventing mould — all without harsh chemicals or guesswork.
Conclusion: It’s time to rethink how you treat mould
Bleach might seem like a quick fix — and it’s often the first thing people reach for when they spot mould — but as we’ve explored, it rarely solves the real problem. Surface-level treatments can give the illusion of cleanliness, while mould continues to grow behind the scenes, or worse, in the air you breathe.
To truly tackle mould, you need a solution that goes deeper. One that treats the root cause, addresses airborne spores, and helps prevent the cycle from starting again — all without exposing your home to harsh fumes or toxic residue.
That’s exactly what SAN-AIR’s biostatic, plant-based system is designed to do. Whether you’re testing for hidden mould, cleaning up visible growth, or protecting your space long-term, SAN-AIR provides a safer, science-backed approach that works with your environment — not against it.
Shop the full range of SAN-AIR Mould Removal solutions at Aussie Pharma Direct.