Why Is Black Mould Considered More Toxic Than Other Types?

The "Black Mould" Myth — and the Reality

The term "black mould" has become a catch-all phrase that strikes fear into homeowners and tenants alike. But what does the science actually tell us? At Mould UK, we believe in evidence-based education — so let's separate fact from fiction.

The mould species most commonly referred to as "black mould" is Stachybotrys chartarum. It thrives on materials with high cellulose content — think plasterboard, wallpaper, cardboard and timber — and requires sustained moisture to grow. It's this dependency on persistent dampness that makes it a strong indicator of serious water damage in a property.

Why Does It Have Such a Fearsome Reputation?

S. chartarum is capable of producing mycotoxins, including macrocyclic trichothecenes (such as satratoxins), which can be harmful when inhaled in high concentrations over prolonged periods. Research published in the International Journal of Molecular Sciences has noted that this species is considered one of the world's ten most hazardous fungi, owing to these secondary metabolites.

The species gained widespread public attention following infant pulmonary haemorrhage cases in Cleveland, Ohio during the 1990s. While the CDC has stated that a direct causal link was not conclusively established, the vast majority of documented cases were associated with the presence of S. chartarum in the affected homes.

Health effects associated with prolonged exposure include respiratory issues such as asthma exacerbation, bronchopulmonary disorders, respiratory infections and allergic reactions. People with compromised immune systems, young children, and the elderly are particularly vulnerable.

A Critical Point: You Cannot Identify Species by Sight

One of the most important things Mould UK emphasises to both landlords and tenants is this: mould species cannot be identified through visual inspection alone. The colour, texture and structure of mould depend on its age, the substrate it's growing on, lighting conditions and even mutation. Analytical testing — such as microscopy or ITS sequencing — is required for species-level identification.

As the BSRIA Topic Guide TG 26/2024 states, even Aspergillus versicolor — a moisture-damage indicator mould — can appear transparent when young before turning white and then dark green as it matures.

All Visible Mould Is a Problem

The CDC's position is clear: all mould should be treated the same with respect to potential health risks and removal. Whether it's black, green, white or grey, visible mould growth indicates excess moisture and should be addressed promptly.

At Mould UK, our lab-verified reports identify the specific species present in a property, giving housing providers the evidence they need to act decisively. Don't wait for a species identification to take action — if you can see it, it's a problem.

Sources

CDC — Facts About Stachybotrys chartarum

International Journal of Molecular Sciences — Update on S. chartarum

BSRIA TG 26/2024 — Mould in Buildings (Efthymiopoulos & Aktas, UCL)