Is black mould especially dangerous?

Any mould can affect health, especially for people with asthma, allergies, or weakened immunity — you can't identify risk by colour, and testing usually isn't needed because the response (remove it, fix the moisture) is the same. Escalate by size and health, not by colour.

On hard, non-porous surfaces, detergent and water plus thorough drying is enough; bleach isn't required. On porous materials, cleaning often can't remove it — replace them instead.

Mould — clean it yourself, or call a pro?

As a rule of thumb (US EPA): if the mouldy area is under about 10 square feet — roughly a 3-by-3-foot patch — a healthy adult can usually clean it safely. Larger than that, or if it's from sewage/flood water, hidden inside walls or HVAC, or anyone in the home is immunocompromised or has asthma or allergies, call a professional. And whatever you do, fix the moisture source — clean mould comes back if the damp does.

The size rule, and its exceptions

The US EPA's guideline is that mould under about 10 square feet is generally DIY, while larger areas warrant a professional. That size is a US EPA figure — Canada and the UK frame it qualitatively as 'small area vs large area', but the practical line is similar. The exceptions override the size: sewage or flood-water contamination, mould inside walls or the HVAC system, or a vulnerable occupant (asthma, allergies, weakened immune system, infants, elderly) all mean call a pro regardless of how small it looks.

How to clean a small patch — and stop it returning

For a small, hard-surface patch: wear gloves and an N95, scrub with detergent and water, and dry thoroughly. You don't need bleach on non-porous surfaces — water and detergent work, and the key is drying. Porous materials that stay mouldy (drywall, carpet, ceiling tiles) usually can't be fully cleaned and should be replaced.

The step people skip is the one that matters most: find and fix the moisture — a leak, poor ventilation, or condensation. Keep indoor humidity around 30–50% and dry any wet materials within 24–48 hours. Without that, it comes straight back.

When to call a professional

• Mouldy area larger than ~10 sq ft, from sewage/flood water, or inside walls or HVAC.
• Anyone in the home is immunocompromised or has asthma/allergies — professional regardless of size.
• It keeps returning after cleaning — there's an unfixed moisture source to find.

Common questions

Is black mould especially dangerous?: Any mould can affect health, especially for people with asthma, allergies, or weakened immunity — you can't identify risk by colour, and testing usually isn't needed because the response (remove it, fix the moisture) is the same. Escalate by size and health, not by colour.
Should I use bleach?: On hard, non-porous surfaces, detergent and water plus thorough drying is enough; bleach isn't required. On porous materials, cleaning often can't remove it — replace them instead.

Sources

You can generally clean mould yourself if the area is under about 10 square feet; larger areas warrant a professional (US EPA). Tier 1 (official)
U.S. Environmental Protection Agency (EPA) · verified 2026-07-26
Use a professional regardless of size if occupants are immunocompromised or have asthma/allergies, or if mould comes from sewage or is inside walls or HVAC (EPA). Tier 1 (official)
U.S. Environmental Protection Agency (EPA) · verified 2026-07-26
Mould always grows from a moisture problem — unless the water source is fixed, cleaned mould returns (EPA / Health Canada). Tier 1 (official)
Health Canada · verified 2026-07-26
Keep indoor humidity around 30–50% and dry any wet materials within 24–48 hours to limit mould growth (EPA). Tier 1 (official)
U.S. Environmental Protection Agency (EPA) · verified 2026-07-26