Is mould dangerous — and how worried should you be?
Mould can cause or worsen health problems — especially for people with asthma, allergies, or weakened immune systems, and for infants and the elderly — so it shouldn't be ignored, but a small patch in an otherwise healthy home is usually manageable. What matters isn't the colour or a lab test; it's the size, whether anyone vulnerable lives there, and fixing the moisture behind it. Under ~10 sq ft and no vulnerable occupants is generally a DIY clean.
Who's most at risk
Health agencies (EPA, CDC, Health Canada) agree that damp and mould can trigger or worsen respiratory symptoms, allergies, and asthma. The people most affected are those with asthma or allergies, weakened immune systems, and the very young or old. For them, even smaller amounts warrant a professional and a prompt fix.
You don't need to identify the species or pay for testing — the recommended response is the same for any indoor mould: remove it safely and fix the moisture that caused it.
When it's more than a clean-up
Treat it as a professional job if the area is larger than about 10 square feet, if it followed sewage or flood water, if it's inside walls or the HVAC, or if a vulnerable person lives there. Otherwise, cleaning a small patch and correcting the humidity (aim for 30–50%) usually resolves it.
When to call a professional
- • A vulnerable occupant (asthma, allergy, immune-compromised, infant, elderly) — professional regardless of size.
- • Large area (>~10 sq ft), sewage/flood origin, or mould inside walls or HVAC.
Common questions
- Do I need a mould test?
- Usually no. Testing rarely changes what you do — if you can see or smell mould, the answer is to remove it and fix the moisture. Spend the money on fixing the damp instead.
- Can mould make a healthy person sick?
- It can cause irritation, congestion, or coughing in some people, but serious effects concentrate among those with asthma, allergies, or weakened immunity. Don't ignore it, but don't panic over a small patch.
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 - 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 - 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