First, a reassuring note. Not everyone who's around mould gets sick, and finding some mould in your home does not mean any of the things below are happening to you. Most mould issues are far more manageable than the internet makes them feel. This page is here to explain what the research says - calmly - so you can make informed decisions, not to frighten you.

When a building gets damp, it grows a mix of biological agents - mould, bacteria, and the by-products they release. Exposure tends to be cumulative: it builds up over time until the body struggles to keep up. Some people are affected far more than others, and symptoms vary a lot. Here's the range of ways mould can affect health:

1. CIRS (Chronic Inflammatory Response Syndrome) - "toxic mould illness"

This is the big one, and the one closest to my heart - I've been through it myself.

Around 1 in 4 people (roughly 25%) are genetically predisposed to CIRS, also called toxic mould illness or biotoxin illness. In these people, the biotoxins from a water-damaged building aren't cleared properly by the immune system - so the inflammation just doesn't switch off, even after you've left the mould. That persistence is what makes CIRS different from a simple allergy.

Because it's a whole-body (multi-system) illness, symptoms are wide-ranging: fatigue, brain fog, headaches, muscle and joint pain, mood changes like anxiety or depression, sleep problems, gut issues, weight changes and chemical sensitivities, among others.

A simple starting point: if you recognise symptoms across 8 or more of the 13 symptom clusters listed below (6 for children), it's worth getting assessed for CIRS by a mould-literate practitioner. Six clusters with known mould exposure is considered "suspicious" and worth investigating.

The good news: once the exposure is found and addressed, most people recover well.

2. Mould allergies

This is mould's most familiar health effect. When susceptible people breathe in spores, they can react - watery eyes, a runny nose, sneezing, congestion, and sometimes skin rashes (like eczema), coughing, wheezing or worsened asthma. Around 6–10% of people generally, and 15–50% of those who are allergy-prone, are sensitive to mould allergens.

Worth knowing: an allergy is a separate thing from CIRS. In fact, most people with CIRS are not allergic to mould - which is exactly why a "you're not allergic" test result doesn't rule mould illness out.

3. Respiratory and irritant effects

In 2009, the World Health Organization reviewed the evidence and found that damp and mouldy buildings were linked to a 30–50% increase in respiratory and asthma-related problems. Mould can irritate the airways of allergic and non-allergic people alike — coughing, wheezing, chest tightness, a blocked or runny nose, headaches - and it can worsen existing asthma or contribute to new asthma developing.

4. Recurrent infections

Spending a lot of time in a damp building can make you more prone to picking things up - recurrent colds, sinus infections, bronchitis and tonsillitis (kids especially feel this). Damp environments can also encourage fungal issues. If you or your family seem to catch everything going around, a damp home is worth ruling out.

5. Mycotoxins

Some moulds produce mycotoxins - chemical by-products - as they grow. These can be potent, and with significant or prolonged exposure they've been studied for their effects on the immune, nervous and other body systems.

Two things to keep in perspective, though: context and dose matter enormously, and finding a small patch of mould does not mean you're being harmed by mycotoxins. This is about understanding why serious water damage is worth taking seriously - not a reason to panic over every spot of mould.

6. MVOCs — that "musty" smell

That distinctive musty smell? Those are mVOCs (microbial volatile organic compounds) - gases moulds release as they grow. Breathing them in can cause immediate irritation of the eyes, nose and throat, plus headaches, dizziness, nausea or fatigue, and research suggests possible effects on concentration, memory and mood.

If a room smells musty, that smell is a real signal worth paying attention to - it usually means there's active moisture and mould somewhere, even if you can't see it.

Why do some people get sick and others don't?

One of the hardest parts of mould-related illness is that two people can live in the same home and react completely differently. This is very common - often it's the mum and one or two kids who react well before the male partner does.

A few reasons for this:

• Genetics - roughly 1 in 4 people carry the HLA-DR/DQ "mould gene" that makes it hard to clear biotoxins.

• Immune differences, and differences between men and women.

• Cumulative exposure - how much mould someone's been exposed to across their life.

• Overall load - gut health, stress, past illness and inflammation all fill up the "bucket."

This is also why some people suddenly react to a place they used to tolerate: the body gradually reaches a tipping point. It's rarely as simple as "mould present = everyone instantly sick."