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Mould

Mould can be an indoor air pollutant, but there’s good news: you can prevent it, clean it, and keep it from coming back.

What is mould, and why does it grow?

Mould is a type of fungus that grows in damp areas. Outdoors, it helps break down leaves and wood. Inside, it can grow on walls, ceilings, carpets, and other surfaces if there’s too much moisture.

Common causes of mould:

  • Leaks in your roof, walls, or plumbing

  • Humidity from cooking, showering, or drying clothes indoors

  • Poor airflow in damp spaces like basements or bathrooms

  • Flooding or water damage

Mould spreads through tiny particles known as spores in the air. These can get into your home through windows, doors, or even on your clothes and pets.

How can you prevent mould growth?

Since mould needs moisture to grow, keeping your home dry is key.

Easy ways to prevent mould:

  • Fix leaks in roofs, walls, or plumbing as soon as possible

  • Use fans or open windows when cooking or showering

  • Run a dehumidifier in damp areas, like your basement

  • Clean your gutters regularly and make sure water drains away from your home

  • Avoid drying clothes indoors, or use a room with good airflow

  • Check for damp spots, especially in bathrooms and kitchens

How can you tell if you have mould?

Common signs of mould include:

  • Discolouration: Mould can appear as black, white, green, or orange patches.

  • Musty smell: A strong, earthy smell could mean mould is growing.

  • Damp spots: Water stains, condensation, or past flooding may lead to mould.

Can mould make you sick?

Mould can affect everyone differently. Some people may not notice any symptoms, while others could experience:

  • Itchy eyes, a runny nose, or a sore throat

  • Coughing, wheezing, or trouble breathing

  • Worsening asthma symptoms

People more sensitive to mould include:

  • Children and seniors

  • People with asthma, allergies, or breathing problems

If you think mould is making you sick, talk to your doctor.

What should you do if you find mould?

If you find mould, it’s important to clean it up and fix the moisture problem that caused it. Small areas (3 or less patches of mould with a total area of less than 1 square meter) can often be cleaned yourself. For bigger areas, call a professional.

What you will need to clean small areas:

  • Safety glasses

  • A disposable dust mask (an N95 mask works well)

  • Gloves

  • Soap and water

Steps to clean small areas:

  1. Scrub washable surfaces with soap and water. Dry the area quickly with fans or a dehumidifier.

  2. For drywall or similar materials, use a damp cloth with a little water and baking soda. Avoid using too much water.

  3. Throw away single-use cleaning supplies and damaged items, like mouldy carpets, that cannot be cleaned.

  4. Fix the moisture problem, like repairing a leak or improving airflow.

When should you call a professional?

  • If the mould covers a large area (greater than 3 square meters)

  • If mould is growing in your heating, ventilation, or air conditioning system

Frequently asked questions

If mould keeps returning, the moisture problem is likely still present. Check for hidden leaks, poor ventilation, or high humidity. You may need to call a professional to identify the source.

No, soap and water are enough to clean most small mouldy areas. Bleach is not recommended because it doesn’t stop mould from coming back and can harm some surfaces.

Health Canada does not recommend testing the air for mould. If you can see or smell mould, the best approach is to clean it and fix the moisture problem.

Yes, mould can grow in cold areas if there is moisture. For example, mould might appear on cold windows or walls where condensation forms.

Protect yourself by wearing safety glasses, gloves, and a disposable mask to avoid breathing in mould particles. Keep children and pets away from the area you are cleaning.


What should I do next?

Here’s a quick summary of what to do:

  1. Check your home for mould: Look for damp spots, musty smells, or patches of discolouration.

  2. Clean small areas safely: Use soap and water, and fix any moisture issues.

  3. Call a professional for bigger problems: They can remove mould safely and keep it from spreading.

  4. Keep mould away: Stay on top of leaks, humidity, and ventilation to prevent mould in the future


Additional information

Check out these resources for more information:

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