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Mold in the home: How big a health problem is it?

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Whether it is a cold, damp winter or a warm, humid summer, activities at home can result in moisture indoors and the appearance of mold.

Mold can grow on walls, clothes, books, toys, and even CDs. It can turn prized possessions into musty relics that only look fit for the garbage.

But is it also a health hazard? What effects can mold have on a person’s body?

This article looks at what mold is, why it grows, its impact on a person’s health, and how to stop it.

What is mold?

Mold in the home
Indoor mold growth is most likely to occur during the fall and winter months.

Molds are a form of fungus. There are many different types, and they can occur both indoors and outdoors.

Molds produce spores, which spread by floating around in the air. Mold spores are present in all indoor environments. There is no way to prevent spores, and they can persist in conditions where mold itself cannot grow.

Mold spores thrive in environments that are moist and warm, so when they land on a damp spot, they begin to grow.

Molds can grow on a variety of different surfaces, including fabric, paper, wood, glass, and plastic. As they grow, they may digest the material they are growing on.

Types of mold

Nobody knows how many kinds of mold there are, but experts estimate that there may be 300,000 or more different types. Some are more likely than others to appear in the home.

Common indoor molds include:

Alternaria: This occurs in damp places indoors, such as showers or under leaky sinks.

Aspergillus: This often grows indoors, on dust, powdery food items, and building materials, such as drywall.

Cladosporium: This can grow in either cool or warm areas. It tends to appear on fabrics and wood surfaces.

Penicillium: This tends to grow on materials with water damage. It often has a blue or green appearance.

Molds take a variety of forms and textures. They can be white, black, yellow, blue, or green and often look like discoloration or stain to a surface.

They can also have a velvety, fuzzy, or rough appearance, depending on the type of mold and where it is growing.

How does mold get into a house?

a cat walking through a cat flap
Mold can attach itself to pets.

Mold spores are everywhere, both indoors and outdoors, but they are not visible to the naked eye.

Spores can enter the home:

Through the air: They can enter through open windows, doorways, and ventilation systems.

By attaching to objects or people: Vehicles include clothing, shoes, and pets.

Mold will only flourish if spores land somewhere that has the ideal conditions for growing, such as moisture and a supply of suitable nutrients. If the environment is unsuitable for the spores, they do not usually develop or cause a problem.

Places where mold often appears include:

  • areas where leakages and flooding have occurred
  • windows where condensation builds up
  • places where the air does not circulate, for example, behind a closet

Wet cellulose materials are most supportive of mold growth.

Examples include:

  • paper products, including wallpaper
  • cardboard
  • ceiling tiles
  • wood products
  • insulation materials
  • upholstery and other fabrics

Mold growth is usually visible and often produces a musty odor. It can damage household items, and it can also have an impact on health.

Mold and health

Mold can pose a health problem, especially for people with an allergy, an existing respiratory problem, or a weakened immune system.

Breathing problems

As mold grows, spores, cells, fragments, and unstable organic compounds can enter the air. They can produce allergens, irritants, and mycotoxins. Some of these can be toxic, especially to individuals who have a sensitivity to them.

Also, dampness encourages materials to break down, increasing the volume of particles, or dust, in the air.

These particles can irritate the lungs, nose, and throat, especially in a person who already has a breathing problem, asthma, or a chronic lung condition.


A person with a sensitivity or allergy to any mold-related particles may react.

Mold allergies can produce similar symptoms to other allergies, such as hay fever, or seasonal allergy. In these, too, airborne substances can affect the upper respiratory tract.

Symptoms include:

  • a blocked or runny nose
  • an itchy nose
  • an itchy throat
  • sneezing
  • watery eyes

People with a mold allergy as well as asthma have a higher chance of having an asthma attack when there is mold in the environment.

A higher volume of dust can increase the risk of dust mites, which can also trigger an allergic reaction in some people.


Some types of mold, such as Aspergillus, can cause a serious health problem, known as aspergillosis, in some people.

Most people can breathe in the spores of this fungus without becoming sick, but people who have a weakened immune system or an existing lung disease can have a severe reaction.

There are different types of aspergillosis:

Allergic bronchopulmonary aspergillosis (ABPA): This affects the lungs and can cause breathing problems.

Allergic aspergillus sinusitis: This affects the nose and can involve a headache.

Aspergilloma, or fungus ball: This can cause a cough, which may produce blood, as well as breathing problems.

Chronic pulmonary aspergillosis: Symptoms include breathing problems, a cough, and weight loss.

Other conditions

Mold can also trigger the production of microbes and bacteria. Exposure to these bacteria may trigger an inflammatory response in some people, according to the World Health Organization (WHO).

The WHO also note that mold and the microbial agents it produces may increase the risk of bronchial and fungal infections.

There is some evidence that it might lead to:

  • hypersensitivity pneumonitis
  • bronchitis
  • allergic alveolitis
  • chronic rhinosinusitis
  • allergic fungal sinusitis
  • lower respiratory tract problems in previously healthy children

Some evidence from the WHO and the Centers for Disease Control and Prevention (CDC) suggests that people have experienced the following symptoms after spending time in an environment where mold is present:

  • skin and eye irritation
  • wheezing
  • fever
  • fatigue
  • nausea
  • headache
  • insomnia
  • mucous membrane irritation
  • sick-building syndrome

Factors affecting the likelihood of having health problems due to mold include:

  • the person’s immune or respiratory health
  • how much mold is present

Protection and prevention

Controlling moisture is the key to preventing mold from growing indoors. It is also important to keep the home clean and well ventilated.

Causes of humidity in the home include:

  • the breath of people and pets
  • use of water for washing and cooking
  • moisture in the air, for example, on rainy or humid days
  • water leaks
  • living in a building with tightly-sealed windows and doors

The Environmental Protection Agency (EPA) advise people to aim for a humidity level of below 60%.

Reducing humidity

People can reduce the risk of a buildup of moisture and mold by:

  • acting quickly if a leak or spillage occurs
  • using a dehumidifier to reduce indoor moisture
  • leaving windows open when possible, to allow air to circulate
  • using extractor fans to remove moisture during cooking
  • avoiding indoor activities that produce moisture, such as drying clothes or using kerosene heaters
  • ensuring that all fabrics are thoroughly dry before storing them
  • emptying and airing rarely used drawers and closets from time to time
  • regularly cleaning so that mold cannot build up on surfaces, dust, or other matter
  • using mold-killing products when cleaning the bathroom
  • avoiding laying carpets in bathrooms and basements
  • maintaining buildings, facilities, gutters, and drains to reduce the risk of leaks

It is not always possible to prevent mold from growing, but regular cleaning and wiping can reduce the risk of it appearing or getting worse.

Removing mold

a person wearning gloves cleans mold off the wall.
There are a number of commercial products available to help remove mold.

To clean or remove mold:

Wipe hard surfaces with a suitable commercial product, soap and water, or a heavily diluted bleach solution. Always dry the surfaces after use to prevent mold from reappearing.

Wash or wipe porous surfaces and check regularly to see if mold has returned, as it can infiltrate these materials. If mold continues to reappear, consider disposing of these items.

Ask at the local hardware store about antifungal paints and other products that can help prevent mold from developing.

Call in a professional to tackle large areas of mold, like Certified Disaster Services.

See a doctor if you believe mold is causing a health problem.

A range of items to help deal with mold are available for purchase online.

These include:

  • dehumidifiers
  • humidity gauges
  • cleaning and other products

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