Good Mold VS Bad Mold: How To Tell The Difference

Mold isn’t the most appealing substance to look at, and it smells, it can be slimy and, of course, some strains can be potentially harmful to humans. However, you may be surprised to know that there are plenty of strains that are used to make some of your favorite foods and even important medicines.

Fungi are often misunderstood, so in this article, I’ll be explaining the differences between what constitutes bad mold, normal mold, and good mold. So, for all you need to know the good vs the bad, keep reading.

Good mold vs bad

What is mold?

Mold is a type of fungus, and makes up just one of the many forms fungi can make, which also include mildew, yeast, truffles, and mushrooms. It is neither plant nor animal and is known as an eukaryotic organism, which means it contains visibly evident nuclei.

Much as mold is something we usually see as being a nuisance and even rather foul, it plays a very important role in nature in that it breaks down and digests organic matter such as dead plants and creatures.

After decomposition, some nutrients are left behind and form soil which new plant life can use for sustenance. This is the important part that fungi play in nature.

What is good mold?

Particular strains of mold can be incredibly useful to humans and have a multitude of uses in the food and medical industries. The following examples are uses of “good” mold, that have been used by humans for centuries.



Antibiotics are used to prevent and treat bacterial infections, they both kill bacteria and prevent their spread and reproduction. Antibiotics were first discovered by Alexander Fleming in 1928, however, their use precedes this, as ancient Egyptians documented the placement of moldy bread on infected wounds in order to heal them faster.

The anti-bacterial (penicillin), that Alexander Fleming discovered, was extracted from the penicillium strain of fungi, whereas many other natural antibiotics are formed from bacteria.


Probiotics can be made from bacteria, mold, and yeasts. These are all banded together under the term “lactic acid bacteria”.

These are said to aid digestion and digestion and restore the balance of “good” bacteria within the gut, which are said to help people with conditions such as IBS.


Aspergillus terreus is the strain of fungi from which the drug Lovastatin is derived. It plays an important role in regulating the levels of LDL or low-density lipoproteins which can cause high cholesterol levels.

The fungi itself can cause skin irritation and a serious condition known as aspergillosis when in its natural state, but when processed and the chemical compounds used for the treatment of cholesterol are extracted, they can be used as an effective treatment.

Food and drink

Many foods and drinks use fungi and yeasts as an important part of the fermentation process. Beer, wine, bread, and cocoa products all use fungi such as Saccharomyces cerevisiae, Zygomycota, and Ascomycota to create the characteristic tastes and textures that make the items so well-loved.

Specific strains such as penicillium roqueforti are also added to soft-ripened cheeses to enhance their flavor and textures.


Mushrooms are one of the most well-known forms of fungi in our everyday diets, and are just one of the edible forms of fungi, with truffles being another example.

Whilst not all types of mushrooms are edible, as some can be quite poisonous, many strains are widely consumed throughout the world.

Which strains are considered “good”?

So, now that we know that not all strains of fungi and mold are bad, let’s take a look into some of the specific strains we have spoken about that are used within the previously mentioned medications and foods. These are the strains that we can consider “good”, as they play important roles in our diets and health.

Geotrichum candidum – Used in cheese making and fermented milk products.

Penicillium Roqueforti – Also used within the cheese industry and creates the characteristic veining of blue cheeses.

Penicillium chrysogenum – Used in the manufacture of antibiotics, as well as bioremediation of pollutants.

Aspergillus oryzae – Used for centuries throughout Asia to create soy sauce and other fermented foods.

What is a normal level of mold?

When we state that a level or degree of mold is “normal”, what we are discussing is the level of spores present in the air at any given time. As fungi spread by releasing spores into the air that then travel and settle to form new colonies, there is always a certain number of them present within the air around us.

This volume is usually very small, and not enough to cause any real harm unless you were severely allergic, in which case you may suffer from some symptoms of exposure.

The number of mold spores that are considered to be “normal” within the air in a property is calculated at fewer than 500 sp/m3. Results higher than this would take the number out of the normal range.

What types are harmless?

As we have seen, not all fungi are toxic, in fact, you may come into contact with many strains on a daily basis without suffering from any health issues whatsoever.

However, despite not being toxic, they can still be allergens, and so could pose a threat to those with suppressed immune systems or allergies.

The following is a list of commonly seen non-toxic fungi strains.

  • Aureobasidium
  • Mucor (with the exception of Mucor indicus)
  • Ulocladium

These fungi may not cause the kind of health conditions that toxic strains can, however, if you have allergies, they can still cause asthma attacks, skin irritation, coughing, itching, sneezing, and more. It is for these reasons that any mold discovered growing in a property should be remediated as soon as it is noticed.

What is bad?

“Bad” mold refers to strains that produce mycotoxins that can be very harmful to the health of humans and animals. These mycotoxins can cause acute poisoning as well as longer-term illnesses such as immune deficiencies and even cancer.

Whilst there are thousands of strains of fungi that have little to no effect on human health, there are many that have the ability to produce mycotoxins as a defense mechanism.

Within the fungi world, there are three main species that produce the highest number of mycotoxins, which include: Aspergillus, Fusarium, and Penicillium.

Despite its reputation, “black” mold, or Stachybotrys chartarum, is no more toxic than any other strain that also produces mycotoxins. It is more likely that this strain is more commonly referred to as bad mold because it is more commonly seen.

With these factors in mind, there really is no “bad” mold as such, as even the strains that are considered “harmless”, could still cause issues for those with allergies.

Different colors and textures of bad mold

Aspergillus, Fusarium, and Penicillium strains each have varying colors and textures which can make them slightly easier to identify, however, bear in mind that it is incredibly difficult to accurately determine a strain of fungi from its color or texture, and kits that include a swab that is sent to a lab for testing is a far more accurate method.

With that being said, here are a few features of each of the three that may help you to tell which type you may have growing.


Has a black to dark brown surface, a yellow-green underside, and comes in a dry, powdery texture.


Fusarium strains can be far more colorful than other toxic strains and can appear in white, yellow, pink, or red coloration. Its texture is most often described as having a cotton-like texture.


This is probably the most well-known of all strains thanks to the development of the drug penicillin which is derived from the fungi. Despite this, it can still be toxic to humans and animals and it’s typically brightly colored, with initial white coloration turning to shades of blue, blue-green, gray-green, yellow, or pink. Its texture is said to resemble wool or velvet.

Symptoms of mycotoxin exposure

It is important to note that inhalation of spores from “toxic” strains has not been conclusively linked to brain fog, fatigue, memory loss, or any symptom referred to as “toxic mold syndrome”. Should the mold be accidentally eaten, the mycotoxins could certainly cause considerable harm, but this is not proven to be the case with inhalation.

Any person with allergies will be just as likely to suffer from symptoms from a “toxic”, strain, as they would a non-toxic alternative. The most common symptoms of mold exposure to those with immune deficiencies or allergies include:

  • Runny nose
  • Skin irritation
  • Watery eyes
  • Sinusitis
  • Breathing difficulties
  • Coughing
  • Tiredness (in rare cases)

Is all bad mold toxic?

No, even strains of fungi that are non-toxic can result in allergic reactions and sensitivity in those with allergies or suppressed immune systems if they are exposed to either a large number of spores or the exposure is over a considerable amount of time.

So, in this sense, all strains could be considered to be bad, as they have the ability to cause your symptoms. It is also said that nearly everyone would suffer some degree of Ill health if exposed to spores for long enough, so it is vitally important that it is removed as soon as it is spotted.

How to test for mold

Using visual cues to determine a particular strain of fungi is particularly difficult, even for professionals. This is why if you want to be more certain of what you are dealing with and the possible toxicity levels associated with the strain, you are much better off either testing it yourself with a home testing kit or hiring a professional.

If you would like to test whether what you are looking at is mold or simply dirt and grime accumulation, a very simple test is to dip a Qtip (cotton bud), in household bleach and gently dap it onto the suspected fungi. Dirt and grime will have no reaction to the bleach, whereas mold will begin to lighten after a few minutes.

In most cases, testing is not necessary, as you would not want to have any form of mold accumulation in your property, as they can cause allergic reactions and can even devalue a property.

Is it safe to remove fungal growth yourself?

Removing very small patches of fungi yourself can be safe as long as you have no allergies and use the correct protective equipment. However, for larger projects where there is a substantial amount of growth, it may be safer to hire a professional, (more on this later).

To make things as safe as possible for home remediation, you should always make sure you wear PPE before beginning to tackle any fungal growth, such as:

It is also advisable to make sure there is plenty of ventilation within the room you are working in, so doors and windows should be opened to allow a fresh supply of air into the room before work begins.

How to remove mold

One of the most simple methods to kill and remove mold from walls and surfaces within a property is to use distilled white vinegar. It is an effective fungi killer that can be applied to many surfaces without the risk of damage being incurred, is cheap and is also environmentally friendly.

In order to kill mold and mildew using white distilled vinegar, use the following steps.

Step 1. Pour undiluted white distilled vinegar into a spray bottle.

Step 2. Liberally spray the vinegar over the patches of growth and allow it to sit for at least 20 minutes to allow the acid to break the fungi down.

Step 3. Using a dry, clean cloth, wipe the area clean of any residue. The smell of vinegar will dissipate over a few hours, but if you find it unpleasant, you can clean the area with warm water and detergent.

Step 4. Whether you have used detergent to remove the scent or not, try to ensure the area dries as quickly as possible. Additional moisture being applied to the walls can worsen an existing mold issue if left, so during winter times, turning on the heat or turning on fans to increase airflow during the summer will speed this up.

How to prevent mold

Preventing mold may be easier said than done under some circumstances, but there are certain things you can do to limit the potential for its growth.

Fungi require moisture, nutrients, and oxygen in order to live, it also prefers to be out of direct sunlight as the UV rays the sun emits will sterilize them.

So, in order to prevent it, you need to keep moisture levels within your property below 55% and ensure it is kept as clean as possible, with dust and debris being removed as quickly as possible as this is what fungi use as a source of nutrition.

Allowing plenty of airflow into each room of a property will prevent moisture buildup and condensation formation, whilst opening blinds and curtains will allow natural sunlight to spread through rooms, making it a less hospitable environment for fungal growth.

When should a professional be called?

Small patches of growth can be tackled safely with home remediation techniques (as long as the correct equipment is worn).

But for much larger patches or continued re-emergence of fungi after removal, hiring a professional may be a better option.

The reason for this is that a professional will be able to assess the current condition of the property and investigate any causes for the growth. They may for instance be able to identify areas of damp that may not be immediately obvious, then treat the growth and recommended services to correct this to prevent re-occurrence.


Some strains of mold have been used in both food and medication production for centuries, and many of our favorite produce would not be possible without it. These strains are often considered to be “good”, but in reality, all strains have the ability to produce allergic reactions in those sensitive to them, and some contain mycotoxins which if consumed can cause serious illness. It is for these reasons, that all mold growth should be treated with caution.

Chris Walker

Chris Walker has struggled for several years with mold after buying his own property. After finding the solutions to several issues around his home, he decided to create this site in order to answer as many questions about mold and mildew as possible to help others dealing with the same problems.

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