Moulds in Homes, Schools and Workplaces

The so-called "toxic" moulds have had a lot of publicity over the last several years because of their reported occurrence in school portable classrooms. There has been a lot of money spent by many school boards to address the concerns of   parents and others who believe that school portables are not a healthy environment for their progeny.  That part is probably true but I don’t think, that the "toxic" moulds are nearly as big a problem as they are made out to be although they are certainly an excellent indicator of poorly designed and unhealthy facilities.

Biology of Moulds

Firstly lets say a little about the factors that influence mould growth and why fungi grow so readily in homes and other buildings in Ontario.

Factors Essential for Growth of Fungi

There are certain essentials  necessary for moulds to grow.

1. Suitable temperature.

2. Availability of water.

3. Organic substrate for energy.

4. Presence of oxygen for respiration.


Most fungi grow well in the temperature range 10-30C.  Mould growth will slow down above and below this range and eventually growth will cease for most fungi below 5C or above 35C.  A few fungi however have adapted to either very low temperatures (cryophiles) or very high temperatures (thermophiles).

Low Temperature Fungi:

Some fungi grow best at low temperatures and can make slow but inexorable growth at refrigeration temperatures (about 5C).  A few fungi can even grow below 0C.

You will probably have already noticed that if you leave foodstuffs for a prolonged period in your refrigerator they will be overrun sooner or later by fungi particularly species of the blue mould Penicillium. It usually takes a week or two for the colonies to be big enough to see and to turn blue.  The blue colour of Penicillium is due to a pigment in the spores.  So, if the colonies are blue you are looking at hundreds of thousands and possibly millions of microscopic spores that will fly around the house at the slightest disturbance (click here for spore heads of Penicillium).

Penicillium is a common allergy inducing mould and so its release into the atmosphere as floating spores (bioaerosols) could cause problems for sensitized individuals.

High Temperature Fungi:

Some fungi favour high temperatures and can grow very well at 37C (blood temperature) and up.  Some of these high temperature moulds can grow at 50C or higher.

Many of the fungi that attack humans and other mammals and/or birds are able to grow at high temperatures. Aspergillus fumigatus causes a pulmonary infection in birds (turkeys, penguins etc.) and has been known to affect lungs in horses and cause eye infections in humans.  It has an optimum growth temperature of 37C and can grow up to 50C.

True Cryophilic Fungi or Thermophilic fungi do not grow well in the normal temperature range.


Most fungi require free water available before they can grow. No water - no growth. There are a few fungi, however, that can pluck water vapour molecules out of humid air to make their growth.  A good trick!  Moulds that do not require free water are sometimes called xerophilic fungi.

Xerophilic Fungi:

Xerophilic fungi can grow if the air is above 60-65% Relative Hunidity.    Rate of growth of these fungi will increase as the relative humidity increases. With the exception of some specialized plant pathogenic fungi none of the Xerophilic fungi grow below 60% RH.

In basements in homes in Ontario during the summer the relative humidity can be considerably higher than 60%.  As a result fungi will grow on any suitable organic material stored in basements e.g. clothes, boxes, leather goods or even on the walls. This gives basements their characteristic musty odour. In damp basements, you may well have hundreds of millions of spores growing on stored stuff. In the fall when the furnace starts up and the hot air system kicks in these spores will be blown all over the house and sensitive individuals can have serious problems. The common Xerophilic fungi belong to the genera Aspergillus and Penicillium. The spores of these fungi are very small (only a few microns) and are inhaled to reach the tiniest alveoli of the lungs to cause problems.

If someone in your home has mould allergies, it is important to start off right away with a dehumidifier in the basement. If you can keep the relative humidity in the basement below 60% (dehumidifier in the summer) NO XEROPHILIC MOULDS WILL GROW IN YOUR BASEMENT IN THE SUMMER.

Free Water:

Bathrooms are one of the commonest spots in the home for free water. During showers or baths the water vapour condenses on the colder spots in the room and ceilings, walls, windows etc. may be running with water.  Water often collects and persists around the edge of the bath.

This results in mould growth in many places in the bathroom.  The fungus Cladosporium is possibly the most commonly observed household mould.  It likes free water at room (ambient) temperature.   The caulking around the edge of the tub becomes stained brownish to olive-brown or blackish.  This indicates growth and fruiting of  Cladosporium. Cladosporium will also cause spotting on ceilings.   Cladosporium will only grow where there is free water but in bathrooms this is a common occurrence.

In very cold weather condensate also occurs on the inside of windows in bedrooms and other rooms in the home. Water runs down the window and soaks the wooden frames.   Mould growth of Cladosporium here will appear a olive-black staining of the wood and in time will rot the frame of pave the way for other wood rotting fungi.

Cladosporium is not only a very common household mould it is one of the fungi known to be a major cause allergies in humans.


1. The blue mould Penicillium commonly grows on food in refrigerators at 5C. It will also grow on bread kept in plastic bags, at room temperature, so the relative humidity stays high.  Penicillium is an allergy inducing mould.

2. Aspergillus commonly grows on organic materials (clothes, cardboard, leather goods etc) stored in the basement at high %RH even if no free water is available.   Aspergillus is an allergy inducing mould.

3. Cladosporium requires free water and commonly grows in bathrooms, shower stalls and on wooden windows where condensate has soaked the wooden frame. It does not grow at low temperatures so is not a problem in refrigerators (unless there is a power cut and then watch out!).  Cladosporium is an allergy inducing mould.

Organic Substrate (Food Source):

Fungi require already elaborated organic materials for energy (food). Some fungi prefer fruits (apples, oranges) others break down keratin and grow on leather goods, feathers etc. Many fungi use cellulose as their primary food source and produce potent cellulases to break down cellulose to glucose units which are then absorbed by the fungal threads (hyphae). The infamous toxic mould’ Stachybotrys chartarum (= Stachybotrys atra) is a very aggressive colonizer of cellulosic materials but only IF FREE WATER IS AVAILABLE.

Thus, the solution to the Stachybotrys problem is just design a portable well enough that condensation (free water) is not available on any surface composed of or containing natural cellulose fibres. Perhaps easier said than done! More on this later!

Special Situations in the Home

Concrete Pads

Most homes in our area have basements. For some homes this is not true and the houses are essentially built on a concrete slab at more or less ground level. Or sometimes, on sloping lots, basement walkouts are at ground level. In the springtime these concrete slabs and the ground below them hold the cold. If there is a gradual warming up over time then everything is OK.  If, however, a warm humid front comes up rapidly from the south and there is a high ambient temperature at high humidity then there a rapid and excessive condensation on these concrete slabs.  I have been in homes where you can actually splash your way across the carpet.  The floor is virtually swimming in water.   I have seen colonies of fungi a foot across lacing their way through the carpet fibres and  sporulating vigorously as they go.  The solution?   Soak up the water as quickly as possible with a wet/dry vacuum, newspapers etc. Turn up the furnace to provide hot dry air, and get a dehumidifier or two going.    Do all of this as quickly as possible.  There is a lag time before fungi really get going so if you stop them in the first 72 hours you can save yourself a lot of grief.

Basement Apartments

Because of the inherent high humidity in basements in summer it is essential to be aware of potential mould hazards.  I had a student from Waterloo who was concerned about moulds in her apartment, for which she had signed a lease.  She was especially concerned about Stachybotrys which she had read about in the local papers.   I asked her to take  a six inch strip of clear sticky tape and stick it to the wall where she could see moulds.  Repeat this at different locations with six pieces of tape.  Label the source e.g. bathroom, bedroom,  outside wall, inside wall etc.  The student brought back six pieces of tape and I examined them microscopically.  I was honestly amazed at the amount of mould on the tapes and one was from the wall above the headboard of her bed!  Moulds included Aspergillus, Penicillium, Cladosporium and Alternaria - all known allergenic fungi.  I wrote a letter for the young lady which supported the breaking of her lease for health concerns.

The problem is that once you have this spore load in the home, it's hard to get rid off.  A vaccum cleaner will suck the spores up at one end and blow them out the other.  The spores are so tiny they won't be effectively filtered out.   Also you can circulate them as part of the house dust for years.   A dead spore will still have the surface proteins that cause allergies.  So what do you do?   A central vacuum system has some advantages here.  The power unit is often outside the home e.g. in the garage.  Thus, all the stuff sucked up inside the house is eventually transported outside the house and sooner or later you'll get ahead.  Also, buy yourself a dehumidifier for the summer months.  No problem in the winter.  As most of us know the furnace dries out the air so that we need a humidifier to bring it up to  30-40%.  Too low for mould growth!

So what about School Portables and the toxic mould Stachybotrys?

I’ll get back to you on this soon!