When describing a black or white patch developing on a wall, ceiling or floor in our building, should we say that it’s “mould” that’s causing these concerns, or is it “mildew” that does it?

We can see it’s spreading, so how alive is this stuff? Is it affecting our health? If only we had a way of recognising it and monitoring it.

Mould and mildew create an undesirable environment indoors. Their little spores are hazardous to the lungs and irritating to the eyes, nose, and throat. Additionally, the secondary metabolites that some moulds and mildews can make can mess with our brains, making us unable to concentrate. In small amounts, exposure to these may be no more harmful than normal living or breathing or eating, but in larger amounts, these organisms and the substances they create are definitely something to screw our noses up at.

So which is it that’s on my walls – mould or mildew? Most people looking at mould on a wall, may regard these two labels to be equivalent. Both terms are certainly in common use in the built environment setting. The built environment comprises all buildings of any kind including, houses, offices, schools, hospitals, warehouses, high rises, and so on. Having defined the context where we are using the term, we can then ask which of the two words, mould or mildew, correctly applies in that environment. Sure, we could look up the meaning and origin of each word to decide which one is best, but why when you can just search with Google Image Search!

The most popular Google Image Search terms (words) that follow after the word “mildew” are about buildings, while not all of the words accompanying “mould” are about buildings. The images shown upon searching with each of the two individual key words tell a similar story about the difference between these two terms. While the results of an image search for the single word “mould” will return you any number of pictures showing mould on food, and not much else, an image search for “mildew” reveals that the term mildew is used especially for the built environment.

As well as moulds growing on spoiled food, there are moulds for shaping jelly, moulds for making plastic car parts, moulds as jigs. You can have a mould for woodworking, and there are a bunch more of those kinds of moulds for shaping. Mould is in our vocabulary. The living kind of mould, on the other hand, is not related to those static pieces of hardware, and so we remember the difference, and our mind then switches meanings according to the “life” context.

It’s clear that in terms of an appropriate single word descriptor, mildew wins for the built environment. People’s common definition is what is interesting here, not what a dictionary says, and people’s clicks are what determines Google Image rank. It’s the democracy of definitions.

So how useful is this distinction? Which term is more memorable? Imagine you are a householder wanting to eliminate the mildew in their home, or that you are a professional, such as a tradesman, who the householder can call on for products or services or both, to fix mildew. Now let’s say there’s something down at the supermarket that can be used by a trained person to fix this. So you, or the tradesmen who you hired, are down at the supermarket, and you see a shelf stacker, who you want to ask for help in order to find the product you are looking for. Should you ask for something to kill mould or to kill mildew?

The word mildew is a specific word for one industry mainly. The shelf stacker may not know what you were asking them for if you ask for something to rid your place of mildew because they may be unsure of the word’s technical meaning and so they may not have stored that word in their random access memory. You may just receive a blank look! It’s worth a try to find out!  On the other hand, if you ask them for something to clean mould, they know exactly what you’re talking about. That difference in positive response is because the word mould has several meanings and several competing uses. It’s because it’s such a handy, well-used word that we regard it as a valuable one, and we remember it because it adds in many ways to our vocab.

A useful mnemonic is that “mildew” is “mould” plus “dew”. Dew is moisture. Mildew is mould that loves moisture. Dampen your indoor environment and you’ll get mildew. You can rightly call it mildew, to differentiate it from mould that grows on fruits or bread, foods which naturally contain their own moisture. Fruits and bread are not walls, so yeah, the stuff on walls deserves a different word of its own. Accordingly, over the long linguistic time of a civilisation developing, the stuff on the walls has gotten its own word: mildew.

The actual origin of the difference between the two terms is that mildew means melon-dew and comprises a living dusty discoloration found on leaves of certain plants such as melon plant leaves. It occurs in low moisture sites and so requires rain or dew to then grow. On the other hand, “to moulder” means to crumble, to turn to dust. So the concept of earth, specifically dirt, whether formed or unformed, seems to link our two current uses of the word mould. If you make a mould from mud, you can form a shape in it. If you reverse the process you get dust. So “to moulder” has come to symbolise decay, and from that we derive the word mould meaning fungus, which is found on decaying objects and has the environmental function of decaying materials.

When mould particles are found indoors in air, how alive are they? Some mould spores are “viable” meaning they can grow, while others may have missed the germination opportunity and have died. Viability is an easier idea to grab onto than naming species of mould. After all, what each spore’s biological name or label is can be complicated. Residents may not care for the level of detail involved in detecting and sorting species, and enumerating each one, and many tradesmen don’t either. Other than these people, no one else goes into the residence for any length of time, and since being inside comprises exposure to the particles, these are effectively the people who matter.

Viability, or growth potential, is determined by microbiologists, just as identification is. The viability assay is done in a lab, and involves amplifying the number of mould particles exponentially, by turning each spore into a “colony.” It requires trained people to undertake the process. They take alot of care for their own health while handling petri dishes full of the stuff. The microbiologists’ analyses of viability are achieved by counting “colony-forming units” (cfu) on petri dishes. One cfu may be an individual spore or a chain of spores, floating through the air.

Methodologies which are aimed at counting only viable spores, have been used to generate published guidelines on how many spores should be expected or tolerated indoors. Because of that method used, the amounts given in such guidelines have generally been set too low to be useful, because there are also non-viable allergens present. Furthermore, a wider application of that viability method in the built environment would require that each situation allow for costly growth assays to be conducted, and for considerable investment of time to be possible, which is not always the case.

The pass levels prescribed by spore viability guidelines have tended to be set anywhere from 30 to 3,000 colony forming units per cubic metre of air. Such guidelines generally have not taken into account that mould particles are found everywhere naturally, including in both indoor and outdoor air. The count of total coarse particles (sized 2.5 to 10 microns) in a healthy environment is often at least 10 times greater than the viable counts, the remainder being dry forms of dust, biologically and non-biologically derived.

Whether all of the particles present are viable is a moot point whenever mould actually starts to take over, because at that point the vast majority of the particles are viable. So to err on the safe side, instead of growing individual spores and spore chains into visible colonies on dishes, and then counting each as one, it can be estimated rather that every coarse particle is potentially a spore. The total number of particles is easy to measure with a meter.

In the end, it is the building owner or manager’s responsibility to monitor any mould, by means of visible occurrence, and by considering occupants’ health indications in the case of hidden mould. We simply don’t often find total coarse particle counts higher than 300,000 particles in a cubic metre of air, not indoors at least, nor even outdoors if the environment is clean and unpolluted. By the time the number of airborne particles in indoor air approaches those numbers, it’s time to investigate.