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Your home is full of fungi. Mould image from

Hidden housemates: meet the moulds growing in your home

Home alone? Hardly. Our homes are positively swarming with creatures of all kinds. In our new series, we’ll be profiling the “hidden housemates” that live with us.

Our offices and homes are full of airborne spores from fungi, and for the most part we never even notice them.

Whether you like to think about it or not, you’re covered in microorganisms. Absolutely teeming with them from head to toe. Your body is covered and filled with bacteria called commensals, which inhabit the microscopic valleys of your skin and recesses of your gut. These organisms for the most part never cause you any harm, and in fact protect you from being colonised by disease-causing organisms.

In the same way that you’re a walking zoo of microbes, the world around you is peppered with invisible microorganisms.

Ancient relationship

This isn’t a new relationship though. Humans have been cohabiting with fungi for a very long time.

Ancient Egyptian bakers and brewers were harnessing natural yeasts more than 4,000 years ago, but it was only in the 1850s that we realised it was microbes that were responsible for leavening bread and making alcohol.

We’ve also known for a very long time that unpreserved foods spoil, growing conspicuously fuzzy tufts of blue and green mould. The kinds of moulds that make our bread and make forgotten oranges go fluffy are really the weeds of the fungal world.

Penicillium (this is the same fungus involved in the discovery of the first antibiotics, but that’s another story) and Aspergillus are the microscopic equivalent of soursobs and dandelions, and look fairly similar in a lot of ways.

Aspergillus niger, the fungal dandelion. Michael Taylor
Penicillium, the source of the antibiotic penicillin. Michael Taylor

Walk through any park, or into any building the world over and you’ll probably be picking up spores from Penicillium and Aspergillus; up to a several hundred per cubic metre of air is normal. In fact when you’re looking at indoor fungi, if you don’t find these two floating around you often question if you’ve taken your samples correctly.

Is your house ‘killing you’?

Indoor airborne fungi have become implicated in “sick building syndrome” and claims that our homes are “killing us”.

There is some sense mixed in with the scare here. These kinds of organisms can colonise our houses and cause serious illness but it’s unlikely that you’re in imminent danger.

Mould becomes a problem when there is moisture, or the inability for it to escape. After large rainfall or flood events, porous materials in buildings like wood, insulation, carpet and furnishings absorb a lot of water.

This water can then support the growth of fungi and fill cavities and hidden areas with very humid and stagnant air – perfect conditions for problem moulds such as Stachybotrys, the toxic black mould.

Stachybotrys, or Black mould. Unknown
If your bathroom is looking like this, you may have a problem… Black mould image from

Most of the time though the fungi that turn up after water damage shouldn’t poison you or cause infection, but will probably smell musty and cause allergy-like symptoms until the problem is fixed.

In many cases fixing the root cause may be relatively simple, with the first step always being to ensure that whatever caused the water to accumulate is fixed and any excess moisture is dried out. Non-porous surfaces are often simply able to be wiped clean of all visible mould with a detergent or cleaning spray.

Soft furnishings, clothes and carpets should be thoroughly vacuumed and washed if possible, or thrown out if extensively contaminated. Porous surfaces are increasingly more difficult as wiping the surface clean may not actually remove the mould and will likely need to be replaced to fully solve the problem. Extensively damaged homes after a flood may be beyond remediation, and any clean-up operations on this scale should always involve a professional.

But it’s not just leaky roofs that encourage fungi to come indoors though, our push towards ultra-efficient green buildings can cause similar problems.

To reduce energy costs, we often design our air-conditioning systems to recycle as much of the indoor air as possible, which over the course of the day can slowly push up carbon dioxide and moisture in the air.

If this isn’t removed, it can leave you feeling sleepy and the air feeling heavy whilst providing an opportunity for fungi to take over.

The fungal garden in your home

We’re often told to aim for a lifestyle with “balance”. The same is true for our microscopic housemates.

If you end up with one single species dominating you may have a problem. On the other hand a mixture of species shows that everything is relatively in order and is an indicator of a healthy environment.

The mixture of airborne fungi does change from place to place, but not as dramatically as you’d expect. The same specimens tend to turn up the world over: Penicillium, Aspergillus and Cladosporium, alongside a handful of other common fungi.

If you live near agricultural pastures, you may find a greater abundance of plant pathogens like Alternaria, Stemphylium and Fusarium. The species may change if you’re in different regions of the world, but overall your lungs probably contain similar spores to your relatives in Spain or Japan.

If you live in California’s San Joaquin Valley, however, you are in the unlucky position of being tens of thousands of times more likely to be exposed to infectious spores from the fungi Coccidioides immitis, which cause the otherwise relatively rare condition of fungal pneumonia.

But if it makes you uncomfortable to think about the invisible world pulsing with life around you, relax. Generally a healthy mixture of fungi can indicate a healthy home, and I promise you that life is better with fungi in it than without.

Are you a researcher with an idea for a “hidden housemates” story? Get in touch.

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