After months of heavy rainfall and increased humidity along Australia's east coast, many people are now dealing with an unwelcome and often stubborn house guest: mould.
A type of fungus, mould is present almost everywhere. It lives on plant and animal matter and plays an important role in our ecosystem.
But mould growth inside the home, often brought on by flood damage, plumbing leaks or damp and humid conditions, can be damaging to our health, especially when left unchecked.
"There's always mould in every house, but it needs dampness to grow, and there's a lot [of dampness] at the moment because of the floods and rain," said Holly Jones, a researcher in the healthy housing unit at Melbourne University's school of population and global health.
"What causes illness is certain species of mould being more concentrated … so by keeping your house dry, you're keeping that in check."
Mould can affect everything from clothing to furniture, and is often noticeable as a stain, smudge or discoloured patch.
But it can also be harder to find — behind furniture, at the back of wardrobes, and even inside wall cavities, says architectural scientist Tim Law of Victoria University.
"The problem is mould is not always visible: it tends to grow in the dark, interstitial spaces of a building, so you may not be able to see it," said Dr Law, who researches how the microbiology of buildings can affect human health.
"You might be able to smell it, but otherwise, it's usually quite hard to detect."
What are the health risks of mould?
In order to reproduce, mould spawns tiny particles called spores which are carried in the air.
Sometimes, these can trigger allergy and respiratory symptoms such as nasal congestion, wheezing, watery or itchy eyes, coughing, and respiratory infections. Mould can also worsen asthma and allergic conditions.
"All of our organs and surfaces have barriers, and mould is something that can irritate those barriers and open up that junction," immunologist and allergy specialist Sheryl Van Nunen said.
For most people, exposure to a very small amount of mould does not have serious health consequences. But how you respond depends on your genetic makeup, underlying health, and how sensitive you are to mould.
Professor Van Nunen, who is also a spokesperson for the National Asthma Council, says approximately 40 per cent of Australians are atopic, meaning they have "an inherent ability to be allergic".
People with allergies, asthma, and other respiratory conditions are more vulnerable to the health effects of mould, as are infants, children and older people.
"The thing that concerns me particularly is children … asthma has been clearly established as being associated with visible mould and mould odour," Professor Van Nunen said.
"One of the other really important findings is that if a pregnant woman is living in a mouldy home, [her] child has an increased risk of asthma."
People with weakened immune systems — such as cancer patients undergoing treatment or organ transplant recipients — are also at risk of more serious complications, including rare but severe mould infections in the lungs.
Dr Law stressed that in large quantities, mould spores can cause ill health in almost anyone.
"At some point, you can have enough mould in the building so that nobody is going to remain healthy under those conditions," he said.
What about other physical and cognitive effects?
In addition to established respiratory symptoms, mould exposure has also been linked to a biotoxin-related illness known as Chronic Inflammatory Response Syndrome, or CIRS.
CIRS is a progressive multi-system illness characterised by exposure to biotoxins, Dr Law said.
Symptoms are reported to include headaches, chronic fatigue, mood swings, allergies and night sweats.
"For a lot of people who have suffered this condition, it took them a long time to make the connection between their condition and the building [they inhabited]," Dr Law said.
"One of the clearest indicators is if people go on holiday and live in a different environment and then suddenly feel very refreshed, and when they come back to their house or office, they feel it all over again."
But CIRS is not a widely recognised medical condition, and many doctors treat it with "a certain amount of suspicion", Dr Law says.
A 2018 parliamentary inquiry into CIRS called for more research into the potential health effects of mould, after medical experts noted a lack of evidence showing a causal relationship between "exposure to biotoxins and unexplained complex symptoms".
Ms Jones said one of the key barriers to understanding CIRS was the difficulty researchers faced when trying to measure mould exposure.
"It's not like doing a blood test — [the amount of mould is] going to be different based on the season, who lives in the house, whether there is a cat or dog, whether someone's been for a walk in nature in the last week," she said.
"So that's what I'm focusing on, because once you can measure how much people are exposed … you can see whether there is causal evidence."
Dr Law is also part of a research team funded by the National Health and Medical Research Council (NHMRC) investigating whether there are clear biomarkers associated with CIRS.
"We hope to really gain some traction with the NHMRC funding and work out a way to screen for it."
Removing mould should be a priority, experts say
While some of the health impacts of mould exposure are still to be determined, Ms Jones says it's widely recognised that poor housing quality, including mouldy and damp living conditions, increases the risk of poor health outcomes.
The World Health Organization has previously estimated that mould affects between 10 and 50 per cent of homes in Australia. Research suggests these are more likely to be in low-income communities.
Well-designed and well-maintained homes are critical to the prevention of mould, but Dr Law says a lack of agreed exposure standards means measures to ensure adequate ventilation and to limit condensation aren't always enforced.
"Very often, these things are not dealt with, and the occupant is the one that bears the consequences of mould and so forth," he said.
"Given how prevalent mould already is … we really need to look at this more carefully."
Where possible, it's important people remove any mould growth in their home, and take steps to prevent it from growing back, Ms Jones says.
"The number one thing is to keep your house dry: prevent moisture in any way you can … that could be with dehumidifiers, or with ventilation.
"And check the tenancy regulations in your state if you're a renter.
"They vary state by state, but a few states do say if it's a structural problem that causes mould, it's the landlord's responsibility to fix it."
How do I get rid of mould?
In addition to reducing the amount of moisture, Professor Van Nunen recommended people use a vinegar solution to remove mould instead of bleach.
"In most cases, if you use bleach, a background level of mould spores will remain and you're not really getting rid of it. Whereas vinegar will do it."
It's also important to remember that in some cases, when the mould is widespread or your health is being compromised, it may be necessary to bring in professional help.
The ABC has more detailed advice on how to remove mould here and here.
