Kitchens are dangerous places – apart from the risk of both sharp and blunt instrument trauma there’s a host of foodborne bugs that can screw up your day. But did you know your dishwasher is trying to kill you too? One doing the rounds again on social media and – naturally – done to death in the Daily Mail is that there are deadly, toxic fungi in your dishwasher that are potentially fatal. But, provided you scrub the inside with vinegar or baking soda you’ll be fine.
Not More Bloody Toxins…
This latest ‘scare’ concerns two species of “black yeast”, Exophiala dermatitidis and E. phaeomuriformis. These, as well as E. jeanselmei and E. spinifera have been associated with skin conditions including Madura Foot and have been known to colonise the lungs of cystic fibrosis patients, for example. Exophiala is also one of a bunch of organisms that can cause hypersensitivity pneumonitis, aka saxophone lung.
So, Exophiala not unknown as an agent of disease but it’s a black yeast – a toxic mould – so it must be bad, right? It certainly sounds pretty icky. And in this latest attempt to reanimate a paper dating back to 2011, the release ends:
“The discovery of this widespread presence of extremophilic fungi in some of our common household appliances suggests that these organisms have embarked on an extraordinary evolutionary process that could pose a significant risk to human health in the future.”
Except it suggests no such thing. That interpretation is just Utter, Utter Bollocks (µ²B) and clearly written by someone who understands neither evolution nor basic mycology. And I slept through most of Prof Butler’s mycology subsid almost 30 years ago; fungal taxonomy and life cycles had me contemplating self-harm and I’ve always been a pro-prokaryota kinda guy.
The most common environmental moulds are Cladosporium, Penicillium, Aspergillus, and Alternaria and – depending on where you are and the time of year – there can be tens of thousands of fungal spores in every cubic centimetre of air you breathe. There are always some present in the air we breathe (indoors and out) but usually there are loads. And at rest we breathe in about six litres of air a minute – so that’s tens of millions of fungal spores we breathe in every minute of every day. Which is quite a lot. Multiply it up. You get a very big daily number very quickly. (Also consider the number of allergens that’s presenting your immune system which it brushes off without a murmur – and some people think a couple of dozen over many years in the vaccination schedule is ‘too many, too soon’).
Yes, spores can cause allergic reactions, they can grow, they can produce toxins, they can be fatal… And just look at a loaf of bread – it seems fine when you go to bed but in the morning it’s transformed into a bag of mouldy ick due to those pesky fungi getting jiggy while you sleep.
But despite this constant and prolific exposure we don’t get sick. This is for a number of reasons, the main one being our immune system making pretty short work of them what we inhale. Second, one of the benefits of being warm blooded is we are far less susceptible to mould infections. In general moulds prefer cooler temperatures – 25ºC or less – which is why reptiles, fish and insects are far more vulnerable to these infections than we are.
But Exophiala can tolerate extremes of temperature and pH for short times and a dishwasher can get pretty extreme; the detergents used are highly alkaline with a pH of 12 or more, rinse aids contain alcohol and the rinse cycle can reach 80ºC – indeed this is a legal requirement in commercial machines to ensure thermal disinfection of foodborne bugs. In tests Exophiala survived 47ºC, a pH range between 2.5 and 12.5 and 17% NaCl salinity. That’s pretty ‘kin hardcore for a bug to survive in.
While it’s true Exophiala is found colonising cystic fibrosis patients and in dishwashers there is no compelling evidence this presents any risk whatsoever for most of us to be worried about, given the above and the ubiquity of fungi everywhere else in the domestic environment – not just in dishwashers. The conditions in a dishwasher will kill just about anything except some spores which is why our Microbial Overlords were mostly found in the rubber seals in the studies cited above. Incidentally, the pink stuff you sometimes see isn’t limescale; it’s Serratia marcescens, one of my favourite bugs. But that’s for another post.
Aaaargh! Toxins! (Again)
The other ‘toxic’ mould people bang on about is Stachybotrys which has been blamed for all sorts of non-specific symptoms and many claim an association with ‘sick building syndrome’. Yet there is no plausible mechanism for its toxin getting aerosolised and no convincing data to back up the scary claims people make of it. If you’re immunosuppressed or have a chronic respiratory illness that’s different – and there are allergic and invasive diseases caused by moulds, but the term ‘toxic mould’ is simply not accurate. Some moulds can produce toxins but the moulds themselves are neither toxic nor poisonous. Despite some reports of toxins from ‘toxic moulds’ causing illness a causal link between the presence of any toxigenic mould and the illnesses ascribed to a supposed toxic aetiology has not been proven.
But that doesn’t give domestic appliances a clean bill of health. Washing machines (and domestic dishwashers) now frequently develop odour problems, usually due to continued low-temperature wash cycles. Here’s why.
Low Temperature Cycles
If a machine is consistently run at 40°C or less you are unlikely to achieve much in the way of thermal disinfection. This is especially true of laundry machines where some domestic detergents suggest a 15°C cycle, and while this temperature may leave clothes visibly clean they will not be hygienically clean. Detergents can be a rich source of nutrients for bacteria and it is reckoned the average washing machine load contains over 100 million E. coli at any one time. If – for example – a food worker launders their uniform with the rest of the family’s dirty laundry including dirty underwear it could end up carrying all sorts of foodborne bugs. Thermally eliminating faecal bacteria requires a minimum temperature of 60°C.
Also, washing at 30°C or 40°C kills just 6 % of house dust mites, compared with 100% at 60°C – and dust mites do produce significant quantities of allergens.
Most laundry liquids – unlike powders – do not contain a bleaching agent and so are less effective at disinfection. A recent survey found living bacteria including Staphylococcus aureus and E. coli, on 83% of laundered underwear samples: of the samples tested, 89% had been washed at temperatures of 40°C or lower. While many people will be resilient to such infections those with reduced immunity, such as the elderly and patients recently discharged from hospital, will be at greater risk.
So, any machine that is only ever run at low temperatures will carry a significant load of mould and bacteria and it is recommended that a weekly or monthly very hot cycle should be carried out to thermally disinfect it.
So, don’t worry about ‘toxic’ moulds lurking in your dishwasher. The chemical and thermal landscape in the business part of the machine is incredibly inhospitable to bugs of most persuasions. While Exophiala is pretty hardy it’s not something that need concern the overwhelming majority of us. It has no form for jumping from dishwashers to the otherwise healthy.
But washing your toxic pants with the tea towels at 40ºC is a very different matter. You’re incubating poop with stuff that will come into contact with hands and other stuff involved in food prep. This is far more likely to get you into trouble with our Microbial Overlords.