“Household dust is made up of 95% human skin.”
In various places throughout the discworld books, Terry Pratchett makes reference to “What everybody knows,” Things that are so obvious or so well known, that no one even considers that they might be wrong. Such as Witches wear black pointy hats for instance.
Except that household dust is not made up of 95% human skin, nor 80% not 70%. It is in actual fact, rather difficult to determine what household dust is made up of, and it can vary enormously depending on location and lifestyle.
This topic was addressed by the wonderful, informative and thankfully very funny, BBC TV show hosted by Steven Fry, QI Fortunately for me I can save writing time by quoting the script note formulated on the QI Talk Forum, link provided below:
If memory serves this script note was used in the show.
“The composition of house dust is studied because of its rôle in allergies. However, it’s difficult to get meaningful data, in part because dust varies so widely from country to country, house to house, and even room to room, as well as varying by season and in response to lifestyles (whether you have a pet, how often you clean, whether you open the windows, etc). What is clear from a review of the studies that have been done, however, is that the often-quoted figure of 70% human skin is a) a complete invention and b) wildly exaggerated.
The components of house dust include: animal and human skin (‘dander’), soil, sand, pollen, clothing and carpet fibres, soot, cigarette and wood smoke, brick and concrete dust, dust mites, bits of dead insects, rat and mouse droppings, industrial pollutants, styrene, pesticides, tiny bits of plastic, PVC and vinyl, flour, crumbs, hair, fungal spores and plant parts. A US study released in March 2005 found seven groups of toxic chemicals in the dust in vacuum cleaners, 90% by concentration being phthalates, which are used in the manufacture of PVC / vinyl.
Much of the dead skin shed by humans is actually drained away in bathwater.
There’s a story promulgated by mattress manufacturers (particularly in the USA) that as much as half the weight of a mattress can be made up of dead mites and their detritus; this isn’t true either.”
All of which reinforces my lifelong committment to be cautious about ‘what everybody knows,’ which has caused all sorts of unnecessary problems throughout human history. (‘What every body knows,’ having caused the trouble of course, not my committment to caution, I have an ego, but it’s not that big.)