The Science of House Dust

 

Let’s venture into the science of house dust so that, at the very least, we will make the act of dusting seem far less prosaic. Since 40 000 tons of space dust falls on our planet’s surface, some of it makes up the dust in our homes along with Earth’s own stony fragments and soil minerals. The powdery residue on forgotten books and collectibles, along with the dust bunnies hiding under the bed, also contain a heterogeneous assortment of construction materials, synthetic fibers, skin flakes, hair, insect and plant debris, dust mites, fungi, bacteria and fragments of household items.

The bacteria and fungi are present because of dust’s organic matter. The type of fungi in dust is largely determined by where one lives. In contrast, the variety of bacteria depends on who lives inside the house of dust.  Nondiphtheiroid Corynebacterium prefer the skin of men over that of women, so the amount of that bacteria reflects the ratio of men to women in a home.

Corynebacteriumdiphteriae
Households with more men have dust with higher concentrations of Corynebacterium bacteria.
Lactobacillus sp 01.png
Lactobacillus, mostly from vaginal walls, is found in higher concentrations in dust from households that have more women.

Thanks to their saliva and feces, pets in the home also influence what bacterial species thrive in the dust. We are all too aware of a city’s compromised variety of animals. Similarly, microorganisms in dust from urban areas shows less biodiversity than those from rural environments. Of interest to housekeepers is that bacteria can make cleaning more difficult, especially after weeks or months of neglect. With time, bacteria in dust secrete sticky exopolymers, causing the dust to stick to the surface. Exopolymers are part of the biofilm that help bacteria survive by anchoring them, and the thicker the film becomes, the more difficult it is to scrub off.  Humidity and evaporation in the home also lead to cycles of expansion, contraction and crystallization. These processes help dust particles penetrate into imperfections of wood and other receptacles.

Fungi in dust can concentrate metals. The metallic elements can originate from outdoor minerals, or from within the home itself. Paint, for example, especially if old layers from less regulated eras are exposed during renovations, can make significant contributions. One study of 50 residences located in 10 neighborhoods across the city of Ottawa, Canada revealed that the multi-element profiles of indoor dust differs from dust of surrounding streets and yards. For example home dust had an average of 222  ppm of lead versus 33 and 34 ppm for street and soil dust, respectively.  The contents of vacuum cleaner bags had 1.6 ppm of mercury versus 0.018  and 0.048 ppm in street and soil dust. There was 4 ppm of arsenic in house dust versus an average of 1.7 ppm for outdoor sources.

Tris(2,3-dibromopropyl) phosphate.svg
tris(2,3-dibromopropyl) phosphate

In general, dust receives a variety of compounds from consumer goods brought into homes. These substances come from multiple classes of chemicals: phthalates which occur in the highest concentrations, followed by phenols, resorcinol-formaldehyde resins , fragrances, and polyfluoroalkyl substances (PFAS). In the dust of 75% of the homes in California, there was TDBPP (tris(2,3-dibromopropyl) phosphate), a compound once widely used as a flame retardant in plastics and textiles. The use of this group 2A carcinogen was banned from children’s sleepwear in 1977.

330px-Diisononyl_phthalate
di-isononyl phthalate. Compared to that of  adults, children’s urine has a concentration that is 5 times higher.

Critics of such studies invariably point out that many compounds are detected only because of extremely sensitive instrumentation which picks up concentrations of 1 to 100 parts per billion, the upper range being that of the phtalates. They also point out that even though di-isononyl phthalate can lead to male genital birth defects, the effect is very concentration-dependent. What the non-precautionary attitude overlooks is that the phtalates are only one small part of the environmental soup. In addition to dealing with natural plant secondary metabolites and geological minerals (some of which we have evolved protective defenses against), our bodies must now cope with a host of other compounds from work and home environments, from air, food, pharmaceuticals, drinks and water. We can’t look at each contaminant in isolation and comfort ourselves with the small concentration each one contributes. Nor can we forget that all of the compounds have appeared in under 20 seconds relative to the total time of mammalian evolution. It is more sensible to be concerned about the likely significant sum of seemingly small effects. It is advisable to stop synthesizing the more pernicious compounds entirely and to reduce exposure to the other necessary evils.

Meanwhile let’s make an effort to keep the chemistry in our homes simpler and to dust frequently.

Sources:

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