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.

Postscript:

From the abstract of an interesting investigation into the relationship between compounds lie triclosan (an antibacterial compound added to cleaning products) and antibiotic resistance in house dust-bacteria

The ubiquitous use of antimicrobial chemicals may have undesired consequences, particularly on microbes in buildings. This study shows that the taxonomy and function of microbes in indoor dust are strongly associated with antimicrobial chemicals—more so than any other feature of the buildings. Moreover, we identified links between antimicrobial chemical concentrations in dust and culturable bacteria that are cross-resistant to three clinically relevant antibiotics. These findings suggest that humans may be influencing the microbial species and genes that are found indoors through the addition and removal of particular antimicrobial chemicals.

https://msystems.asm.org/content/3/6/e00200-18

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Lessons Learned From The Sandoz Disaster

Circled in red is the site of the 1986 Sandoz disaster.
Circled in red is the site of the 1986 Sandoz disaster.

In 1986, on the border of Switzerland and Germany, 1350 tonnes of highly toxic compounds suddenly went up in flames at a warehouse belonging to Sandoz (now part of Novartis). The fire brigade responded promptly and put out the fire in about five hours. But to do so they used millions of liters of water. Due to inadequate catch basins at the factory, 20 tonnes of a pesticide-brew tagged along and flowed into the Rhine. Eventually within a couple of weeks, along a 400 km path, fish and birds were killed, and so were most of the eels in the river. The Ijssel River as far as the Netherlands was affected, even though they closed floodgates. Initially a Sandoz spokesperson had dismissed the 70-km long red slick as “a harmless dyestuff” Understandably the safety director of the company was later pelted with dead eels by protestors.

The most problematic compounds in the mixture released into the river were dinitro-orthocresol, propetamphos and parathion. Until 1991, the first compound was used as a pesticide. It’s toxic to aquatic organisms at low concentrations(0.07 to 5.7 ppm). The latter two are organophosphate pesticides, which are cholinesterase inhibitors and which are also moderately to highly toxic to fish. Parathion, specifically, is lethal after 96 hours of exposure to 50% of fish at concentrations ranging from 0.02 to 2.7 ppm, depending on the species.

As a result of the public outcry from the disaster, the Rhine Action Program came into effect in the following year. It set goals to cut 1985 discharge levels by half. It increased safety regulations for industries. Adequate catch basins had to be set up to prevent leaks into the river. Spawning grounds for salmon had to be restored in the Rhine’s tributaries with the hope of having salmon again in the river by the year 2000. Finally, shoreline ecosystems had to be revived with indigenous species. Fourteen years later, three years ahead of schedule, salmon returned to the Rhine. Nitrates and phosphate levels were cut by 50% and there was a 80 to 100% reduction in some other forms of water pollution.

A second program came into effect in 2001 when the ministers in charge of the Rhine adopted “Rhine 2020“. Here’s an outline of its main goals.

(1)The presence of salmon in the Rhine is still dependent on human intervention. One aim of the new program is to get wild salmon from the ocean to return and to increase population to self-sustaining levels.

(2) A second commendable goal is to keep improving water quality. A number of target values have been set, and currently the elements and compounds whose concentrations are still above desired levels are copper, cadmium, zinc, diurone and benzopyrene. Diurone was a mercury-based diuretic Benzopyrene is a group 1 carcinogen formed from the combustion of oil, wood and tobacco.

(3) Since lowered groundwater tables  pose a problem in parts of Moselle/Saar, the Lower Rhine and the Delta Rhine, in particular in mining zones, Rhine 2020 also aims to protect drinking water in those areas.

Here in Quebec, we have something comparable to the Rhine 2020 program known as the St. Lawrence Action Plan, but unfortunately it does not include specific goals with regard to reducing contaminants. And yet the sediments of the St.Lawrence are moderately contaminated as revealed by this 2012 map:

from http://planstlaurent.qc.ca/en/state_monitoring/overview_of_the_state_of_the_st_lawrence_2014.html#c2747
from http://planstlaurent.qc.ca/en/state_monitoring/overview_of_the_state_of_the_st_lawrence_2014.html#c2747

beluga-whale_458_600x450We also have a serious problem with belugas, whose population in the gulf of the St. Lawrence River has declined from 8000 individuals in 1920 to 886 in 2012,  Hunting of belugas was banned in 1979, yet the species continued to suffer. Although the concentration of many contaminants declined, there was a doubling of PBDE levels in male belugas between 1985 and the 2000s. PBDEs are polybrominated diphenyl ethers, compounds used as flame retardants in many household goods. A number of toxicological studies have demonstrated that exposure to PBDEs may have critical endocrine disrupting effects during fetal development of belugas. Biologists realize that a number of other stress factors are involved, many of which are, however, also caused by mismanagement of the river. For instance, toxins released by some algal blooms could very well be involved. There were also 334 spills involving ships in the St. Lawrence River between February 2002 and November 2012. Meanwhile Environment Canada is at least currently considering prohibition of PBDE compounds.

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