Why is house dust always gray?

Why is house dust always gray?

The dust on cars and garden furniture often has colors that you can easily trace: yellow during the time of abundant pollen, red-brown after dust storms from the Sahara, sometimes black in industrial areas. But the dust in our homes is often gray, from the thin layer on photo frames to the airy dust mice under your bed. (In Norwegian they are called hybelkaniner: student house bunnies. The English call them dust bunnies.) What gives all that dust its gray color?

A microscope shows that the dust is not uniformly gray at all. It consists of all kinds of particles, each with its own color: yellow pollen; brown organic and inorganic material from outside; and fibers from clothing and furniture in all colors of the rainbow. Plus a lot of gray stuff from very close sources: dead skin flakes from the human and animal inhabitants of the house. These flakes consist of keratin (horn dust) and are ashen, just like calluses and cut nails. But is that enough gray to make all the dust look gray?

Opinions differ about the share of dead skin cells in house dust. Percentages of up to 80 to 90 percent circulate on the internet, but there is no clear source. The underlying question of how many skin cells we actually lose is also divisive. That is therefore very difficult to measure.

Vlogger Derek Muller, physicist, found out in a video on the website Nerdist. On average, we lose about 480 million dead skin cells every day, he concluded from books and articles. That is as many stars per year as there are stars in the Milky Way – together about half a kilo. Of course, not all of them end up in your house: some of them are lost outside the home, some of them are washed away with shower and washing machine water. But Muller still arrives at the conclusion that half of the house dust consists of dead skin cells.

But what about the other half? An argument on the forum MadSci Network brings in a physical phenomenon: perfect light scattering. The house dust particles are up to a few hundred micrometers (thousandths of a millimeter) in size – exactly the size of the tiny floating water droplets in the atmosphere that together form clouds. Clouds are not white: they are transparent because they consist of water, but we see them as white because they scatter all incoming white sunlight. The same goes for milk: a suspension of tiny fat and protein globules that scatter all the white ambient light. Physicists speak of Mie scattering, named after physicist Gustav Mie (1868-1957).

Does house dust also contribute to Mie scattering? We call Willem Vos, professor of applied nanophotonics at the University of Twente. “We only use the term Mie scattering for that specific phenomenon with spheres,” he says, “where the scattered light shows very characteristic phenomena, such as certain resonances. Gustav Mie described this brilliantly in 1906. It’s a shame that he didn’t get a Nobel Prize for it.”

House dust particles are far from spherical. They therefore do not scatter the light as structured as cloud droplets. They absorb some of it. Of all colors, that is, because the fabric consists of all kinds of materials, including all those colorful fibers. And part of the light of all colors is returned and ends up on our retina. “Something appears white if it scatters all colours,” Vos summarizes, “black if it absorbs all colours, and gray if it partly scatters and partly absorbs all colours.”

Exactly what you would expect from such a colorful, dusty mixture.