Picture a juice glass sitting on a porch railing in the sunshine. It may look empty, but churning inside that glass are twenty-five thousand microscopic pieces of dust - at least. And these dusts are a little bit of everything on Earth. One minute, they are tiny crumbs chipped off Saharan sand, and invisible shreds of camel hair. Then the wind shifts, and there are spores of forest fungi and fragments of desiccated violets. A bus stops nearby to take on passengers, and invisible flakes of human skin, mixed with miniscule specks of black soot momentarily dominate the mix. Every time you inhale, thousands upon thousands of motes swirl into your body. Some lodge in the maze of you nose. Some stick to your throat. Others find sanctuary deep in your lungs. By the time you have read this far, you may have inhaled one-hundred-fifty thousand of these worldly specks - if you live in one of the cleanest corners of the planet. If you live in a more grubby region, you've probably just inhaled more than a million. Although these dusts have been waved aside for most of human history, in this book we'll see that dust is terrifically consequential. Some dusts menace the planet and its living residents. Some are beneficial to people, plants, and animals. Many are merely fascinating. All are going under the microscope. And the secret lives of dust are being revealed.

One of the most impressive revelations is how much of it surrounds us - the sheer tonnage of stuff rising off the face of the Earth. Because dust is so small and shifty, the estimates are still rough. Nonetheless, irrefutably huge amounts of small things take to the wind each year.
Between one and three billion tons of desert dust fly up into the sky every year. One billion tons would fill fourteen million box-cars, in a train that would wrap six times around the Earth's equator.
Three and a half billion tons of salt flecks rise off the oceans.
Trees and other plants exhale a billion tons of organic chemicals into the wind, perhaps one-third of which condenses into tiny, sailing beads. Plankton, volcanoes, and swamps leak twenty to thirty million tons of sulfur compounds, about half of which forms little airborne specks. Burning trees and grasses throw up six million tons of black soot. The world's glaciers slowly grind their host mountains into dust that takes to the wind - but in what quantities? No one knows.
Likewise, how many glassy bits of volcanic ash are blasted into the ether? And the dusts of life - flying fungi, viruses, diatoms, bacteria, pollen, fibers of rotting leaves, eyes of flies and legs of spiders, scales from the wings of butterflies, hair fragments from polar bears, skin flakes from elephants - how many tons of these roam the atmosphere? About four million years ago, our ancestors began to augment the dusty exhalations of Nature.
At first, we supplemented the soot, as we mastered the mesmerizing tool of fire. Then, when we learned about the miracle of metals, our smokes grew richer with microscopic beads of hot bronze, iron, copper, gold, and silver. The advent of spinning and weaving created invisible fragments of animal and plant fibers, which the wind lifted out of our encampments. Finally, with the Industrial Revolution, our dust output shifted into high gear.
Ninety to a hundred million tons of sulfur now rise annually from the world's fossil-fuel burners - mainly, coal-fired power plants, but also oil-fired plants, and diesel engines. Every natural sulfur bead in the sky is now accompanied by between three and five human-made beads. And the Earth hosts more fuel-burners every day.
More than a hundred-million tons of nitrogen oxides, which like sulfur gas is prone to form dusty particles in the sky, flow upward from our farms, automobiles and other fuel-burning inventions. Eight million tons of black soot in the sky is attributable not to burning trees and grasses, but to the conflagration of fossil fuels - especially coal. Even of the six million tons of soot that rain upward from fires, most are lofted by the fires of humanity.
Whether the skies carry one billion or three billion tons of desert dust, fully half may be our responsibility. Our agriculture and other assaults on the landscape may have doubled the amount of desert dust naturally present in the air.
And the miscellaneous dusts of the twentieth century - nerve-wracking mercury and stupefying lead, carcinogens from dioxin to PCBs, the radioactive dusts of nuclear disasters, pesticides, asbestos and poisonous smokes - how many tons of these roam the skies each year? That is undetermined.

If the quantities of dust are hard to gauge, dust scholars have an easier time pinning a size on various dusts. Generally, the dusts that whirl around us are so small that gravity has to fight to get control of them. Forces on the surface of a piece of dust - static electricity, even the interaction of one atom with another - can overpower the call of gravity. Dust can perch on the ceiling as easily as on the tabletop.
Scientists measure dust in microns, a twenty-five thousandth of an inch. Consider the hair on your arm. A single hair might be one-hundred microns wide. Now imagine taking up scissors, and snipping off a section one-hundred microns long. That tiny snippet, visible only if you know where to look for it, is too big to be dust. From a scientist's perspective, that snippet falls in the family of sand.
The very biggest grains of dust are, technically, only two-thirds as wide as a hair. These fat dusts are usually the work of Nature. The diameter of pollen grains, for instance, ranges from a full hair's width to one-tenth of a hair's width. If you sift a handful of sand from the beach or the desert, the faint powder that sticks to your palm will be a range of sizes, with lots of grains in the fatter category. The flakes of dead skin that float out through the weave of your shirt to form an invisible halo around you, are rectangles one-tenth of a hair wide, and two-tenths of a hair long. Many of the salt flecks that blow off the oceans are upward of five microns wide. And those are still some of the larger dusts. Health scientists fret more about