POSTED 7 APRIL 2005
The biology of dust
What's in the air? You'd think we'd know by now, after throwing umpteen bezillion
bucks into watching the atmosphere from Earth, space, and all points in between.
Now comes Ruprecht Jaenicke of the University of Mainz, Germany, telling us something
new about the particles we breathe every day.
Until now, scientists thought nearly all the airborne particles were mineral.
According to the International Panel on Climate 
Change (2001 report), about
2 billion metric tons of dust blow off deserts and degraded land each year;
about 3.3 billion tons of salt blows off the oceans; and 316 million tons comes
from various human-made sources like smokestacks. And a puny 1 percent of the
atmospheric particles were thought to have biological origin.
That, at least, was the conventional wisdom. Now comes Jaenicke, claiming that as many as a quarter of all particles in the air were made by life -- various cells, pollen, even dandruff of giraffe, for all we know. That's a big difference -- enough to make even The Why Files take notice. Are we really breathing so much biological junk, and is that healthy? Could bio-particles affect how the atmosphere works?
How he done it
To find out exactly what's in the atmosphere, Jaenicke collected particles less than 10 microns in diameter from various locations and altitudes. His article in Science last week (see "Abundance of Cellular ..." in the bibliography) includes data from Germany and Lake Baikal, Russia, although he's also collected crud over the South Atlantic Ocean, a Swiss mountaintop, and the Amazon forest.
After separating the particles in the lab, Jaenicke stains them with a blue dye that gloms onto the protein that most cells contain. He may also look for the peculiar shapes of biological particles, or blast the samples with an electron beam, which destroys biological, but not mineral, material.
Under a scanning electron microscope, bio-particles can
be spotted based on their shape. X-ray analysis of elements in the particles
can help pin down a biological origin. Courtesy Ruprecht Jaenicke
The result could be considered the first survey of life aloft -- although much of this life was already dead when collected. The biological components of the atmosphere, Jaenicke wrote, "include fur fibers, dandruff, skin fragments, plant fragments, pollen, spores, bacteria, algae, fungi, viruses, protein crystals, and more, ranging in size from tens of nanometers to millimeters."
Overall, he says, by mass and by the number, biology accounts for 20 percent to 25 percent of the fine particles in the atmosphere, with individual monthly readings ranging from 19 percent over the South Atlantic to 80 percent over Lake Baikal. Unexpectedly, the proportion of biological material is not higher in summer, although levels do meander quite a bit from month to month.
Dust is a must!
Just as the biological origin of so many particles flouts conventional wisdom,
so does Jaenicke's take on dust in general. 
You and I may think dust is a bad
thing, but he doesn't think particles are inevitably pollution. "Particles are needed for life, for rain, for clouds," he says. Water vapor has difficulty forming the tiny water droplets that make up clouds unless they can gather on a nucleus of dust. Similarly, rain at middle latitudes occurs when water droplets gather on ice nuclei -- and ice nuclei form most easily on certain dust particles.
Dust in the air -- from all sources -- makes sunsets red by blocking out blue light. Photo: NOAA
And while atmospheric scientists generally figure these nuclei are made of minerals, Jaenicke has shown that clouds and ice can form on pollen. (What about snowflakes?)
Don't ask Jaenicke to talk dirt about dust. Dust, he argues, helps fertilize the wild lands of the world. "The Saharan dust clouds you often see going over the Atlantic are the nutrition of the Brazilian tropical forest; if there's no desert, there's no tropical forest." Dust carries nitrogen and phosphorus to the nutrient-poor soils of the Amazon forest.
Click
to see a movie of global dust movement in 2001. Giant dust storms carry fertilizing
compounds for long distances. Use the arrow keys to view one month at a time.
In January, immense amounts of dust left West Africa for South America. In
April, a giant dust cloud from Asia jumped the Pacific to North America. 180KB
Movie: NASA Earth
Observatory
Particles also determine whether incoming sunlight is absorbed or reflected back to space, Jaenicke points out, and help set the course of global warming.
Atmospheric scientists have admitted that many airborne particles are unidentified, Jaenicke says. "They are seeing 60 percent of the picture -- perhaps more in a polluted city and less in other places. It's not uncommon to accept that 40 percent of the mass is chemically unknown."
Biological particles collected from the air are stained
blue to distinguish them from mineral dust. You are looking through a light
microscope at particles less than 2 microns across. Courtesy
Matthias-Maser & Ruprecht
Jaenicke
Here's to your health
What are the health considerations of all this flying bio-debris? Particle pollution is linked to high death rates, and as we'll see later in this story, pollen and mold trigger allergies and asthma.
But if we don't know exactly what's in the air, it's hard to pinpoint the health effects.
Tiina Reponen, an associate professor of environmental health at the University of Cincinnati, says her studies of indoor air confirm that "we have a lot of previously unaccounted-for material that is of biological origin in the air." The exact health effects, she says, are uncertain, but she hopes Jaenicke's work will stimulate a more comprehensive look at particles and health. Presently, she says, scientists are split. "Epidemiological studies have so far focused on fungal spores, or on pollen, or on particulate mass, but almost nothing is known" about combined exposures. Jaenicke's work, she says, "is looking beyond counting pollen."
Do hog farms put antibiotic resistant bacteria in the air?
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive
