The Air We (You & I) Breathe

I've been struggling with an upper respiratory infection, with bronchitis-like symptoms, for nearly three weeks now.  I finally went to see a physician last week, who explained that my condition was quite common, particularly with the return of the dry season, and "the dust."

Things have gotten pretty dusty around here lately.  Surfaces are gritty: floors, table tops, railings.  We knew it was coming: off-shore winds transporting fine Saharan sands -- dust -- over West Africa, and the Atlantic, toward the Americas.  We'd been told that it could get pretty nasty around here, thick with haze, as a sand storm from the interior passes over us.  (Note the photos above.)

My physician speculated that "the dust" was more than just dust.  She was puzzled why minor cuts could become infected overnight during the dry season, and wondered whether pathogens might be hitchhiking along with the dust particles.

So what's in "the dust"?

There's a growing body of research on the transport of African dust, its composition, and the affects on ecosystems downwind, including human health.  Here's what we now know:

It has been estimated that 2.2 BILLION metric tons of soil and dried sediment is transported from Africa across the Atlantic each year.   The largest of the desert dust storms are capable of dispersing this "dust" across large areas of the sea and land.  Studies have shown that from February to April, the trade winds carry up 13 MILLION tons per year to the northeastern Amazon Basin alone.  These numbers have increased over the past 20 years, tied to climate change and the desertification of North Africa.  A long-term drought in the Sahel region of begun in the early 1970s, along with the drying of Lake Chad and poor land-use practices, are believed to have contributed to this increase.

About 50% of the African dust that reaches the United States affects Florida, while the rest may scatter as far northward as Maine, and as far westward as Texas.  Does Mickey know this?

Here's the rub:  it's not just dust, inert particulate matter.   One gram of desert soil may contain as many as one BILLION bacterial cells.  It has been found that the thicker the dust, the greater the concentration of microbes, and that samples collected in the Caribbean and from ship in the middle of the Atlantic match 100% with samples collected in Mali, the country immediately east of Senegal.

It is also thought that the composition of the dust has changed as a result of human activity, such as the burning of trash, the use of antibiotics, pharmaceutical and pesticides, and increased industrialization.  A few of the resulting nasties in "the dust" include polyaromatic hydrocarbons, polychlorinated biphenyls, dioxins and furans, each quite toxic.  

According to USGS microbiologist Dale Griffin, a leading voice in the field, it was originally thought that any hitchhiking microbes would be killed off by UV rays in the atmosphere, but not so, as they can be shielded in the cracks and crevasses of the dust particles.  It's also been shown that that the moderate temperatures and higher humidity over the Atlantic makes for happy and healthy pathogens.

In samples taken during an African dust event in Tampa, Florida, 6500 km from the west coast of Africa, Griffin found that 2.6 million particles were contained in one one-thousandth of a meter of air, 99% of which are small enough to penetrate deeply in the lung environment.  Of course, nose hairs and the mucus glands that line our airways are designed to block much of the larger particles.  It's the little stuff that's worrisome.  They're with us for the duration.

Areas impacted by desert dust storms, such as communities in the Caribbean, are known to have some of the highest incidences of asthma on the planet.  On the Caribbean island of Barbados, the incidence of asthma increased 17-fold between 1973 and 1996, coinciding with a period of increased transport of desert dust.  The incidence of pediatric respiratory illness on the island of Trinidad has also been linked to desert dust, which commonly contains allergens such as fungal spores, plant and grass pollen, pesticides, herbicides, industrial emissions, and toxic metals such as arsenic and mercury.

Remember that next time you're planning your Caribbean holiday.  

The transport of African dust is certainly not new --  it's been going on for hundreds of thousands of years -- and not entirely negative:  the dust fertilizes the Atlantic Ocean with nutrients, stimulating massive plankton blooms, and contributes key nutrients (especially phosphate) to the Amazon Basin.  Still, the composition of "the dust" has changed.  It has become a microbial magic carpet.  Griffin has identified of over 300 kinds of microorganisms cultured from air samples collected on St. Croix, St. John and Trinidad, during dust and non-dust conditions.  There were 2-3 times as many culturable microorganisms per volume during dusty days and non-dust conditions,  25% of which were known plant pathogens, and 10% were known opportunistic pathogens of humans.

Other consequences of African dust transport are addressed in the literature, from coral bleaching, to blooms of toxic algae (red tide), to sea fan disease throughout the Caribbean.

So here I am, coughing, dislodging phlegm, at the head winds of the African dust bowl.  Understanding why I'm coughing leads to being reminded that the Earth is a vastly complex, interactive system, for good, and for ill.

For more information on this topic, check out the following sources:

http://soundwaves.usgs.gov/2002/09/research2.html

http://svs.gsfc.nasa.gov/vis/a010000/a010000/a010008/index.html (a terrific animation)

http://www.ehponline.org/members/2002/110-2/EHP110pa80PDF.PDF

http://www.rsmas.miami.edu/divs/mac/People/Faculty/Prospero/Publications/ (a list on publications by a leading researcher in the field, Joseph Prospero, at the University of Miami)

http://discovermagazine.com/2005/mar/cover

http://coastal.er.usgs.gov/african_dust/  (a key source for USGS research, including Dale Griffin)

http://www.nasa.gov/centers/goddard/mpg/97435main_africandust.mpg  (an excellent short video)

http://www.sciencedaily.com/releases/2006/05/060526180915.htm

http://www.sciencedaily.com/releases/2001/06/010615071508.htm

http://www.sciencedaily.com/releases/2008/02/080206192436.htm

http://cmr.asm.org/cgi/reprint/20/3/459.pdf

http://nicholnl.wcp.muohio.edu/WCP221Fall2005Folder/DustsIllWinds.pdf

http://www.sgm.ac.uk/pubs/micro_today/pdf/110506.pdf

http://www.cprm.gov.br/pgagem/puerto/MedGeo.Griffin.pdf  (a terrific slide show by Dale Griffin)

http://earthobservatory.nasa.gov/Features/Dust/