The Washington Post - - FRONT PAGE - BY BEN GUAR­INO More at wash­ing­ton­ news/ speak­ing-of-sci­ence

in Chicago used a novel strat­egy to de­ter­mine the ups and downs of U.S. air­pol­lu­tion lev­els over time: Mea­sur­ing soot lev­els on birds kept in mu­se­ums.

Soot on birds’ bel­lies tells a story of air pol­lu­tion more than a cen­tury in the mak­ing. Re­searchers at the Univer­sity of Chicago mea­sured black car­bon that clung to birds kept in Rust Belt mu­seum col­lec­tions and found a strik­ing record of filthy air.

Over the course of 135 years, the dirt­i­ness of the birds’ plumage rose and fell in line with coal leg­is­la­tion and fol­lowed so­cial changes such as the switch in res­i­den­tial heat­ing from coal to nat­u­ral gas.

The soiled bird spec­i­mens also sug­gest that the air above Chicago, Pitts­burgh and Detroit in the 1880s was even sootier than his­to­ri­ans re­al­ized.

“We’re track­ing a piece of the en­vi­ron­ment that has lit­er­ally been ac­cu­mu­lated by the spec­i­men,” said Carl Fuld­ner, an art his­to­rian who spe­cial­izes in pho­tog­ra­phy. Fuld­ner and Shane DuBay, an evo­lu­tion­ary bi­ol­o­gist, are au­thors of the new study, pub­lished in the Pro­ceed­ings of the National Academy of Sci­ences.

Track­ing en­vi­ron­men­tal pol­lu­tants via col­lec­tions at nat­u­ral his­tory mu­se­ums has some prece­dent. Re­searchers have mea­sured the thin­ness of ea­gle-egg shells, DuBay said, as a proxy for ex­po­sure to poi­sons such as DDT. But what made this study dif­fer­ent was that ev­i­dence of pol­lu­tion was not sec­ond­hand — the soot it­self col­lected on feath­ers, as though the birds were feather dusters float­ing above smoke­stacks.

DuBay and Fuld­ner stud­ied five species: horned larks, red­headed wood­peck­ers, field spar­rows, grasshop­per spar­rows and East­ern towhees. All were housed at the Field Mu­seum in Chicago, the Carnegie Mu­seum of Nat­u­ral His­tory in Pitts­burgh and the Univer­sity of Michi­gan’s Mu­seum of Zo­ol­ogy in Ann Ar­bor. Records back to 1880 in­di­cated the time and place the birds were col­lected.

These birds molt each year, re­plac­ing their feath­ers with a fresh set. That molt es­sen­tially wiped out the soot that had ac­cu­mu­lated in their ear­lier plumage. The dirt on the birds at the time of their death was a snapshot of that year in in­dus­trial his­tory, the re­searchers said.

“I’ve never heard of any­thing like this. I think it’s quite clever,” said Tami Bond, an en­vi­ron­men­tal-en­gi­neer­ing pro­fes­sor at the Univer­sity of Illi­nois who was not in­volved with this re­search.

The re­searchers, both grad­u­ate stu­dents, used mi­cro­scope im­ages to re­veal globs of car­bon stuck to the birds’ feath­ers. Us­ing a pho­to­graphic tech­nique, they mea­sured how poorly the birds’ feath­ers re­flected light — the less rel­a­tive light a bird re­flected, the dirt­ier it was.

The au­thors con­structed a time­line of air­borne black car­bon, al­though they em­pha­sized that the birds re­vealed only rel­a­tive trends in at­mo­spheric soot, not di­rect con­cen­tra­tions of black-car­bon emis­sions.

The birds were dirt­i­est from 1880 un­til 1929, when the Great De­pres­sion hit. Coal con­sump­tion plum­meted, only to rise again dur­ing World War II. The mid-1940s birds dark­ened in re­sponse. As power plants be­came more ef­fi­cient and nat­u­ral gas sup­planted coal in homes, the birds light­ened. A pe­riod of leg­is­la­tion — 1955’s Air Pol­lu­tion Con­trol Act, 1963’s Clean Air Act and 1970’s Clean Air Act ex­ten­sion — vis­i­bly bright­ened the birds’ feath­ers. The birds from the 1980s on­ward are the least sooty in re­cent decades.

Rel­a­tive dirt­i­ness fol­lowed trends Bond and her col­leagues mod­eled in a 2007 pa­per that traced black-car­bon emis­sions from 1850 to 2000 us­ing his­tor­i­cal records. “One of the shock­ing find­ings that we found was what we re­cover is very sim­i­lar to these pre­dic­tive mod­els,” DuBay said. With one ex­cep­tion: The birds were far dirt­ier from 1880 to the early 1900s than Bond’s car­bon emis­sion study would have sug­gested.

“There is a lot we don’t know about emis­sions dur­ing that pe­riod,” Bond said. “You can use ice cores, but of course that only works if you have glaciers.” (Re­searchers have es­ti­mated car­bon emis­sions by the soot trapped in Green­land ice.)

She noted that the study was con­strained to ur­ban ar­eas, “and it would take some work to un­der­stand how that would con­nect to regional or global av­er­age con­cen­tra­tions.”

De­spite the blue skies above cities such as Chicago to­day, the re­searchers said we’re not in the clear yet. Al­though coal is de­clin­ing in the United States, glob­ally hu­mans are burn­ing more coal than ever. Cities in Asia have par­ti­cle pol­lu­tion that the re­searchers likened to old Rust Belt hubs. DuBay said he has seen birds in Bei­jing dark­ened by air pol­lu­tion.

And while vis­i­ble par­tic­u­late mat­ter has largely been scrubbed out in North Amer­i­can and Euro­pean cities, DuBay cited a re­cent study that Lon­don­ers were ex­posed to tiny air pol­lu­tants — named PM2.5 for their di­am­e­ters of 2.5 mi­crom­e­ters or less — at con­cen­tra­tions 1½ times the limit sug­gested by the World Health Or­ga­ni­za­tion.

In a cen­tury of mu­seum work, no one could have an­tic­i­pated the birds would be used like this, not the nat­u­ral­ists who col­lected the an­i­mals nor the cu­ra­tors who pre­served them. DuBay and Fuld­ner em­pha­sized that nat­u­ral-col­lec­tions have un­tapped po­ten­tial be­yond these birds.

Some mu­se­ums keep pick­led bird lungs that could re­veal what the birds breathed in while alive. Draw­ers full of dead plants or lichens could, hy­po­thet­i­cally, con­tain the heavy met­als that the liv­ing or­gan­isms ab­sorbed from the soil.

“We’re study­ing at­mo­spheric sci­ence,” Fuld­ner said, “and the birds were a pathway to that.”


Horned larks col­lected dur­ing the early 20th cen­tury. The birds at left were col­lected in Illi­nois, inside the U.S. man­u­fac­tur­ing belt. The birds at right were from the West­ern coast, away from in­dus­try.

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