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Zara Zhuang/MEDILL

Once a year, the Chicago River goes from murky green to Irish green. And people love to watch the transformation.

Turning the Chicago River a lucky shade of green

by Zara Zhuang
March 19, 2014


Zara Zhuang/MEDILL

The Plumbers Union 130 works magic on the Chicago River.


Zara Zhuang/MEDILL

The Chicago River in all its green glory.

On St. Patrick’s Day in Chicago, everyone is Irish. Even the river. But before the city got to admire the brilliant color of a river dyed green, spectators were treated to the sight of motorboats releasing a yellow-orange powder into the water. Which makes one wonder, what is that stuff?

For the 52nd consecutive year, the Chicago River was transformed from its usual murky green to a brighter shade associated with Ireland and shamrocks. The tradition is a trademark of the city’s annual St. Patrick’s Day parade and celebration the Saturday before March 17.

Organizers of the event are tight-lipped about what is in the dye. Kevin Sherlock, parade coordinator in the St. Patrick’s Day Parade Committee, declined to discuss the ingredients used in the dye. Coloring a river green for St. Patrick’s Day “is something unique to Chicago,” he said. He added that the dye used is safe for the environment and dissipates in six to 10 hours, depending on the flow of the river.

Sherlock is also the vice president of Chicago Journeymen Plumbers Local 130, which sponsors the parade and dyeing the river. Most of the people who work on the parade are plumbers.

Checking with the Illinois Environmental Protection Agency turned up the name of one chemical: fluorescein. “We get asked about that dye every year,” said Kim Biggs, Illinois EPA spokeswoman. A concerned citizen contacted EPA to evaluate the dye about 10 years ago. “We didn’t do a lab test on a sample of the dye (intended for the Chicago River). EPA contacted the Plumbers Unions on what was in the dye. Then, EPA’s toxicologist reviewed the dye’s ingredient and deemed it safe.” According to Biggs, there have been no changes in the content within the last decade.

Fluorescein is an orange powder that turns green in alkaline waters. It is a synthetic coloring agent used as a stain in medical applications and as a water tracer. In 2006 the U.S. Food and Drug Administration reviewed and approved Fluorescite, a diagnostic drug containing fluorescein sodium intended for retina or iris examinations. Fatal cases of anaphylactic shocks following fluorescein angiography — examinations done after introducing a tracer — are extremely rare. There may be discomfort or reactions if fluorescein is directly injected as a tracer, Biggs said, but at the concentration used to dye the river it is safe.

Gazing at a river dyed green for St. Patrick’s Day isn't an experience unique to Chicago residents and visitors. Nancy Hunt, executive director of the Paseo del Rio association in San Antonio, Texas, said the city has been dyeing its river annually for 20–25 years for the city’s St. Patrick’s Day boat parade. Instead of a powder, the San Antonio River takes a liquid dye.

A connection to Chicago’s river-dyeing tradition could not be confirmed. “Not that I’m aware of,” Sherlock said. “They might have talked about it before my time here. It’s just a matter of time before people figure out how to do this.”