<P> As part of a more than fifty - year - old Chicago tradition, the Chicago River is dyed green in observance of St. Patrick's Day . The actual event does not necessarily occur on St. Patrick's Day and is scheduled for the Saturday before March 17, unless the 17th falls on a Saturday (when the event takes place on St. Patrick's Day). </P> <P> The tradition of dyeing the river green arose by accident when plumbers used fluorescein dye to trace sources of illegal pollution discharges . The dyeing of the river is still sponsored by the local plumbers union . </P> <P> The United States Environmental Protection Agency (EPA) outlawed the use of fluorescein for this purpose, since it was shown to be harmful to the river . The parade committee has since switched to a mix involving forty pounds of powdered vegetable dye . Though the committee closely guards the exact formula, they insist that it has been tested and verified safe for the environment . Furthermore, since the environmental organization Friends of the Chicago River believes the dye is probably not harmful, they do not oppose the practice . </P> <P> In 2009 First Lady Michelle Obama, a Chicago native, requested that the White House fountains be dyed green to celebrate St. Patrick's Day . </P>

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