what do they use to dye the chicago river green

They use a top-secret, vegetable-based, powdered dye that looks orange when it goes in but turns the river bright green in the water.

What the dye actually is

• The exact formula is kept confidential by the local plumbers’ union that does the dyeing, and they compare it to a “Coca-Cola recipe” in terms of secrecy.

• Public statements describe it as a vegetable-based, food-grade dye that is considered non-toxic at the concentration used in the river.

• The powder itself is orange; it appears green only after it disperses in the river water and interacts with light and pH.

Was it always this way?

• In the early 1960s, they used fluorescein, a synthetic tracing dye that is orange-red as a powder and turns green in water and sunlight.

• Environmental concerns in the mid-1960s pushed Chicago to switch from fluorescein to a more natural, plant-based formula.

• Modern analyses and expert commentary suggest the current dye is likely a xanthene-type dye, chemically similar to fluorescein but formulated as a “vegetable-based” tracer dye.

Safety and environmental notes

• Illinois environmental authorities have described the dye as food-grade and non-toxic at the levels used for the St. Patrick’s Day event.

• It is similar to dyes also used in medicine, antifreeze coloring, and water-tracing studies, all applications where safety at low concentrations is important.

• The green color typically lasts only a few hours before fading as the dye disperses and dilutes downstream.

TL;DR: When people ask “what do they use to dye the Chicago River green,” the honest answer is: an orange, vegetable-based, food-grade tracer dye whose exact recipe is secret, probably related to fluorescein-type dyes, designed to be vivid but non-toxic at the amounts used.

Information gathered from public forums or data available on the internet and portrayed here.