How purple came to be a scarier color than red on the Air Quality Index

JUANA SUMMERS, HOST:

Smoke from Canadian wildfires is again blanketing parts of the U.S. in a thick haze. Midwestern cities like Chicago, Minneapolis, Detroit and Milwaukee posted some of the worst air quality in the world yesterday and today. As the smoke spreads south and east, millions of Americans are anxiously scanning air quality maps and focusing on two colors - purple and red. Red is obviously terrible. Purple is even worse. NPR's Neda Ulaby tells us how purple came to be a scarier color than red.

NEDA ULABY, BYLINE: Red is the traditional color of danger, of stop signs, of warnings. So it's also the color on the government chart showing the air quality index is more than 150, meaning unhealthy. When it's more than 200, it's purple, very unhealthy. This makes sense to information designer Giorgia Lupi.

GIORGIA LUPI: From yellow to orange to red and purple is the next color in the spectrum.

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ULABY: But isn't purple a positive color? - royalty, luxury, the aggressively lovable dinosaur Barney, the LA Lakers, the Minnesota Vikings. But Lupi says purple can be dark, livid and sinister.

LUPI: Think about bruises and the color purple on skin when talking about a disease.

ULABY: None of this was in the air, so to speak, when the Environmental Protection Agency held a conference back in the 1990s. There was a lot of controversial stuff on the agenda, including a brand-new color-coded air quality index chart. Scientist Susan Stone was there, along with a number of advocates and state, local and tribal officials.

SUSAN STONE: And I was just totally surprised that colors was the topic that really blew the whole discussion up. They were getting so heated that we were saying we need to call a break because otherwise people are going to start shoving each other.

ULABY: Back then, Stone says, the idea of using even red for air quality was somewhat theoretical.

STONE: It looked like, at the time, looking back at the data, that if we were to put red at hazardous, it would never occur. These were the days in the late '90s and early 2000s before the huge wildfires out west. So it was extremely rare to get into the hazardous range.

ULABY: So rare, the EPA thought purple might never be used. Now, even purple is not bad enough. The very, very worst color is maroon. That's partly because black does not read well on maps, and you cannot see the borders. Still, purple clearly indicates a royal mess.

Neda Ulaby, NPR News.

(SOUNDBITE OF RONNIE FOSTER'S "MYSTIC BREW") Transcript provided by NPR, Copyright NPR.