How purple became an Air Quality Index cautionary color: NPR

A version of this story originally ran February 5, 2021

This week, millions of Americans anxiously scan air quality maps focusing on two colors: red and purple. Red indicates “unhealthy” air quality, and purple? “Very unhealthy.”

When purple become the color most associated with danger?

“Red is the color of alert, of stop signs,” confirms information designer Giorgia Lupi, partner at Pentagram. But she sees the choice as logical. “Purple is the next color in the spectrum, from yellow to orange to red.”

Lupi’s job is to translate data into visual images that are easier for our minds to process. Color, for her, is a vital tool. While purple often carries positive associations in Western culture – such as lavishness and royalty – Lupi also points to the color’s unsettling lividity. “Think of bruising and purple skin color when talking about illness,” she suggests. “It’s another level. It’s darker, and a more advanced stage, if you will.”

As for how purple came to officially represent “very unhealthy” air quality: In the 1990s, the Environmental Protection Agency held a conference in Baltimore. There was a lot on the agenda, including a brand new color-coded Air Quality Index.

Scientist Susan Stone was there, along with a number of attorneys and state, local and tribal officials.

Color designation was a topic “that really blew up the discussion,” Stone recalls. “They were really starting to warm up. We were all saying we need to take a break or people are going to start jostling.”

In 2021, a spokesperson for the Environmental Protection Agency offered the following background:

When developing the AQI we have today, the most heated discussions were about colors. At a big meeting in Baltimore (in 1997 or 1998), we took an unplanned break during the color discussion because we thought the attendees were going to start jostling. The focus was entirely on the standard level and the color red. It was the days before the huge wildfires in the West, so it was extremely rare to enter the Hazardous range. We mainly reach very unhealthy levels with ozone. Although we didn’t have many continuous PM monitors at the time, we looked at filter-based PM data to assess the number of days in different categories.

There were two factions. Environmental groups wanted red in the Unhealthy for Sensitive Groups (USG) category to show levels were above NAAQS levels. The EPA and many state, local, and tribal officials wanted red in the unhealthy category, because that’s when the AQI indicates air quality can pose a risk to everyone. We were also concerned about message fatigue. At that time, it was not uncommon to have 30 days during which the ozone was above the standard level.

We’re not sure anyone knows for sure how the final decision was made, but in the end DC decided to go with red in the unhealthy category. The highest colors were decided by the AQI team to show that as the air quality deteriorates, it may be unhealthy for some people before it is for everyone. And even once the air quality becomes unhealthy, higher levels may dictate different actions. At orange, members of sensitive groups can have effects; at red, some members of the general population may be affected and effects on sensitive groups may be more severe. Purple is an alert, and the risk is increased for everyone. Brown – dangerous – represents emergency conditions. We don’t usually see this except for forest fires and sometimes dust storms.

Stone told NPR that she never suspected how often purple would be used as an alarm color.

“Looking at the data,” she says, “if we put red as ‘dangerous,’ it would never happen.”

Now, of course, dangerous days are not uncommon, and at least in some places the AQI is turning to an even worse color: brown. (It turns out that black is less readable on maps and it’s hard to see the borders.) For now, purple continues to show how in a royal mess we are.