Marine One takes off in hazy air over the White House on Tuesday. ( Erik Cox Photography )

Washington’s improving air quality in recent decades is one of its great environmental success stories. But on Monday, the pollution level climbed into the “unhealthy” range for the first time in six years, showing there is still work to be done to clean up the air.

Monday’s dirty air, rated Code Red, may come as a surprise since skies were clear and the humidity was low. But it also marked the third day in the row without much wind. When the air is stagnant, pollutants tends to collect and remain in the atmosphere.

Elevated concentrations of ground-level ozone were the culprit for the unhealthy air, the Metropolitan Washington Council of Governments (COG) said. “[It] is created by chemical reactions between oxides of nitrogen and volatile organic compounds in the presence of sunlight,” the Environmental Protection Agency explains. “Emissions from industrial facilities and electric utilities, motor vehicle exhaust, gasoline vapors, and chemical solvents are some of the major sources.”

As stagnant air lingered Tuesday, the COG issued a Code Orange air quality alert, which is one step below Code Red.

(Environmental Protection Agency)

Code Red alerts indicate the air is unhealthy for everyone; Code Orange alerts signify that sensitive groups, particularly children, older adults and those with respiratory issues, may experience adverse health effects.

During Code Red conditions, the COG advises residents to:

  • Turn off lights and electronics when not in use and follow tips from your electric utility about how to use less electricity to cool your home.
  • Avoid lawn mowing or use an electric mower.
  • Fill your vehicles’ gas tank after sunset.
  • Take transit, carpool or work from home.

Monday’s Code Red conditions come less than a year after Washington’s lowest summer air pollution levels in decades, when there was not a single Code Red day.

Pollution levels have settled at relatively low levels for the past five years — averaging 11 unhealthful Code Orange or Code Red days, compared with an average of 52 in the previous 16 years.

“We’ve seen a dramatic improvement in the region’s air quality thanks to more than a decade of action and coordination at all levels of government,” Hans Riemer, chairman of the Metropolitan Washington Air Quality Committee, said in a press release last fall. “Still, area residents continue to breathe unhealthy air too often. We must work together to continue to reduce emissions that contribute to ground-level ozone.”