For the first time since 2009, and only the second time in the last 16 years, Washington, D.C. had zero code red days for unhealthy air quality in the summer of 2013. This reprieve from suffocating air represents another data point fitting into a a recent trend towards cleaner air.

(Metropolitan Washington Council of Governments)

The District had just four days – the lowest since 2009 – that exceeded the “standard” for ozone (a concentration of 75 parts per billion, ppb). Above the standard, air quality begins to become unhealthy for sensitive groups (older adults, children, and people with respiratory problems), reaching the code orange level.

Ozone forms when air pollutants react on hot, sunny summer days. At high concentrations, ozone can cause respiratory issues, fatigue, headaches, nausea, chest pain, and eye and throat irritation.

The lack of hot weather certainly contributed to the lower ozone levels in 2013; however, in past years with similar temperatures, there were many more unhealthy days.

From 1997 to 2005, almost without exception, air quality levels climbed into the unhealthy range when the temperature was 90 or higher, and frequently on cooler days as well.

(Metropolitan Washington Council of Governments)

But since 2006, increasingly, ozone levels on days above 90 have not exceeded health standards.  In 2013, just 4 of the 18 days above 90 (during the period analyzed) were levels in the unhealthy (code orange, for sensitive groups) range.

(Metropolitan Washington Council of Governments)

“[S]nce 2006, 90 degree and higher temperatures no longer guarantee that the region will exceed federal health standards,” the Metropolitan Washington Council of Governments said in its press release on 2013’s summer air quality.

Link: 2013 Ozone Season Summary and Historical Trends

Particulate matter pollution is also trending downward.  The summer of 2013 matched 2009’s record low of zero days at unhealthy levels.

(Metropolitan Washington Council of Governments)

“This is the cleanest summer since 2009,” said D.C. Council Chairman Phil Mendelson, chair of the Metropolitan Washington Air Quality Committee. “Progress has been made through the combined efforts of governments as well as individuals.”

Mendelson stressed it’s not time to get complacent about the state of the air. “The region can’t lose sight of the challenges ahead,” he said. “Even as the region’s air quality improves, EPA is considering lowering the health standard for ozone.”

EPA has been incrementally tightening ozone standards since the 1970s and is due to announce a revised standard in 2014.