In spite of all the traffic congestion around town, Harris County's air quality appears to be improving, according to a new report from the American Lung Association.

The State of the Air 2015, released earlier this week, found that Houston ranks as one of the cleanest cities in the nation for short-term particle pollution. The number of days spent with unhealthy ozone levels and year-round particle pollution improved, as well.

The city has seen slight improvements in year-round particle pollution (soot) levels from 2014, which is in keeping with the national trend, according to the report.

According to the report, the Houston area experienced 69 days with unhealthy ozone levels, which is 47 fewer days than reported in 1996.

Houston also experienced fewer unhealthy days of high ozone (smog) and has shifted from the No. 5 to No. 6 most polluted city in the country. The American Lung Association advises that on designated unhealthy days, everyone should limit prolonged outdoor exertion, especially children, active adults and people with respiratory disease, such as asthma.

According to the 2015 report, more than 4 in 10 Americans – nearly 138.5 million people – live in counties where ozone or particle pollution levels make the air unhealthy to breathe.

The 16th annual report, which looks at national air pollution data collected from 2011 to 2013, shows that improvements to the nation’s air quality was mixed. While many cities experienced significant improvements, others suffered from increased episodes of unhealthy air quality, with a few even experiencing the highest number of unhealthy days.

"Houston can certainly be proud of the progress we've made in cleaning up our air since the first State of the Air report 16 years ago. However, there's still a lot of work to be done to make our air healthy for all of us to breathe," Sandra Curphey, executive director of the American Lung Association in Texas, said in a statement.

This year, Harris County received an "A" grade for short-term particle pollution as there were no days with unhealthy levels.

While the city's ozone levels improved over the past year, Harris County still received a "D" grade for high levels of ozone. According to the report, the area experienced 69 days with unhealthy ozone levels, which is 47 fewer days than reported in 1996. Ozone is the most widespread air pollutant, created by the reaction of sunlight on emissions from vehicles and other sources. Ozone inhalation can cause wheezing, coughing, asthma attacks and even premature death.