The air quality in Northern Portugal substantially improved in 2008 compared to 2006 and 2007, according to conservation association Quercus. Their study found that in 2007, 18 points of analysis exceeded the acceptable limits for PM10 (non-inhalable pollutant) particles. In 2008 however, only 3 points of analysis remained at excessive levels for more than the accepted number of consecutive days (35 days). In the Lisbon and Tagus Valley region however, Quercus reported that the same air quality problems from a couple of years ago still persist.