air quality monitors is considered by the ALA, without any context or averaging. The lone monitor used is immediately downwind and adjacent to U.S. Steel's Clairton Coke Works, the nation's largest coke mill, and several municipalities outside the city's jurisdiction of pollution controls, leading to possible confusion that Pittsburgh is the source or center of the emissions cited in the survey. The region's readings also reflect pollution swept in from Ohio and West Virginia. Although the county was still below the "pass" threshold, the report showed substantial improvement over previous decades on every air quality measure. Fewer than 15 high ozone days were reported between 2007 and 2009, and just 10 between 2008 and 2010, compared to more than 40 between 1997 and 1999. ACHD spokesman Guillermo Cole stated "It's the best it's been in the lifetime for virtually every resident in this county ... We've seen a steady decrease in pollution levels over the past decade and certainly over the past 20, 30, 40, 50 years, or more." In the summer of 2017, a crowd sourced air quality monitoring application, Smell PGH, was launched. As air quality is still a concern of many in the area, the app allows for users to report odd smells and informs local authorities. The city contains 31,000 trees on 900 miles of streets, by the last count conducted in 2005. A 2011 analysis of Pittsburgh's tree cover, which involved sampling more than 200 small plots throughout the city, showed a value of between $10 and $13 million in annual benefits based on the urban forest contributions to aesthetics, energy use and air quality. Energy savings from shade, impact on city air and water quality, and the boost in property values were taken into account in the analysis. The city spends $850,000 annually on street tree planting and maintenance. Water quality The local rivers continue to have pollution levels exceeding EPA limits. This is caused by frequently overflowing untreated sewage into local waterways, due to flood conditions and antiquated infrastructure. Pittsburgh has a combined sewer system, where its sewage pipes contain both stormwater and wastewater. The pipes were constructed in the early 1900s, and the sewage treatment plant was built in 1959. Due to insufficient improvements over time, the city is faced with public health concerns regarding its water. As little as a tenth of an inch of rain causes runoffs from the sewage