see Schantz 2006 The water in Stockholm is so clean that people can dive and fish in the centre of the city. The waters of downtown Stockholm serve as spawning grounds for multiple fish species including trout and salmon, though human intervention is needed to keep populations up. Regarding emissions, the government's target is that Stockholm will be free before 2050. Air quality Stockholm used to have problematic levels of particulates (PM10) due to studded winter tires, but by the 2010s they were below limits, after street-specific bans. Nitrogen oxides emitted by diesel vehicles were a problem in the 2010s, but by 2021 they were again below limits, after electric cars had started to replace diesel-driven ones, and pollution regulations for lorries had tightened. As of 2021, the pollutant that exceeds limits is ozone, due to global pollution. In 2021 the average levels for urban background (roof of Torkel Knutssonsgatan on Södermalm) were: NO2 9.7 μg/m3, PM10 9.5 μg/m3, PM2.5 5.1 μg/m3, soot 0.36 μg/m3, ultrafine particles 6100/cm3, SO2 0.4 μg/m3, ozone 53 μg/m3. For urban street level (the densely trafficked Hornsgatan on Södermalm) the average levels were: NO2 23 μg/m3, PM10 17 μg/m3, PM2.5 6.0 μg/m3, soot 0.55 μg/m3. Transport Public transportation Stockholm has an extensive public transport system. It consists of the Stockholm Metro (), which consist of three colour-coded main systems (green, red and blue) with seven lines (10, 11, 13, 14, 17, 18, 19); the Stockholm commuter rail () which runs on the state-owned railroads on six lines (40, 41, 42, 43, 44, 48); four light rail/tramway lines (7, 12, 21, and 22); the 891 mm narrow-gauge railway Roslagsbanan, on three lines (27, 28, 29) in the northeastern part; the local railway Saltsjöbanan, on two lines (25, 26) in the southeastern part; a large number of bus lines, and the inner-city Djurgården ferry. The overwhelming majority of the land-based public transport in Stockholm County (save for the airport buses/airport express trains and other few commercially viable bus lines) is organized under the common umbrella of Storstockholms Lokaltrafik (SL), an aktiebolag wholly owned by Stockholm County Council. Since the 1990s, the operation and maintenance of the SL public transport services are contracted out to independent companies bidding for contracts, such as MTR, which operate the Metro. The archipelago boat traffic is handled by Waxholmsbolaget, which is also wholly owned by the County Council. SL has a common