An explosion at the Horizon oilsands site in northern Alberta cast black smoke over the region and burned four workers, including two who went to hospital. The explosion happened Thursday afternoon at a site owned by Canadian Natural Resources Ltd. Nearby residents of the Fort McKay First Nation told CBC News they were worried about what was spilling into the sky over their community as they watched the fire burn Thursday. Testing equipment showed that air quality was “good” as of 6 p.m. MT and Alberta Environment is on site today to assess risks. The fire started in an upgrader that converts bitumen into crude oil.