Due to the wildfires raging in Northern California, San Francisco, Stockton and Sacramento were the world's three "most polluted cities" on Friday morning, according to Berkeley Earth, a nonprofit that aggregates data from air-quality monitoring sites.
With the shocking loss of thousands of homes and dozens of lives in the Camp and Woolsey fires in Northern and Southern California, people are looking for answers as they try to understand how a tragedy such as this can be prevented in the future.
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