The world's worst air this week is not in pollution hot spots like India or China. It's in the western United States, where record-breaking wildfires are blanketing the region with smoke.
Portland, Oregon, has topped the air pollution charts for major cities this week at monitoring company IQAir. com.
The San Francisco Bay area on Thursday emerged from 30 consecutive days under an unhealthy-air alert. It's the longest stretch on record. The previous record was 14 days, also during wildfires in 2018.
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From the United States to Brazil to Australia to the Arctic, climate change is helping drive an increase in wildfires and the pollution they generate.
Scientists know air pollution can be deadly. One major study found exposure to fine particles in air pollution, known as PM2.5, is the fifth-leading risk factor for death worldwide.
But studies have looked mostly at lifelong exposure to unhealthy air. Scientists know much less about what lasting impacts to expect from a few days or weeks of breathing wildfire smoke. It's an area of active study, but big gaps remain, experts say.
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What's increasingly concerning is that these huge wildfires are no longer isolated incidents.