Tiny smoke particles could make wildfires particularly harmful to human health

When the San Francisco Bay Area experienced a record 30 consecutive days of worrisome air quality alerts in August and September, Mary Prunicki began taking blood samples from firefighters.

The sky had turned orange from nearby wildfires. Thousands of firefighters would spend months battling the blazes, which would eventually scorch more than 4 million acres and kill 31 people.