California officials have said this week's wildfires are collectively the deadliest in state history.

Far From California’s Fires, Smoke Poses a Hazard, Too

More than 20 wildfires have been burning through Northern California this week, killing 31 people and destroying 3,500 homes and businesses. Residents were ordered to evacuate from communities in Napa, Sonoma, and Solano counties, though smoke caused by the fires also poses a threat to residents farther afield.

California officials have said this week’s wildfires are collectively the deadliest in state history. Visual: JOSH EDELSON/AFP/Getty Images

On Thursday, officials around San Francisco and the Bay Area — up to 100 miles away from the blazes — reported the region’s worst air quality on record. Of express concern is a category of particulate matter known as PM2.5, which, at less than 3 percent the diameter of a human hair, can be inhaled into the lungs and easily make its way into the bloodstream. The composition of this particulate matter varies, but at elevated levels, even short-term exposure can pose health risks for elderly people and those with heart or lung disease.

While the EPA’s air quality index website lists anything up to 50 micrograms of PM2.5 per cubic meter of air as “good,” the level around San Francisco was 158 as of Thursday, which is considered “unhealthy.” At this level, public health officials are advising people to limit outdoor activity and set air conditioners to recirculate to keep outside air from entering.

Also in the news:

• Puerto Rico’s death toll continues to rise, as the island struggles to recover from Hurricane Maria. According to officials, 45 people have died since the hurricane struck in September, but the real count is likely much higher. (Vox)

• A 34-year-old man tested positive for pneumonic plague, a severe lung infection caused by the bacterium Yersinia pestis, in the Seychelles after returning from a trip to Madagascar last Friday. Though the plague is treatable with antibiotics, 42 people have died in Madagascar since an outbreak began in late August. (New York Times)

• Following mounting sexual harassment allegations against film executive Harvey Weinstein, news also came last week that Boston University is investigating sexual harassment allegations against Antarctic geologist David Merchant. (The Atlantic)

• In August, the FDA approved a new kind of treatment for childhood leukemia, in which immune cells are genetically modified to attack the cancer. Though the therapy, known as CAR-T, has been hailed as a breakthrough, researchers offer new insight into the possible side effects, including neurotoxicity, in two new papers. (Washington Post)

• And finally: A project in New Mexico is helping families understand a disease-causing genetic mutation that can be traced to the Baca family, who came to the area more than 400 years ago. (Undark)