Smoke

 NY Times: See How Wildfire Smoke Spread Across the United States By Nadja Popovich and Josh Katz

As the smoke moved eastward across Toronto, New York and Philadelphia on Tuesday, concentrations of dangerous microscopic air pollution known as PM2.5 (because the particles are less than 2.5 microns in diameter) reached highs in the “unhealthy” range for most of the day.