There was a thread yesterday on the Philly sub about people reporting breathing difficulties. Lots of speculation about early allergens, but one of the posts included a satellite map of carbon monoxide concentrations in our region. Yesterday Eastern PA/NJ/NY was the second largest hotspot of CO on the planet next to all of eastern China, and the plume is forecast to return Sat-Sun:

https://www.reddit.com/r/philadelphia/comments/119phph/comment/j9nnt6u/
https://www.windy.com/-CO-concentration-cosc?cosc,40.424,-76.965,8

CO is odorless but I wonder if there's any correlation with other industrial particulates?