Many plants, particularly conifers, emit abundant terpenes, especially in
hot weather. When combined with pollution, terpenes radically increase
ozone to levels considered dangerous. This is probably why Ronald Reagan
told the tree huggers that trees pollute.

I was a greenie, 1976-84, during which time I personally planted 1,000
trees. Of course, this area at the time was exurbia, not urban, and now I
guess you'd call it somewhere between suburbia and exurbia, altho this
particular valley is still mostly agri. I wonder how much my efforts have
contributed and will contribute to pollution. Maybe should've left the
area remain as meadow.