While visible injury symptomatology is one aspect of indicator value of plants, the other is leaf epidermology. The rationale behind the latter is that it is the leaf epidermis that is most exposed, and stomata are the easy entry points of pollutants in higher plants.

[…] asked the young Brother Benedict, early one Good Friday morning, as he carried off, on a board, one batch after another of the loaflets of white “lord's bread.”

I do not, for that cause, or any other cause, propose to destroy, or alter, or disregard the constitution. I stand to it, fairly, fully, and firmly. Abraham Lincoln

The dirty secret of the internet is that all this distraction and interruption is immensely profitable. Web companies like to boast about[…]and so on. But the real way to build a successful online business is to be better than your rivals at undermining people's control of their own attention. Partly, this is a result of how online advertising has traditionally worked: advertisers pay for clicks, and a click is a click, however it's obtained.