If and when the 8-hour standard becomes enforceable, presumably early in the 21st Century, the challenge to meet this standard will become more formidable, if in fact, we have already wrung all of the pollution we can out of motor vehicles.
(the 8-hour standard; becomes; enforceable) (the challenge to meet this standard; will become; more formidable; T:when the 8-hour standard becomes enforceable; T:presumably early in the 21st Century) (we; have wrung; all of the pollution; out of motor vehicles; T:already)
the 8-hour standard the challenge to meet this standard we
becomes will become have wrung
enforceable
more formidable T:when the 8-hour standard becomes enforceable T:presumably early in the 21st Century
all of the pollution out of motor vehicles T:already