[Shotimes] emissions question

[van Oss]

I teach public speaking.  A student gave a speech that has raised questions
about emissions testing.  Please help.  I live in a small city with no
testing.  To those who live in an area with emissions testing:

Test results, and numbers required to pass, are all expressed in terms of
percentages, are they not?  I mean, percentages as opposed to absolute
amounts of various compounds per mile?  So a vehicle with a larger volume of
exhaust output might score well on percentages, but actually put out more
total volume of these compounds --- when compared with, say,  a vehicle with
a smaller exhaust volume but percentage-wise "dirtier" emissions?

Any help appreciated.

VO