Bob writes:
Is it possible to stop the stack effect (rising up of warm air causing
dirty crawlspace air to be sucked into living area) by installing an
exhaust fan in the crawlspace (obviously all vents are covered except
the exhaust vent) and creating a negative air pressure?

Or would this just create a convection effect where in cooler part of
the house, air migrates down into the crawlspace, and in warmer area of
the house, air rises upwards and pulls the crawlspace air into the
living area?

Why is the crawlspace vented into the living area in the first place?