The PCV valve regulates the amount of air drawn through the crankcase. The air is the filtered air after the MAF (so it is metered). From the boot (Which is "high pressure" before the Throttle body)through the crankcase, through the PCV valve and then to intake manifold ("Low Pressure" or "high vacuum", if you prefer). Except for some a small amount of blowby (exhaust gases getting past the compression rings on the piston) and a little bit of oil carried over, the air really isn't that nasty. The oil is the real problem and can be helped with a better oil separator (Cover somewhere in this site). Before PCV, not only did the exhaust gases escape untreated, the environmnet in the crankcase was miserable. Hot, lot's of sulfur, and moisture could not escape easily. Besides the air quality issue, PCV is actually good for the engine (as long as you don't carry over the oil, anyway)
