Engine air filters are designed to separate particulate matter from the air, not gasses[FONT=tahoma, arial, helvetica][SIZE=-1] (such as [/SIZE][/FONT][FONT=tahoma, arial, helvetica][SIZE=-1]N₂ [/SIZE][/FONT][FONT=tahoma, arial, helvetica][SIZE=-1]or [/SIZE][/FONT][FONT=tahoma, arial, helvetica][SIZE=-1]O₂[/SIZE][/FONT][FONT=tahoma, arial, helvetica][SIZE=-1]). [/SIZE][/FONT]
If they were designed to filter gasses, I would wholeheartedly agree with everyone.
Try breathing through an air filter and then through a gas mask.
Breathing through a gas mask takes an obvious effort. An air filter takes zero effort until it begins to lose it's porosity or is too small (low cfm rating) to begin with. I purposefully use the smaller filters for aesthetic reasons with no difference in performance. Proved out with a stroboscope at various RPM, load or no.
Holds true for 2 or 4 cycle.