This is from ASHRAE "Parkng garages" :
The design ventilation rate required for an enclosed parking
facility depends chiefly on four factors:
• Acceptable level of contaminants in the parking facility
• Number of cars in operation during peak conditions
• Length of travel and the operating time for cars in the garage
• Emission rate of a typical car under various conditions
For many years, the model codes, ANSI/ASHRAE Standard 62,
and its predecessor standards have recommended a flat exhaust rate of either 1.5 cfm/ft2 or 6 air changes per hour (ACH) for enclosed parking garages. But because vehicle emissions have been reducedover the years, ASHRAE sponsored a study to determine ventilation
rates required to control contaminant levels in enclosed parking facilities (Krarti and Ayari 1998).
The study found that, in some cases, much less ventilation than 1.5 cfm/ft2 was satisfactory. However, local codes may still require 1.5 cfm/ft2 or 6 ACH, so the engineer
may be required to request a variation, or waiver, from authorities having jurisdiction before implementing a lesser ventilation system design. NFPA 88A recommends a minimum of 1.0 cfm/ft2.
Because various agencies and countries differ on the acceptable level of CO in parking garages, a reasonable solution is a ventilation rate designed to maintain a CO level of 35 ppm for 1 hr exposure, with a maximum of 120 ppm, or 25 ppm for an 8 h exposure.
Because the time associated with driving in and parking, or driving out of a garage, is on the order of minutes, 35 ppm is considered to be an acceptable level of exposure.
. Though imok2 condensed it all here, I would be glad to send you a good paper on garage ventilation as a token of repayment for helping me in the above said forum(Ref : material selection of Lyophilizer)