U.S. Department of Labor Occupational Safety and Health Administration
What are the permissible exposure limits for
asbestos?
Employee exposure to asbestos must not exceed 0.1
fiber per cubic centimeter (f/cc) of air, averaged over an 8-hour work
shift.
Short-term exposure must also be limited to not more than 1 f/cc, averaged over 30 minutes.
Rotation of employees to achieve compliance with either permissible exposure limit (PEL) is prohibited.
Courtesy of U.S. Department of Labor Occupational Safety and Health Administration

The OSHA standards for Exposure Monitoring