Carpets and HVAC are connected
In addition to walk-off mats another preventative maintenance measure which will prove extremely effective is monthly replacement of HVAC filters.
HVAC filters are inexpensive but, many facilities try to extend the replacement cycle beyond its usable life. They are very effective in trapping airborne soils which are the most difficult to remove from the carpet. They are also very effective in trapping other infectious airborne contaminants. Failure to have an effective filter change schedule increases your buildings maintenance cost.
Have the HVAC system inspected and serviced twice yearly to keep it at its maximum efficiency. Biological contamination could have occurred during periods of non-use. Turning on the system will allow the contaminants to spread throughout the building. When turning on the heating or air conditioning it is a good idea to turn it on over a weekend to allow time for particulates that have built up in the ductwork to settle. Be sure to coordinate with the maintenance staff to insure extra cleaning will take place after the system has been turned on.