After the HVAC s

After the HVAC system is installed and its operation begins,
the particulate accumulation process continues throughout
the life of the system. Poor design, installation and
maintenance practices, low-efficiency air filtration, air flow
bypass, inadequate or infrequent preventative maintenance
practices, humid conditions, and many other factors will
result in contaminated HVAC systems. HVAC systems may
also serve to transport and redistribute unwanted particles
from other sources in the building.