Why should I start to clean air ducts?

This statement is too general. Cleaning of exhaust and ventilation systems is already quite common in industry today. The industrial technology requires clean exhausts for its proper operation and in many cases also for elimination of fire hazard. Cleanliness requirements for industrial exhausts are specified by technology supplier.

A different situation is with air conditioning and ventilation systems for human occupancy. The situation has rapidly changed in this area. For long years when it was not clear whether to clean or not and what are responsibilities of the building owners. New CEN standards and EU directive changed the situation. It is clear now that air conditioning and ventilation systems for human occupancy must be cleaned and what are levels of cleanliness. A similar situation is in many other countries worldwide now.

On the other side regardless of all the standards and directives the economical part starts also to play an important role as well today. Recent experience from the USA shows clearly that the clean air conditioning and ventilation systems allow to slow down ventilators up to 30% when keeping the required air exchange. And this 30 % lower speed can spare a lot of energy, i.e. operation costs of the whole system.