The usefulness of air duct cleaning depends on several factors.
Ducts in commercial kitchens must be cleaned on a regular basis to comply with fire regulations. Airborne fats and other cooking debris settle in ducts and extractors and pose fire and health and safety hazards. They also tend to attract insects and vermin into the ducts looking for the food that produced the odours that remain inside.
For commercial kitchens or other businesses that generate pollutants, cleaning air ducts is always worthwhile. At the very least, they need to be routinely inspected.
Commercial buildings and offices
Commercial HVAC systems usually have proper filters installed. This means relatively little dust should accumulate in ducts or other ductwork parts.
The usual concern in large buildings is airborne micro-organisms. Depending on the design of your system, micro-organisms may be spread from room to room through ducts.
There are air conditioning technologies that destroy or filter micro-organisms. They are common in hospitals but not in commercial buildings. In 1985, the cooling tower above Stafford District Hospital became infected with Legionella, killing 175 people. A whole range of legislation now places responsibilities on employers and landlords to maintain their HVAC systems (https://legionellacontrol.com/compliance/legionella-regulations-duty-holder-responsibilities/).
Ducts in domestic homes
If you have good-quality filters, there shouldn’t be anything in your ducts to clean out. Not all home HVAC systems make sufficient use of filters. Without them, ducts serving living rooms and bedrooms accumulate dead skin and hair cells, pet dander and fibres from clothing and bed linen. That’s not as dangerous as it sounds; if it stays in the duct, it’s not doing much harm.
However, problems arise when it moves. Drafts can lift it back into the air, as can inexpert cleaning. As anyone with bad hay fever or asthma knows, the only thing worse than living in a dusty house is dusting.
Duct contamination is made worse by poor-quality ducts. If all your ductwork parts are made from lightweight spiral steel tubing, you can minimise the problem with help from https://www.dustspares.co.uk/ductwork-parts/). Their surfaces also resist mildew, a serious air quality issue.
To solve the immediate problem, call professionals who know how to clean the ducts without blowing it back into the air. They can also advise you about improving your HVAC system to eliminate the problem permanently.