Indoor air pollution is a concern for anyone who spends the bulk of their time indoors. If your air in your home is heated or cooled with a central air system, you may need to decide if cleaning the air ducts would help avoid the health effects of indoor air pollution.
There is no clear cut recommendation on whether air duct cleaning will have an impact on indoor air pollution. Air duct cleaning has never been proven effective in preventing health problems. Most of the dirt in air ducts is stuck to the duct surfaces and doesn't necessarily enter the air of the living space. And there is no conclusive evidence that a light amount of household dust or particulate matter in air ducts poses any health risk.
There are some instances in which cleaning your air ducts should be considered. If there is substantial, visible mold in the ducts or on components of your heating and cooling system, then you should have it removed if possible. But if you fail to correct the conditions that caused the mold in the first location, it will return after cleaning.
Air ducts should be cleaned if they are infested with rodents or insects. Ducts that are so clogged with dust that particles are actually released into the air should be cleaned. But again, every effort should be made to rectify the situation that caused these conditions.
Some believe that regular cleaning will improve the efficiency of your system, but this usually refers to a complete cleaning of the system components, not just the air ducts.
If you want to have your air ducts cleaned because you think anything that gets dirty over time should be cleaned regularly, then there's no evidence that air duct cleaning is harmful. As with any consumer service, take precautions to locate a competent and reputable air duct cleaning service.
Some service providers may suggest applying chemicals in your air ducts to kill mold and germs, and to prevent their recurence. Or they may propose using a sealant. To date, no research has proven the effectiveness of these methods, and in fact, the use of such chemicals in the air duct system of your home may contribute to indoor air pollution.
To sum up, the most effect method of keeping your air ducts clean is to prevent the entry of dirt and moisture in the first place.