It's normal for HVAC return registers to collect grime as dust-laden air is pulled through the grate. This does not indicate that your air ducts are contaminated with heavy deposits of dust or debris; the registers can be easily vacuumed or removed and cleaned.
Pros and Cons
If no one in your household suffers from allergies or unexplained symptoms or illnesses, and if after a visual inspection of the inside of the ducts, you see no indication that your air ducts are contaminated with large deposits of dust or mold (no musty odor or visible mold growth), having your air ducts cleaned is probably unnecessary.
On the other hand, if family members are experiencing unusual or unexplained symptoms or illnesses that you think might be related to your home environment, you should discuss the situation with your doctor.
While the debate about the value of periodic duct cleaning continues, no evidence suggests that such cleaning would be detrimental, provided that it is done properly.
Failing to follow proper duct cleaning procedures can cause indoor air problems. For example, an inadequate vacuum collection system can release more dust, dirt, and other contaminants than if you had left the ducts alone. A careless or inadequately trained service provider can damage the ducts or heating and cooling system, possibly increasing your heating and air conditioning costs, or force you to undertake difficult and costly repairs or replacements.
If there is substantial visible mold growth inside hard surface (e.g., sheet metal) ducts or on other components of your heating and cooling system, your ducts need cleaning. There are several important points to understand concerning mold detection in heating and cooling systems.
Many sections of your heating and cooling system may not be accessible for a visible inspection, so ask the service provider to show you any mold they say exists.
You should be aware that although a substance may look like mold, a positive determination of whether it is mold or not can be made only by an expert and may require laboratory analysis for final confirmation. For about $50, some microbiology laboratories can tell you whether a sample sent to them on a clear strip of sticky household tape is mold or simply a substance that resembles it.
If you have insulated air ducts and the insulation gets wet or moldy it cannot be effectively cleaned and should be removed and replaced.
If the conditions causing the mold growth in the first place are not corrected, mold growth will recur.
Ducts infested with vermin like rodents or insects should also be cleaned. As with mold, the underlying problem needs to be addressed, or the ducts will quickly become unhealthy again. Mice and other rodents can leave droppings and dander in the ducts, but if the little creatures themselves are not eliminated, there's no point in vacuuming out the system.
When ducts are clogged with excessive amounts of dust and debris, particles are actually released into the home from your supply registers. Obviously, these ducts need to be cleaned. But your HVAC system is designed to prevent excessive buildup in the first place, so if there's a problem in your ducts you need to check your filters and the overall health of your system.
Other Important Considerations...
Health Issues - Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts or go down after cleaning. This is because much of the dirt that may accumulate inside air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to health.
The EPA - The EPA does not recommend that air ducts be cleaned except on an as-needed basis because of the continuing uncertainty about the benefits of duct cleaning under most circumstances. If a service provider or advertiser asserts that the EPA recommends routine duct cleaning or makes claims about its health benefits, you should notify the EPA in writing about the claims of that provider. The EPA does, however, recommend that if you have a fuel burning furnace, stove, or fireplace, they be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning. Some research also suggests that cleaning dirty cooling coils, fans and heat exchangers can improve the efficiency of heating and cooling systems. However, little evidence exists to indicate that simply cleaning the duct system will increase your system's efficiency.
Professional Advice - If you think duct cleaning might be a good idea for your home, but you are not sure, talk to a professional. The company that services your heating and cooling system may be a good source of advice. You may also want to contact professional duct cleaning service providers and ask them about the services they provide. Remember, they are trying to sell you a service, so ask questions and insist on complete and knowledgeable answers.