Less dust and dirt! Cleaner, healthier air!Improved energy efficiency! Ads for air duct cleaning services are everywhere. Every coupon mailer seems to contain at least one, often illustrated with dramatic before and after photos of dust-choked vents, now shiny clean.

You could shell out $50 to $1,000 or more to have the interior of your home’s heating and air conditioning ductwork spruced up, a process that may involve brushing, vacuuming, blasts with compressed air and the spraying of mold- and bacteria-killing chemicals. But should you?

Most of the time, according to the Environmental Protection Agency (EPA), it’s not worth it.

“Duct cleaning has never been shown to prevent health problems,” the EPA writes on its website. “Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. Little evidence exists that cleaning only the ducts will improve the efficiency of the system."

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The EPA reached that conclusion back in 1997, after teaming up with the National Air Duct Cleaners Association (NADCA) to study how vent cleaning affected indoor air. Since then, newer studies have reached conflicting conclusions.

One 2010 review of the scientific data on air duct cleaning raised alarms about the practice. Canada’s National Research Council concluded that in some cases, “post-cleaning air pollutant concentrations can be higher than pre-cleaning levels” and “there are health concerns in the use of biocides, sealants and encapsulants.”

Another study was more positive. When researchers from Florida International University compared air quality in homes before, during and after duct cleaning in 2001, they found that pollutant levels did drop afterward. But, they warn, the amount of “dirt, debris and other pollutants” in the air was higher during the process than before cleaning began — pointing to the need to hire well-trained contractors if you do decide to have your ducts cleaned.

While no research links duct cleaning to fewer health problems, the EPA as well as some asthma experts suggest that duct cleaning by a reputable company may be worthwhile if your vents contain visible mold or are clogged with dirt that spews from vents when you turn on the heat or air conditioning, or if your ducts have become stomping grounds for mice, bugs and other pests.

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If you do opt for duct-cleaning, these five steps can help you get the best results.

1. Hire a reputable contractor. Ask the company that services your heating or air conditioning system for referrals. Or check the NADCA website for companies in your area. Ask about licenses, which are required in some states including Arizona, Arkansas, California, Florida, Georgia, Michigan and Texas. Shy away from contractors who make big health claims or who say they’re endorsed by the EPA — that federal agency doesn’t certify duct cleaners. Make sure they have experience working on systems like yours and that they comply with NADCA cleaning standards. Check references and get several estimates.

2. Insist on a thorough inspection before you sign up for a cleaning. Ask for proof of dirt, mold or vermin problems. The EPA recommends fixing the cause of the problem before having your ductwork cleaned. This may involve working with a heating, air conditioning and ventilation company. 

3. Go with a cleaning plan that protects your home and your air.Ask how the contractor will keep dust and pollutant levels low during the job. This may involve using a vacuum system that exhausts air to the outdoors or that uses a HEPA filter. 

4. Think twice about antimicrobials and other chemicals. Ask for information about the risks and benefits of pesticides, sealants, deodorizers and any other chemicals the contractor suggests using. The National Institutes of Health recommends avoiding them, noting, “even EPA-registered biocides may pose health risks including eye, nose and skin irritation.” 

5. Inspect the work. Ask to look into all ducts before they’re closed up. You shouldn’t see any signs of dirt, mold or vermin. Some companies even provide remote photography to show hard-to-access spots. Use the EPA’s duct-cleaning checklist to help you perform a thorough review with your contractor.

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Sari Harrar is an award-winning health, medicine and science journalist whose work appears in Dr. Oz The Good Life magazine, Good Housekeeping, O--Oprah Magazine, Organic Gardening and other publications.