A remodeler in the 1980s accidentally dropped a sandwich inside an open vent in Myrna Dean's Kansas City home. It's been festering in the back of her mind ever since.

Dean has seen a barrage of air duct cleaning ads in recent years, so she's considering having the service done in her home. But she's heard there can be drawbacks.

Air duct cleaning is a buyer-beware industry, according to the National Air Duct Cleaners Association, a professional organization based in Washington. Some companies, nicknamed "blow and go" businesses, don't have the proper equipment to suck out all the dirt.

The association warns consumers to avoid gimmicks, such as $99 whole-house specials and sweeping claims about health benefits.

Professional cleaners complain about companies that use only Shop-Vac-sized machines with two-inch hoses. In this business, they say, bigger is better. People should ask companies (talk to at least three) whether they use powerful gasoline-powered machines mounted to trucks. Hoses should be at least 10 inches in diameter, similar to a dinner plate.

"Otherwise you're just stirring a bunch of stuff out of the air ducts without removing it," said Brad Kuhlmann, owner of Midwest Duct Cleaning Services in Merriam, Kan. "That will create more damage rather than providing a service."

Homeowners can check their ducts using a mirror and a flashlight. If there are no large deposits of dust or mold, or if no one in the house suffers allergies or asthma, having air ducts cleaned is probably unnecessary, the Environmental Protection Agency said.

Just because you have dirty return registers doesn't mean you have disgusting ducts. It's normal for dust-laden air to be pulled through the grates, which can be vacuumed by homeowners.

Air-system businesses, however, recommend that ducts should be cleaned under the following circumstances:

* The home has been remodeled.

* Water has damaged the home.

* There are pets.

* Someone smokes.

* The carpet is old.

* The windows are open often.

* The home is newly constructed.

"There's drywall, Sheetrock, sandwich wrappers and all kinds of things," said Scott Self, owner of Enviro-Duct in Independence, Mo. "After the home is cleaned once, it won't be built up to that level again."

Still, cleaners find plenty of decaying bits inside the average home. The typical 2,000-square-foot house fills one to three grocery sacks with dirt, cereal, milk cartons and bits of old floor covering, Kuhlmann said. The accidental sandwich in Dean's air duct is, apparently, not such an unusual thing.

Most reputable companies charge for a complete cleaning starting at $225, which includes 16 vents. They charge more for larger houses, starting at $12 per vent.

Homeowners should expect cleaners to spend hours performing the service, which involves vacuuming and brushing. One employee should work four to eight hours; a two-person crew should take three to six hours.

Once ducts are cleaned, homeowners should expect improved energy efficiency with their heating and cooling system, according to the National Air Duct Cleaners Association.

The Environmental Protection Agency said you should expect your service provider to:

* Open access ports or doors to allow the entire system to be cleaned and inspected.

* Use only high-efficiency particle (HEPA) air vacuuming equipment.

* Protect carpet and household furnishings during cleaning.

* Use well-controlled brushing equipment with vacuums to dislodge dust and other particles.

* Take care to protect ductwork, including sealing and reinsulating any access holes that may have been made.