Hot Water Extraction describes a special type of cleaning (commonly known as steam, although no steam is really produced) that is capable of killing bacteria and even pests such as dust mites that are lurking in your carpets.
A growing number of service providers are responding to concerns on the part of the public for cleaner indoor air and a healthier indoor environment. The EPA has stated that hot water in excess of 130 degrees is capable of killing bacteria. A few well informed carpet cleaners are buying top of the line cleaning equipment that can deliver hot water to the carpet during the cleaning process. This equipment also has excellent vacuum extraction capacity which removes sand, dirt, pollen, mold spores and even the majority of the cleaning detergents left by previous cleaners. By the way, this special hot water extraction equipment leaves carpets almost as dry as dry chemical systems.

Having just mentioned dry chemical cleaning may raise the question, can dry systems deliver healthy cleaning? NO. Dry chemical methods don't use hot water, or powerful vacuum extraction to remove dirt, pollens, or mold spores. In fact, dry chemical systems leave the majority of their chemicals in the carpeting. This chemical residue can become a fine powder which ends up as an airborne contaminate. Do not be fooled by clever franchise advertising, or misrepresentations: dry chemical cleaning is not a healthy system of carpet cleaning. Persons with allergies should avoid dry chemical carpet cleaning. In fact, 98% of the carpet manufacturers recommend only using the hot water extraction method to clean their products and maintain their warranty. Many of them REQUIRE hot water extraction at least once a year to keep the warrantee valid! If a bonnet is used on your cut-pile carpeting by a dry chemical cleaner, it may void the warrantee!

Let the manufacturers speak for themselves:

"Do not use spin bonnet or other rotary systems on cut pile carpeting"
Axminster carpet, "How to avoid the most common problems"

"Non-approved methods:...Bonnet Method...Disadvantage: This method only affects the surface of the carpet. The buffing action can distort the carpet pile and grind dirt deeper into the pile, and the spinning action of the pad can distort face yarn."
Collins & Aikman floorcoverings, care & maintenance program for Powerbond Products.

"Bonnet cleaning systems...Shaw Industries does not recommend this cleaning system. The bonnet system has a very limited capability for soil removal and leaves much of the detergent in the pile since it employs no real extraction. As a result, rapid resoiling often occurs. Another disadvantage of this system is that the spinning bonnet may distort the pile fibers of cut pile carpet, leaving distinct swirl marks."
Shaw Industries, The world's largest carpet manufacturer.

The informed carpet cleaner is aware of the valuable service he provides. These cleaners are concerned with your indoor environment and they apply the latest cleaning techniques to protect your health. Many professional carpet cleaners also use and sell HEPA vacuums, some offer air purification equipment. Others provide bed-mattress cleaning and a variety of services and products to help maintain your home. They know that wall-to-wall carpeting is a giant filter that traps dust, pollens and other pollutants. They also understand that this is not a bad thing. In rooms with bare floor paticulates remain airborne, or are easily made airborne just by walking through the room. Having a carpet serve as a filter keeps things out of the air you are breathing. The only problem with having such a large filter in your home is the fact that it has to be cleaned by the correct method.
Carpets that are properly cleaned using the hot water extraction method at least twice each year can actually provide a healthier indoor environment than a bare floor. Persons who suffer from dust mites and pollen might consider having hot water extraction carpet cleaning twice each year to protect their health. By the way, frequent carpet cleaning when done properly will also extend the life of a carpet.