Hot Water Extraction Cleaner
This is the old fashioned steam cleaner. It's the kind of unit that your Mom might have rented from your local supermarket when you were a kid.
In fact, the term "steam cleaner" for these types of units is a misnomer. Some "steam machines" use gallons of water in the cleaning process and virtually no steam. As a result, carpet may be left extremely damp, needing hours to dry completely. For those with allergies to dust mites and mold, this can be a significant problem. Damp and wet carpets may encourage the growth of these pests.
So, why would you ever use a hot water extraction cleaner? The biggest advantage of these cleaners is that they can tackle a larger surface area more effectively and more quickly. Most people will prefer hot water extraction units for cleaning very large dirty areas of carpet.
If you have a large area of carpet to clean, and want to use a hot water extraction unit, consider a 'dust mite' additive in the cleaning solution. There are also special additives for other allergens like pet allergens. Keep in mind that additives may not be right for you, if you have chemical sensitivities. When renting the unit, take a few minutes to discuss what options you might have as an allergy or asthma sufferer.
You can also buy sprays of various sorts that will inhibit mold, mildew, dust mites and fungi that can be applied after cleaning. What if you only have small carpets or area rugs? You could consider vapor steam cleaning. Vapor steam cleaning of carpets, while a slow process, is best used for the upkeep of new or lightly soiled carpets. The advantage is that it doesn't leave carpets wet as a result of cleaning.