Carpet cleaning is performed to remove stains, dirt, and allergens from carpets. Common methods include hot water extraction, dry-cleaning, and vacuuming. Hot water extraction Hot water extraction, sometimes mistakenly called "steam cleaning", uses equipment that sprays hot water and detergent into the carpet and extracts it along with any dislodged and dissolved dirt. Many experts recommend hot water extraction as the most effective carpet cleaning method. Hot water extraction equipment may be a portable electrical units, or for large jobs truck mount carpet cleaner with long hoses may be used as this may be faster and more effective than portable equipment. A common process of hot water extraction begins with preconditioning. Alkaline agents such as ammonia solution for synthetic carpets, or mild acidic solutions such as dilute acetic acid for woollen carpets, are sprayed into the carpet, then agitated with a grooming brush or an automatic scrubbing machine. Next, a pressurized manual or automatic cleaning tool known as a wand passes over the surface to rinse out all pre-conditioner, residue, and particulates. If an alkaline detergent is used on a woollen carpet, a mild acetic acid solution will neutralize the alkaline residues and restore neutral fiber pH. Thorough extraction is important to avoid problems such as mold growth and browning of wool. Carpet cleaning specialists try to find a balance between rapid drying (using less fluid) and the need to remove the most soil (using more fluid). Drying time may also be decreased by fans, de-humidifiers or simply opening windows. Dry-cleaning Many dry carpet-cleaning systems rely on specialized machines. These systems are mostly "very low moisture" (VLM) systems, relying on dry compounds complemented by application cleaning solutions, and are growing significantly in market share due in part to their very rapid drying time, a significant factor for 24-hour commercial installations. Dry-cleaning and "very low moisture" systems are also often faster and less labor-intensive than wet-extraction systems. Heavily soiled areas require the application of manual spotting, pretreatments, preconditioners, and/or "traffic-lane cleaners", (commonly sprayed onto carpet prior to the primary use of the dry-cleaning system) which are detergents or emulsifiers which break the binding of different soils to carpet fibers over a short period of time. For example, one chemical may dissolve the greasy films that bind soils to the carpet, and thus prevent effective soil removal through vacuuming. The solution may add a solvent like d-limonene, petroleum byproducts, glycol ethers, or