useful for campers who carry their equipment as, unlike with normal air mattresses, no pump is needed for inflating. Available brands include Aerobed, Coleman, Therm-a-Rest and others. The U.S. Consumer Product Safety Commission advises consumers not to let infants sleep on air mattresses. This is motivated by reports of deaths, mostly infants younger than 8 months of age, who were placed to sleep on air mattresses, and either suffocated in a face down position on an air mattress or died due to suffocation after falling into gaps between the mattress and bed frame, or the mattress and adjacent furniture or wall. Waterbeds A waterbed is a mattress with water in its interior instead of metal coils or air. Waterbeds can be lined with different layers of fiber to achieve the level of firmness the user desires. Waterbeds are well known for providing support to the spine and other body parts, similar to the other mattress types. There are several options of support which range up to 100% waveless, where the user does not notice that they are lying upon a waterbed. Quality Many parameters determine the quality of a mattress. Laboratory test methods have been established for some of these parameters, such as pressure distribution, skin microclimate, hygiene, edge support, and long-term stability. Some of these have been developed by Duncan Bain, working on behalf of the UK's Medicines and Healthcare products Regulatory Agency. Other parameters, such as firmness, are more specific to the sleeper. In general, firm mattresses are recommended for stomach and some back sleepers, soft mattresses are recommended for side sleepers, and medium mattresses are recommended for the majority of back sleepers. Double mattresses are available with a softer and a firmer part, or with adjustable firmness levels, to accommodate sleepers with different preferences who share a bed. Ergonomics In 2003, a randomized-controlled trial found that medium-firm mattresses assessed using the Hs scale from the European Committee for Standardization were associated with less pain; this study has been cited by clinical practice guidelines on lower back pain. In 2015, a systematic review of studies concluded that medium-firm, custom-inflated mattresses were best for pain and neutral spinal alignment. Lifespan The term mattress lifespan refers to the duration in which all types of mattress can retain their original support and comfort. Mattresses deteriorate over time, and the lifespan of a mattress depends on a variety of factors, notably materials,