During one night, human loses approximately 0.5-0.75 l of water (Grbac 2006). This results in a significant increase in water vapour content inside the mattress (Svennberg and Wadso 2005). Cunningham’s research (1999) indicates that the relative moisture of the mattress both in its lower and middle layer may rise even above 70 %.
Therefore, it is important that the materials that make up the spring, lining and cover layer, as well as bedding, have the ability of very good absorption and giving out of moisture (steam and sweat). Residual moisture at the appropriate temperature may well encourage the development of different kinds of fungi. According to Svennberg and Wadso (2005), due to the ability of the mites to absorb water vapour from the air thanks to the salt compounds, relative moisture above 58 % at a temperature of 20 °C is dangerous. Therefore, it is recommended to use solutions enabling users to change the side of the mattress depending on the season, which is conditioned by the quantity and speed of the given out moisture. A study carried out by Svennberg and Wadso (2005) confirms that with the increase in the thickness of the mattress (made with polyester foam and cotton cover), its quasi-insulation properties increase, and the combination of different materials significantly affects the microclimate in the immediate vicinity of the human body during sleep. Relative humidity of the air in the room, despite the migration of moisture within the mattress, does not undergo significant changes. Together with the loss of moisture, human gets rid of the old cuticle through decortication, which causes that the bed becomes over time an ideal habitat for mites, producing one of the strongest allergens—a protein enzyme. It is therefore necessary to use such materials that can prevent the settling of micro-organisms or at least reduce this phenomenon. An example might be a coconut mat or foam, which has antibacterial and antiallergic properties, or one of the newest materials—a special 3D, light and flexible polyester mesh DryMesh. This material has been designed so as to allow free airflow, reduce moisture condensation and thus reduce the likelihood of the formation of mildew. This mesh, due to its properties, is also used in yachts and boats.
Softness of the mattress
The human changes the position of the body about 40-50 times during sleep, assuming—depending on the gender, age, mobility and preferences—up to 21 positions (Grbac and Domljan 2007). This restless sleep is associated most often with the wrong degree of softness of the materials used in the upholstery layers.
After reaching a certain local border value of pressure, human feels uncomfortable and tries to fix this by changing the body position. According to Grbac (2006), the pressures on the body cannot exceed 28 mmHg. Krutul (2004b) and Defloor (2000) suggest that the pressure above 32 mmHg causes closing the lumen of the smallest blood vessels—capillaries and is so-called border value for the formation of compression ulcers. Beldon (2002), after Fronek and Zweifach (1975), reports that the limit of pressure for the human body amounts to 30-35 mmHg. The same author cites research of Bennett et al. (1981), which confirms that for ill people, compression ulcers already start occurring at the pressure of 22 mmHg. Usually, pressure below 45 mmHg can be treated as so-called relieving pressure, and pressures below 32 mmHg are called “reduced pressures”. Presently—in terms of the value of pressures on the body, unequalled are waterbeds. Studies show that stresses occurring around the pelvis bones have the highest values that amount to 25 mmHg. For comparison, in an ordinary mattress, pressures in this point have a value of about 58 mmHg (Grbac 2006). The disadvantage of this type of solution is the need of using special disinfectants and relatively big weight. A mattress with the size of 2000 x 1600 mm can weigh even about 200 kg (Meier 1998).
During sleep, neuromuscular activity is at a low level and the major force acting on the body and partly on the spine is gravity. This force causes deformations of the soft tissue during the rest on the mattress. In the work of Normand et al. (2005), the method of measuring pressures that occur in the spine has been presented, with and without lumbar support, with the use of a pneumatic mattress in variable laboratory conditions. It was concluded that lumbar support causes more uniform support above the hips, in the lumbar and thoracic area, and causes lumbar lordosis when lying on the back (Table 3.17). Quality of support in six different experimental conditions was examined. These conditions were changing in such a way as to
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Table 3.17 The average pressure force (N) in different places of the body support (Normand et al. 2005)
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Loins
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Hips
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Breasts
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reflect the body support on a bed without mattress, with foam and with mattress as well as when supporting loins with an air cushion or not. Pressures on the body were measured by the sensory compression mat Tekscan. On the basis of the studies, it has been shown that without supporting the loins with an air cushion, the pressures focus mainly in the area of the hips. And during a gradual filling up the cushion with air, a gradual, even increase of pressures in the lumbar area and alignment of pressures around the hips and breasts occurred. Such support reduces the pressures in the lumbar area of the spine.
Park et al. (2001) showed that critical assessment of a comfortable mattress during its use should include the assessment of the shape of the spine line, the size and the distribution of pressures on the human body and the manufacture class of the mattress (including the quality of the mattress). Deformations of the spine line were studied by comparing the spatial (3D) image of the shape of the spine of the user during sleep with the 3D shape of the spine of a standing person, with the method RMS (Root Mean Square). The pressures were studied using a sensometric mat, and the comfort of the mattress was evaluated on the basis of the survey responses of the mattresses users. Studies have shown that the mattress providing shape of the spine line as similar to the shape kept in a standing position as possible is the most comfortable. The larger the differences in the shape are, the less comfortable the mattress is. Properly constructed bed should therefore support the spine in a continuous manner, without unnecessary implementation of a shared sleeping surface. The function of a uniform distribution of reaction forces of the base on the body should be maintained for various positions, and when changing the position of the body protuberances, the deformation should occur on small radii of curvature with simultaneous support of the entire body (Kapica 1993a).
In the construction of an ergonomic mattress, it is therefore important not only to properly support the body in various positions, but also zonal differentiation of stiffness of the mattresses. This involves uneven distribution of pressures generated by the base. Studies have shown that the greatest pressures occur always around the shoulders, hips, and elbows, and the smallest pressure accumulates around the knees and ankles (Buckle and Fernandes 1998). It is also confirmed by a study carried out by Defloor and Schuijmer (2000).
Currently known construction solutions of mattresses require an individual approach to the issue of selection of mattress for the user, which is associated with subjective feeling of its softness or hardness, and softness is not the only criterion of assessing the quality of upholstered furniture. Such a determinant can also be the stiffness coefficient k, clearly determining the hardness of the examined system (Smardzewski 1993a).
Durability
Durability of the mattress depends on the degree of exploitation of the product, which has a direct connection with the properties of the materials used and the quantity of absorbed and retained moisture. The increase in the number of loading cycles (0-5000) decreases the stiffness coefficient by about 6 %, regardless of their construction (Kapica 1993b). Although methods of improving elastic properties of biconical springs are known, which have an impact on reducing permanent deformations (Kapica and Smardzewski 1994), durability of mattresses is estimated for about 10 years, but an exchange (for hygienic reasons) is recommended after about 5 years of use. In addition, by building the so-called layered systems of foams of various compressibility, one can adjust the pliability of the upholstery layer made of polyurethane foam (the degree of regulation of the pliability of the system depends on the diversification of compressibility of the component foams, and is the larger, the bigger this diversification is), and the characteristics of deformation of the systems depend on the array, their thickness and positioning in relation to one another (Kapica and Pechacz 1983).
Dangers
The extensive use of foam materials as the filling of elastic layers (ousting the use of springs) causes certain dangers, related to their chemical structure. Studies of Kozlowski et al. (1988) showed that during combustion of upholstery materials (wool, silk, flax, cotton and polyurethanes), very dangerous products in terms of toxicity are emitted, e. g. carbon dioxide, carbon monoxide, cyanates, isocyanates or hydrogen cyanide (polyurethanes), and depending on the material, the temperature of the glow amounts to 185-340 °C (Navratil and Osvald 1997). It is therefore necessary to choose solutions that will to the highest possible degree minimise the risk of sustaining of glow or fire. The covering layers must be absolutely made of materials with flame-resistance clearance.
