The EU (European Union) also has similar symbols to indicate safety standards for children travelling in a vehicle. As an EU member states, products used in Spain should comply with European Union single market definitions. As it was an EU member state at the time laws were introduced, products used in United Kingdom should comply with European Union single market definitions. From September 18, 2006, all children under the age of 12 have to use some form of child car seat, unless they are taller than 135 cm (4 ft 5in). Though there are hundreds of variations of makes and models in the world of child safety seats, the materials used in the manufacturing process are basically the same. Factories in which the seats are put together receive loads of polypropylene pellets. Foam makes up the padding of the individual seats, while vinyl and fabrics are used to make up the covers for the seats as well as the harnesses. A safety seat increases the safety of a properly restrained child in the case of a motor vehicle accident. The safety seat includes foam padding, fabric covers, a harness, and buckles or attaching mechanisms. Labels and instructions are also attached. Every child safety seat will have an expiration date on it. The Safe Kids USA organization does not recommend using a child safety seat that is more than 6 years old. Periodically, child safety seats are recalled by manufacturers for safety reasons. The National Highway Traffic Safety Administration posts a link to recent recall information at nhtsa.gov. There are different types of child safety seats for children of different sizes and ages. Manufacturers have quality controls to ensure seats are properly put together and packaged. However, it is not guaranteed that the included instructions are always adhered to and correctly followed. Up to 95% of the safety seats that are installed may not be the right seat for the child, may be hooked into the vehicle loosely, may be hooked with an incompatible belt in the vehicle, may have harnesses incorrectly fastened in some way, or may be incorrectly placed in front of air bags. In 1997, six out of ten children who were killed in vehicle crashes were not correctly restrained. Along with the problem of instructions not being followed properly, there are other hazards that can affect children involving these safety seats. A recent study attributed many cases of sudden infant death syndrome (SIDS) to the prolonged sitting or lying position these infants are in when putting the safety seats to use. When researchers reviewed more than 500 infant deaths, it was found that 17 of these deaths occurred while the infant was in a device such as a child safety seat.