Mountain rescue services may be paid professionals or volunteer professionals. Paid rescue services are more likely to exist in places with a high demand such as the Alps, national parks with mountain terrain and many ski resorts. However, the labor-intensive and occasional nature of mountain rescue, along with the specific techniques and local knowledge required for some environments, means that mountain rescue is often undertaken by voluntary teams. These are frequently made up of local climbers and guides. Often paid rescue services may work in co-operation with voluntary services. For instance, a paid helicopter rescue team may work with a volunteer mountain rescue team on the ground. Mountain rescue is often free, although in some parts of the world rescue organizations may charge for their services. But there are also exceptions, e.g. Switzerland, where mountain rescue is highly expensive (some 2,000 to US$4,000) and will be charged to the patient. In more remote or less-developed