Swimming pools range in size from small inflatable pools to large Olympic size pools, and the even larger pools now used at water parks. Pools can be classified as either above-ground or in-ground; however, there are variations of these classifications which are designed and constructed to fit the circumstances. Above-ground pools are generally constructed of vinyl and metal or wood; they are easy to build and less costly to construct than in-ground pools, but are less durable. In-ground pools can be constructed of a number of different products, including fiberglass, steel and concrete. Pools constructed inside buildings require appropriately designed equipment to ensure appropriate humidity levels and the required ventilation.