In backward curved impeller, the blade elements are not radial but actually curve backward from the direction of rotation. The wheels of this type produce very high efficiency but do not have as high a pressure ratio for a given diameter and speed as the 90° radial wheels. Strength is inherently less than that of the 90° radial wheel because the centrifugal force at high speed tends to bend the blades at their toots. Due to the low-pressure ratio for a given speed and the inherently lower strength of this type wheel, it is not normally used at high-pressure ratios. Usually these impellers are used for suction of dusty air, suction of gas/fumes, room ventilation, dust transport and air handling unit.