An air compressor (10) has a reservoir (12), pump (14), motor (16), transmission belt (18) wound around a driving pulley (20) coupled to a motor shaft (16) and around a driven pulley (22) coupled to a pump shaft. Driven pulley spokes (28) are shaped as fan blades transmit torque and cool the pump (14). A belt guard (30) encloses the transmission belt (18), and pulleys (20, 22). The belt guard (18) has a front wall (32) with slits (34) for air drawn by the driven pulley (22), a side wall (36) enclosing the space around the transmission belt (18) and around pulleys (20, 22), and a rear wall (38) having an opening (40) receiving the pump (14). Air drawn through the slits (34) in the front wall (32) passes towards the pump (14). The guard rear wall (38) covers the pump (14) by at least 30% of its depth.