An air filtration system is described for removing oil and grease entrained in air extracted from the region above a cooker. The system comprises a first mesh screen filter leading to a first chamber, in which two nozzles 10 create two separate continuous water curtains 15a, 15b which overlap when viewed in the direction of the air flow to form a combined curtain extending across the region through which the air passes. The extracted air then passes via a second mesh screen filter 11 into a second chamber 12, identical to the first chamber 9, in which two further nozzles 13 are provided which create two further water curtains 15c, 15d. The water curtains 15a, 15b, 15c, 15d cool and condense vaporized oil and grease within the air, which condensed oil and grease droplets fall with the water into a container. The main bore of each nozzle is of circular cross-section, but a profiled plate causes the resulting water curtain to have a width which is narrow relative to the diameter of the nozzle bore. The water contains dissolved ozone and can also be “grey water”.