Mechanical cleaning may involve abrasive blasting, wire wheels, sandpaper/sanding belts, grinding wheels, or abrasive chips in tumbling mills. To capture and entrain the large heavy particles produced by cleaning the required capture velocity is often very high with collection hoods positioned to direct materials towards the hood. Once captured, general practice recommends a minimum duct transport velocity of 1066 m/min / 3500 fpm (but 1219 m/min / 4000 fpm to 1524 m/min / 5000 fpm is often necessary). A hood that encloses as much of the operation as practical is generally desired. In situations where the material removed has toxicity concerns, complete enclosures may be necessary or workers may need to wear respirators in addition to using local exhaust ventilation.