Canopy-mounted spray systems. Most of the dust liberated by shield movement comes from the canopy area of the shields during advance. A canopy spray system that activates sprays discharging into the roof material on top of the shields for a short period of time before and during shield advance has been available for many years. The goal is to wet the material on top of the canopy to lower dust levels during shield advance. Unfortunately, experience has shown that this type of system is hard to maintain and is not effective in distributing moisture to the material on top of the shield canopy.