A small amount of water can go a long way toward reducing dust if you have to cut concrete with a circular saw and a diamond blade. For dust control in these situations, I rely on a 5-gal. drywall-compound bucket with a hole in the side. As shown in the drawing below, I fill the bucket with water and place it adjacent to the cutline. As the water slowly dribbles out, it spreads over the cutline, practically eliminating dust as I make cuts.
I move the bucket as necessary to keep the cutline irrigated, and if I need to stop the flow, I stick a nail in the hole to plug it. If you use this technique, be sure to protect yourself against electrical shock by plugging the saw into a GFCI-protected circuit.
John Lindsay Jr, Philadelphia, PA