How far do I need to be away from someone else creating dust to be safe?

It is very difficult to give an answer that will cover all situations. There are though a few good rules of thumb. If working outside you should stay away from the area around the dust cloud and take care that any wind does not blow this in your direction. If working indoors, this is more difficult as the dust levels will build up. In general it is best to stay away from the area where the dust is until the air is clearer. However, this can take a long time as the fine dust can stay suspended in the air for a long time. The best thing is always to stop the dust getting into the air in the first place. Tell someone in charge that the correct controls are not being used.