It's not smoke, it's dust from sawing up concrete slabs. It's M's clever solution to my paving conundrum but it's definitely bad for the respiratory system. I kept well back and will dig the residue into the soil; M was inevitably close and unfortunately is more concerned about my safety (which he does not need to worry about - I know a fair bit about construction health and safety) than his own. I do comment from time to time, and we've even had a discussion about a friend of his who was working in the HSE car park under the watchful eye of cameras in the office windows, but like most of us (including sedentary keyboardists, those who stand on chairs to change lightbulbs, brick-carriers...), he underplays the risks.