It's slightly complicated, but I'll get onto one of my other hobby horses, for what it's worth. It's not the radon itself that causes the lung cancer; it's the so-called degradation products of the radon. Radon is an inert gas, so it doesn't attach itself to lung linings. It's when radon transforms itself by atomic degradation into a radioactive metal that the problem occurs, because that attaches to your lungs. That attaches to dust, so more dust in your house is a bad thing if you also have radon. That's really what causes the damage.
A becquerel is a unit of disintegration. One becquerel per metre cubed is one disintegration per cubic metre of space, so that 750 becquerels per metre cubed is 750 disintegrations per cubic metre of space.