Well, your second sentence is true. But since the levels seem to rise and fall with conditions in Rome - a known polluter with known health problems from lead -

Well, your second sentence is true. But since the levels seem to rise and fall with conditions in Rome - a known polluter with known health problems from lead - it is a very likely source of those levels. Scientists create hypotheses on the basis of information available to them; if more information arrives, the hypotheses are (eventually) adjusted accordingly. They're not meant to be taken as absolute assertions.