A positive correlation has been found between the amount of soot in the atmosphere of cities and the frequency of a certain ailment among those cities' populations
However, the soot itself probably does not cause this ailment, since in cities where there are large amounts of soot in the air, there are usually also high concentrations of many other air pollutants
In each of the cities where there are large amounts of soot in the air but little other air pollution, the frequency of the ailment is at least as high as it is anywhere else.