Analysis of Maryland appellate case regarding 'substantial factor' anslysis in asbestos litigation. Testimony of plaintiff expert Dr. Laura Welch was thrown out by the Court of Appeal (reversing the lower court) because her opinions that every exposure adds to the risk of disease, the risk of disease is proportionate to the exposure, and there is no safe level of exposure, were not helpful to a jury because the plaintiff had to show that the exposure was more than a negligible exposure, which Dr. Welch's opinions did not address.