Cigarette maker Philip Morris spent years studying whether additives, such as menthol, added to the toxicity of their smokes. And several published studies—conducted by the company—have claimed that the additives had no impact on the danger of their products.

But thanks to lawsuits against the tobacco industry, a trove of previously secret scientific and corporate documents about the research have been made public. And a multidisciplinary team of researchers, led by Marcia Wertz, of the Center for Tobacco Control Research at the University of California, San Francisco, decided to take a closer look to see if the company's findings and methods were more smoke and mirrors than solid science.

Philip Morris's internal analysis examined 333 cigarette additives, as described in a 2001 report by the company, and found no "meaningful effect of the ingredients on the toxicity of cigarettes." Wertz's study, however, found something quite different, as detailed online Tuesday in PLoS Medicine.

Compared with the tobacco company’s published findings, the external review found toxicity and particulate matter increased with additives. The team also found internal documents that indicated that the statistical analyses were changed after initial findings seemed unfavorable. The change adjusted for a jump in toxicity by pegging it to particulate matter concentration, which also increased with additives. This shift "obscured this underlying toxicity and particulate increase," Wertz and her colleagues wrote in their paper.

That large companies work over their internal stats to their advantage might not come as much of a surprise. And many anti-smoking advocates have found claims that the health impacts of cigarette additives were neutral difficult to swallow. But the team behind the new paper noted that their finding should be a reminder that internal studies might not always be airtight—and added that the discrepant internal versus external findings as "a case study of tobacco industry scientific research being positioned strategically to prevent anticipated tobacco control regulations."