Hexavalent Chromium Study’s Unjustified Conclusions
This letter was published in Occupational and Environmental Medicine.
To the Editor: Luippold et al[1] assert that their study of hexavalent chromium, a known lung carcinogen, demonstrates an “absence of an elevated lung cancer risk” in workplaces in which hexavalent chromium exposures had been reduced by changes in production processes. Their data do not support this conclusion for the following three reasons.
First, the impact of the well known selection bias known as the “healthy worker effect” on the results of lifetable analyses is readily apparent in this study. The magnitude of this bias can be assessed by the proximity of the standardized mortality ratio (SMR) for “all cause” mortality to 1.00. In the Luippold study, the SMR for all causes combined is 0.59 (95% confidence interval _ 0.39–0.85), as it is for deaths from heart disease, the largest subcategory. This powerful bias would mask anything but a very large increase in lung cancer risk.
Second, the length of follow-up in this study is too short to permit meaningful conclusions. Not a single worker in Plant 2 had been followed for more than 18 years (the average length of follow-up was 10 years), and 40% of the Plant 1 population had been followed for fewer than 10 years. Because, as the authors themselves state, “the average latency for lung cancer may be longer than 20 years,” this study would be extremely unlikely to detect a real increased risk of lung cancer.
Finally, this study is very small, with a total of 27 observed deaths from all causes. Given that the expected number of lung cancer deaths is a paltry 3.75, the chromium exposure would have to more than double the risk of lung cancer, and nine lung cancer deaths would have had to have been observed before the SMR would have reached a level of statistical significance based on the Poisson distribution (SMR 2.4; P _ 0.05). But even in the Gibb study, the most robust study of hexavalent chromium carcinogenicity, the SMR in the highest exposure group, in which exposures were approximately 15 times greater than the present study, was only 2.24 (95% confidence interval: 1.60–3.03).[2] Because the Luippold study could not have detected an increase in lung cancer even at the highest exposure level in the Gibb study, it is obviously statistically underpowered to detect any increase in cancer at lower exposure levels.
In summary, the conclusions promoted by the authors of this study are not supported by their data. The Occupational Safety and Health Administration currently is promulgating a new hexavalent chromium exposure standard.[3] Several industry groups[4] cited the Luippold study in their comments opposing the OSHA proposal, although it provides essentially no useful information in understanding the lung cancer risk associated with hexavalent chromium exposure.
Altoon Dweck, M.D., M.P.H.
Researcher
Public Citizen’s Health Research Group
Peter Lurie, M.D., M.P.H.
Deputy Director
Public Citizen’s Health Research Group
David Michaels, M.D., M.P.H.
Department of Environmental and Occupational Health
The George Washington University School of Public Health and Health Services
Sidney Wolfe, M.D.
Director
Public Citizen’s Health Research Group
References
1. Luippold RS, Mundt KA, Dell LD, Birk T. Low level hexavalent chromium exposure and rate of morality among US chromate production employees. J Occup Enviorn Med. 2005;47:381–385.
2. Gibb HJ, Lees PSJ, Pinsky PF, Rooney BC. Lung cancer among workers in chromium chemical production. Am J Ind Med. 2000;38:115–126.
3. Occupational Safety and Health Administration, US Department of Labor. Occupational Exposure to Hexavalent Chromium. 69 Federal Register, October 4, 2004, 59306-474.
4. The Society of the Plastics Industry, Inc. Post-hearing comments, (Docket H054A Exhibit #47-24-1), April 20, 2005; Collier Shannon Scott on behalf of the Specialty Steel Industry, Exhibit #47-27-1, April 20, 2005; Surface Finishing Industry Council, comments prepared by The Policy Group, (Exhibit #47-35-1), April 20, 2005.