In a recent paper (1), we related a slight, mostly reversible reduction in nerve conduction velocities (NCV) in the arms of tunnel workers to exposure to acrylamide and N-methylolacrylamide. We would like to thank Dr Cornblath for his suggestion of another possible mechanism, that of ulnar neuropathy from external compression due to the use of handtools and the like. On an individual basis, we obviously cannot exclude that some of the reduced NCV findings could be related to mechanical exposure factors such as the use of handtools and not to acrylamide exposure. On a group basis, however, we do not think this is a likely explanation, for the following reasons. First, the optimal reference group in epidemiology is comparable to the index group, except for the actual exposure under study. Our choice of other tunnel workers, doing the same type of work, but not exposed to acrylamide grout as referents, would fulfill these requirements. In detailed interviews with the participants, the reference group reported the same number of years in construction work as the index group (20.4 years versus 19.3 years), almost all had used vibrating handtools (91.9% versus 100%), and for the same length of time (7.9 years versus 6.0 years). This information gives no support to potential incomparability between the two groups in relation to the use of handtools. Furthermore, after 12 months of follow-up, when both groups were still working as construction workers (and still exposed to vibrating handtools), we found significant improvement in the NCV of the exposed group, which, for both the median and the ulnar nerves, was related to qualitative indicators of acrylamide exposure ("tunnel time" and "exposure time index"). In these intraindividual comparisons, we believe that between-group confounding (eg, from the use of handtools) should be avoided.

In general, tunnel workers are exposed to a variety of exposure factors, which could, in part, confound the observed effects. Although we did not obtain a complete comparability between the two groups in all aspects (see table 1 of the original article), we still believe that the chosen design secured valid inter- and intraperson comparability between the groups, and, in particular, in relation to the aspect of mechanical compression as an alternative explanation of our results.

Key terms grout worker; letter to the editor; neuropathy