  Non-localities of Foldy-Wouthuysen and related transformations, which are
used to separate positive and negative energy states in the Dirac equation, are
investigated. Second moments of functional kernels generated by the
transformations are calculated, the transformed functions and their variances
are computed. It is shown that all the transformed quantities are smeared in
the coordinate space by the amount comparable to the Compton wavelength
$\lambda_c=\hbar/mc$.
