  Khatri and Wandelt reported that change in the value of alpha by 1% changes
the mean brightness temperature $T_b$ decrement of the CMB due to 21 cm
absorption by 5% over the redshift range z $<$ 50. A drawback of their approach
is that the dimensionful parameters are used. Changing of units leads to the
change of the magnitude and even sign of the effect. Similar problems may be
identified in a large number of other publications which consider limits on the
variation of alpha using dimentionful parameters. We propose a method to obtain
consistent results and provide an estimate of the effect.
