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Khatri and Wandelt reported that change in the value of alpha by 1% changesthe mean brightness temperature $T_b$ decrement of the CMB due to 21 cmabsorption by 5% over the redshift range z $<$ 50. A drawback of their approachis that the dimensionful parameters are used. Changing of units leads to thechange of the magnitude and even sign of the effect. Similar problems may beidentified in a large number of other publications which consider limits on thevariation of alpha using dimentionful parameters. We propose a method to obtainconsistent results and provide an estimate of the effect.

Comment on "21-cm Radiation: A New Probe of Variation in the Fine-Structure Constant"