  Small-scale inhomogeneities in the baryon density around recombination have
been proposed as a solution to the tension between local and global
determinations of the Hubble constant. These baryon clumping models make
distinct predictions for the cosmic microwave background anisotropy power
spectra on small angular scales. We use recent data from the Atacama Cosmology
Telescope to test these predictions. No evidence for baryon clumping is found,
assuming a range of parameterizations for time-independent baryon density
probability distribution functions. The inferred Hubble constant remains in
significant tension with the SH0ES measurement.
