Volatile shielding conditions in the B_wd–P_wd parameter space. The shaded area between the solid and dashed lines indicates the zone where the shield mechanism is in effect. The black solid lines represent the magnetosphere condition, above which R_mag is larger than R_cor, ensuring that gas is shielded from the magnetosphere of the white dwarf. The black dashed lines represent the dry-out condition, below which the icy fragments of a comet can be totally dried out before they cross the corotation radius R_cor in the process of orbital circularization. The shaded region could expand according to different assumptions on the accretion rate and the pericenters of fragments. The dashed lines are based on the Alfvén-wave drag-induced orbital circularization, so in other scenarios of circularization, these dashed line are not valid. The shaded region should also depend on the material properties of planetary pollutants, so there is a large uncertainty on the region above the solid lines. DBA white dwarfs WD0041-102 and WD0853+163 are out of the shielding regime, consistent with the detection of hydrogen in their atmospheres. DB white dwarf WD0912-536 is also open to volatiles, but no hydrogen is detected. This might be the result of no planetary system or some unknown mechanism. The blue dotted line denotes the lower limit of the observation on the surface magnetic field of white dwarfs. WD2326+049 has several measurements in the rotational period, two of which are in the volatile-shielding region.