Context. Accreting planetary-mass objects have been detected at H α , but targeted searches have mainly resulted in non-detections. Accretion tracers in the planetary-mass regime could originate from the shock itself, making them particularly susceptible to extinction by the accreting material. High-resolution ( R > 50 000) spectrographs operating at H α should soon enable one to study how the incoming material shapes the line profile. Aims. We calculate how much the gas and dust accreting onto a planet reduce the H α flux from the shock at the planetary surface and how they affect the line shape. We also study the absorption-modified relationship between the H α luminosity and accretion rate. Methods. We computed the high-resolution radiative transfer of the H α line using a one-dimensional velocity–density–temperature structure for the inflowing matter in three representative accretion geometries: spherical symmetry, polar inflow, and magnetospheric accretion. For each, we explored the wide relevant ranges of the accretion rate and planet mass. We used detailed gas opacities and carefully estimated possible dust opacities. Results. At accretion rates of Ṁ ≲ 3 × 10 −6   M J yr −1 , gas extinction is negligible for spherical or polar inflow and at most A H α ≲ 0.5 mag for magnetospheric accretion. Up to Ṁ ≈ 3 × 10 −4   M J yr −1 , the gas contributes A H α ≲ 4 mag. This contribution decreases with mass. We estimate realistic dust opacities at H α to be κ ~ 0.01–10 cm 2 g −1 , which is 10–10 4 times lower than in the interstellar medium. Extinction flattens the L H α – Ṁ relationship, which becomes non-monotonic with a maximum luminosity L H α ~ 10 −4   L ⊙ towards Ṁ ≈ 10 −4   M J yr −1 for a planet mass ~10  M J . In magnetospheric accretion, the gas can introduce features in the line profile, while the velocity gradient smears them out in other geometries. Conclusions. For a wide part of parameter space, extinction by the accreting matter should be negligible, simplifying the interpretation of observations, especially for planets in gaps. At high Ṁ , strong absorption reduces the H α flux, and some measurements can be interpreted as two Ṁ values. Highly resolved line profiles ( R ~ 10 5 ) can provide (complex) constraints on the thermal and dynamical structure of the accretion flow.