Resonant atoms modify the electromagnetic environment that in turn determines their optical response, giving rise to a self-consistent light-matter problem at finite density. We investigate this feedback for the hydrogen Lyman-$\alpha$ transition using a local effective-medium description combined with an electromagnetic Green's function treatment of the atomic self-energy. Within the controlled regime, the self-consistent dressing produces a systematic density-dependent modification of the spectrum, including a shift of the spectral centroid and an asymmetric redistribution of integrated spectral weight. Comparison with a density expansion through fourth order provides an internal validity test: The local description remains quantitatively controlled up to approximately $2.0\times {10}^{21}{\mathrm{m}}^{-3}$ and becomes marginal above this density. Formal continuation to higher densities produces much stronger spectral restructuring, but this occurs together with the loss of perturbative and local-response control and is therefore not interpreted as a quantitative prediction of resolved high-density modes. To assess experimental relevance without assuming a microscopic hydrogen collision model, we additionally introduce a parametric collisional broadening ${\mathrm{\Gamma }}_{\mathrm{coll}}=n\beta$. Collisional broadening becomes comparable to the controlled electromagnetic centroid shift for a characteristic coefficient of order ${10}^{-14}{\mathrm{m}}^3{\mathrm{s}}^{-1}$. The robust prediction of the local theory is thus a density-dependent electromagnetic dressing of the resonant response, whereas its high-density breakdown identifies the regime in which a spatially resolved, nonlocal microscopic description of collective electromagnetic coupling becomes necessary.

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• Electric dipole-dipole interactions
• Bose gases
• Quantum many-body systems