Axions originally emerged as low-energy remnants of the Peccei Quinn solution to the strong CP problem, but they also unavoidably contribute to the energy density of the Universe. In particular, they contribute to the effective number of relativistic degrees of freedom ($\Delta N_{eff}$) which is constrained by cosmic microwave background experiments. In this talk, I will discuss the main axion-thermalization channel at temperatures below that of the QCD phase transition, namely the axion-pion scattering, within the 2-flavour Chiral Perturbation Theory (ChPT). Based on leading order (LO) ChPT, the bound on axion mass from $\Delta N_{eff}$ is found to be approximately below the eV scale. However, considering the impact of NLO corrections, I will show that the perturbative chiral expansion breaks down for temperatures above 70 MeV, making the LO analysis not reliable. I will discuss how to extend the EFT validity via unitarization techniques up to the deconfinement temperature, providing a reliable determination of the relic density of thermal axions decoupling after the QCD phase transition.
