Intensities of the first rotational lines of H₂ predicted by the same PDR model (P = 10⁷ K cm^-3 and χ = 10³) for the four prescriptions for ortho-para conversion on grains: no conversion (purple), rate equation approach neglecting the fluctuations (green), statistical approach taking the fluctuations into account (blue), and full conversion for H₂ molecules sticking to grains (red).