Constraints in the ω_b-Y_P plane from CMB and abundance measurements. The CMB constraints are for Planck+WP+highL (red 68% and 95% contours) in ΛCDM models with Y_P allowed to vary freely. The horizontal band shows observational bounds on ⁴He compiled by Aver et al. (2012) with 68% errors, while the grey region at the top of the figure delineates the conservative 95% upper bound inferred from Solar helium abundance by Serenelli & Basu (2010). The green stripe shows the predictions of standard BBN for the primordial abundance of ⁴He as a function of the baryon density (with 68% errors on nuclear reaction rates). Both BBN predictions and CMB results assume N_eff = 3.046 and no significant lepton asymmetry.