CMB data place independent constraints on the primordial helium abundance, Y _p . Left panel: the contours show the two-dimensional 68% and 95% confidence intervals for Y _p and n _s for three data sets: SPT, WMAP 7, and SPT+ WMAP 7. The BBN prediction for three neutrino species is marked by the horizontal black dashed line at Y _p = 0.2478. The change in the damping scale due to varying Y _p can be mimicked by changing n _s for the SPT data. WMAP 7 provides an independent measurement of n _s , greatly enhancing the SPT+ WMAP 7 measurement of the helium abundance. Right panel: the marginalized one-dimensional posteriors for Y _p in different model and data combinations. We show constraints from the SPT (blue dot-dashed line), WMAP 7 (blue dashed line), and SPT+ WMAP 7 (black solid line). The BBN prediction for three neutrino species is marked by the vertical black dashed line at Y _p = 0.2478. The orange solid line shows the marginally wider BBN prediction from the ΛCDM+ N _eff MCMC chain. The gray area is ruled out by the 95% upper limit from the protosolar helium abundance. The preference of the CMB for higher helium abundances can be reduced by introducing other extensions that affect small scale power, as is demonstrated in Section 9.1.