The primordial helium abundance Y _ { p } is commonly inferred from abundance determinations in low-metallicity extragalactic \mathrm { H } \textsc { ii } -regions .
Such determinations may be subject to systematic uncertainties that are investigated here .
Particular attention is paid to two effects : \mathit { icf } -corrections for “ imperfect ” ionization structure leading to significant amounts of ( unobservable ) neutral helium or hydrogen and “ \mathit { tcf } ” -corrections due to non-uniform temperature .
Model \mathrm { H } \textsc { ii } -regions with a large number of parameters are constructed and it is shown that required corrections are almost exclusively functions of two physical parameters : the number of helium- to hydrogen- ionizing photons in the illuminating continuum Q ( \mbox { $ \mathrm { He ^ { 0 } } $ } ) / Q ( \mbox { $ \mathrm { H ^ { 0 } } $ } ) , and the ratio of width to radius \delta r _ { S } / r _ { S } of the Strömgren sphere .
For clouds of sufficient helium-ionizing photons Q ( \mbox { $ \mathrm { He ^ { 0 } } $ } ) / Q ( \mbox { $ \mathrm { H ^ { 0 } } $ } ) \gtrsim 0.15 and non-negligible width of the Strömgren sphere a significant overestimate of helium abundances may result .
Such clouds show radiation softness parameters in the range -0.4 \lesssim \log \eta \lesssim 0.3 coincident with the range of \eta in observed \mathrm { H } \textsc { ii } -regions .
Existing data of \mathrm { H } \textsc { ii } -regions indeed seem to display a correlation which is consistent with a typical \sim 2 - 4 \% overestimate of helium abundances due to these effects .
In case such an interpretation prevails , and in the absence of other compensating effects , a significant downward revision of Y _ { p } may result .
It is argued that caution should be exercised regarding the validity of commonly quoted error bars on Y _ { p } .