  Lyman-alpha is now widely used to investigate the galaxy formation and
evolution in the high redshift universe. However, without a rigorous
understanding of the processes which regulate the Lya escape fraction, physical
interpretations of high-z observations remain questionable. We examine six
nearby star-forming galaxies to disentangle the role of the dust from other
parameters such as gas kinematics, geometry and ISM morphology in the
obscuration of Ly-alpha. Thereby we aim to understand the Ly-a escape physics
and infer the implications for high-redshift studies. We use HST/ACS to produce
continuum-subtracted Lya maps, and ground-based observations (ESO/NTT and NOT)
to map the Halpha emission and the extinction E(B-V) in the gas phase derived
from the Balmer decrement Halpha/Hbeta. When large outflows are present, the
Lya emission appears not to correlate with the dust content, confirming the
role of the HI kinematics in the escape of Lya photons. In the case of a dense,
static HI covering, we observe a damped absorption with a declining
relationship between Lya and E(B-V). We found that the Lya escape fraction does
not exceed 10% in all our galaxies and is mostly about 3% or below. Finally,
because of the radiative transfer complexity of the Lya line, star formation
rate based on Lya luminosity is underestimated with respect to that derived
from UV luminosity. The failure of simple dust correction to recover the
intrinsic Lya/Ha ratio or the total star formation rate should prompt us to be
more cautious when interpreting high-z observations and related properties,
such as SFRs based on Lya alone. To this end we propose a more realistic
calibration for SFR(Lya) which accounts for dust attenuation and resonant
scattering effects via the Lya escape fraction.
