  We present deep sub-millimetre observations of seventeen galaxies at z=0.5,
selected through being hosts of a type 1a supernova. Two galaxies are detected
directly, and the sample is detected statistically with a mean 850 micron flux
of 1.01mJy +/- 0.33mJy, which is 25% - 135% higher than locally. We infer that
the mean value of A_v in normal galaxies at z=0.5 is comparable to or greater
than the mean A_v in local normal galaxies, in agreement with galaxy chemical
evolution models and indirect observational evidence. Scaling from the local
value given by Rowan-Robinson (2003) gives a mean extinction at z=0.5 of A_v =
0.56 +/- 0.17. The dust in the brightest sub-mm object in our sample is best
interpreted as normal `cirrus' dust similar to that seen locally. The detection
rate of our sample suggests that some sources found in blank-field sub-mm
surveys may not be high redshift starbursts, but rather cirrus galaxies at
moderate redshifts and with lower star formation rates. Finally, an increase in
host dust extinction with redshift may impact the cosmological results from
distant supernova searches. This emphasizes the need to carefully monitor dust
extinction when using type Ia supernovae to measure the cosmological
parameters.
