  We have observed unexpected spectral attenuation of ultraviolet light in
freshly drawn hollow core optical fibres. When the fibre ends are left open to
atmosphere, this loss feature dissipates over time. The loss matches the
absorption spectrum of gaseous (molecular) chlorine and, given enough time, the
transmission spectrum of the fibre recovers to that expected from the
morphological structure of the fibre. Our measurements indicate an initial
chlorine concentration of 0.45 $\mu$ mol/cm$^{3}$ in the hollow core,
equivalent to 1.1 mol% Cl$_{2}$ at atmospheric pressure.
