Thin-film Al filters are very popular owing to their high transmittance in the wavelength range of 17-67 nm and low transmittance in the visible and near-UV regions; however, they are prone to oxidation. The amorphous Al2O3 layers on the Al surfaces have much smaller transmittance than the bulk Al material; therefore, they strongly influence the total transmittance of the filter. This paper not only provides the transmittance of very old Al filters but also maps the transmittance development of Al filters over two years since their delivery (in 2016) in an uncontrolled atmosphere.