Melt-blown fiber having an average diameter of not more than 5.0 μm, said fiber comprises at least 85 wt.-% of a propylene copolymer, wherein •said melt blown fiber and/or said propylene copolymer has/have a melt flow rate MFR2 (230° C.) measured according to ISO 1133 of at least 200 g/10 min, •said propylene copolymer has a comonomer content of 0.5 to 5.5 wt.-%, the comonmers are ethylene and/or at least one C4 to C20 α-olefin selected from the group consisting of 1-butene, 1-pentene, 1-hexene, 1-heptene, and 1-octene, •the propylene copolymer has regiodefects of not more than 0.4 mol.-% determined by 13C-spectroscopy, and •said melt blown fiber and/or said propylene copolymer fulfill(s) the equation (1). wherein Tm [° C.] is the melting temperature [given in ° C.] of melt blown fiber said and/or of said propylene copolymer measured according to ISO 11357-3, C2 [wt %] is the amount [given in weight percentage] of comonomers within said melt blown fiber and/or within said propylene copolymer determined with Fourier transform infrared spectroscopy (FTIR). Tm ⁡ [ ° ⁢ ⁢ C . ] [ ° ⁢ ⁢ C . ] ≥ 160 - C ⁢ ⁢ 2 ⁡ [ wt ⁢ ⁢ % ] × 5.25 [ wt ⁢ ⁢ % ] ( 1 )