A packaging machine for producing sealed packages of a pourable product from a web of packaging material advancing along a web advancement path (P), the packaging machine comprising: - an isolation chamber separating an inner environment from an outer environment ; - a sterilization apparatus for sterilizing at least a first face of the web of packaging material at a sterilization station and being in fluid connection with the isolation chamber ; wherein the sterilization apparatus comprises: - an irradiation device configured to sterilize at least the first face of the advancing web of packaging material by directing a sterilizing irradiation onto at least the first face while, in use, advancing along a sterilization portion (P1) of the web advancement path (P); - a main shielding chamber housing the irradiation device and comprising an advancement channel having an inlet opening and an outlet opening and through which, in use, the web of packaging material advances along the sterilization portion (PI); - a first auxiliary shielding chamber being arranged upstream of the advancement channel along the web advancement path (P) and having a first inner space being fluidically connected to the advancement channel ; - a second auxiliary shielding chamber being arranged downstream of the advancement channel along the web advancement path (P) and having a second inner space being fluidically connected to the advancement channel and the inner environment ; characterized in that it further comprises pressure control means configured to control a first pressure in the first auxiliary shielding chamber , a second pressure in the second auxiliary shielding chamber and a third pressure in the isolation chamber ; wherein the pressure control means are configured to control the first pressure, the second pressure and the third pressure such that the first pressure is lower than the second pressure and the second pressure is lower than the third pressure. - an isolation chamber separating an inner environment from an outer environment ; - a sterilization apparatus for sterilizing at least a first face of the web of packaging material at a sterilization station and being in fluid connection with the isolation chamber ; - an irradiation device configured to sterilize at least the first face of the advancing web of packaging material by directing a sterilizing irradiation onto at least the first face while, in use, advancing along a sterilization portion (P1) of the web advancement path (P); - a main shielding chamber housing the irradiation device and comprising an advancement channel having an inlet opening and an outlet opening and through which, in use, the web of packaging material advances along the sterilization portion (PI); - a first auxiliary shielding chamber being arranged upstream of the advancement channel along the web advancement path (P) and having a first inner space being fluidically connected to the advancement channel ; - a second auxiliary shielding chamber being arranged downstream of the advancement channel along the web advancement path (P) and having a second inner space being fluidically connected to the advancement channel and the inner environment ; <EOS>