A system for filtering fresh air flowing in a tunnel overlying one roadway and / or for filtering exhaust gases flowing out therefrom, comprising: - at least one first port (300, 310) fluidly connectable with the outer environment for the flowing in of fresh air, respectively fluidly connectable with the tunnel for the inflow of the exhaust gases, to be filtered; - filtering means to filter the exhaust gases and / or the fresh air; - at least one second port (300, 310) fluidly connectable with the tunnel for exhausting the fresh air, respectively fluidly connectable with the outer environment for exhausting the filtered exhaust gases; - a first line for the fluid connection of said at least one first port (300, 310) and at least one second port (300, 310) passing through said filtering means; - fan means to force the evacuation of the exhaust gases from the tunnel towards the outer environment or the inflow of fresh air from the outer environment into the tunnel; wherein said filtering means comprise at least one filtering unit (20', 20", 20′′′) that includes: - at least one water reservoir; - pumping means; - a plurality of high pressure nozzles to spray atomized water; - a line for the fluid connection of said at least one water reservoir with said high pressure nozzles passing through said pumping means; wherein said pumping means include at least one high pressure pump; and wherein said plurality of high - pressure nozzles spray atomized water into drops having an average diameter lower than 250 μm, and at a minimum working pressure of 200 bar, and wherein said at least one filtering unit (20', 20", 20′′′) includes at least one porous laminar element for the collection of residues of filtration of the exhaust gases and / or the fresh air, said at least one porous laminar element being assembled in said at least one filtering unit (20', 20", 20′′′) in a removable manner; and wherein said at least one porous laminar element is placed below said high pressure nozzles to collect the mixture of water and filtration residues, the porosity thereof being such as to allow water to pass therethrough retaining the filtration residues; and wherein the porous laminar element has a permeability greater than or equal to 85 l / s m 2 and a porosity not lower than 85%, with an average pores diameter of 100 μm. - at least one first port (300, 310) fluidly connectable with the outer environment for the flowing in of fresh air, respectively fluidly connectable with the tunnel for the inflow of the exhaust gases, to be filtered; - filtering means to filter the exhaust gases and / or the fresh air; - at least one second port (300, 310) fluidly connectable with the tunnel for exhausting the fresh air, respectively fluidly connectable with the outer environment for exhausting the filtered exhaust gases; - a first line for the fluid connection of said at least one first port (300, 310) and at least one second port (300, 310) passing through said filtering means; - fan means to force the evacuation of the exhaust gases from the tunnel towards the outer environment or the inflow of fresh air from the outer environment into the tunnel; wherein said filtering means comprise at least one filtering unit (20', 20", 20′′′) that includes: - at least one water reservoir; - pumping means; - a plurality of high pressure nozzles to spray atomized water; - a line for the fluid connection of said at least one water reservoir with said high pressure nozzles passing through said pumping means; - at least one water reservoir; - pumping means; - a plurality of high pressure nozzles to spray atomized water; - a line for the fluid connection of said at least one water reservoir with said high pressure nozzles passing through said pumping means; wherein said pumping means include at least one high pressure pump; and wherein said plurality of high - pressure nozzles spray atomized water into drops having an average diameter lower than 250 μm, and at a minimum working pressure of 200 bar, and wherein said at least one filtering unit (20', 20", 20′′′) includes at least one porous laminar element for the collection of residues of filtration of the exhaust gases and / or the fresh air, said at least one porous laminar element being assembled in said at least one filtering unit (20', 20", 20′′′) in a removable manner; and wherein said at least one porous laminar element is placed below said high pressure nozzles to collect the mixture of water and filtration residues, the porosity thereof being such as to allow water to pass therethrough retaining the filtration residues; and wherein the porous laminar element has a permeability greater than or equal to 85 l / s m 2 and a porosity not lower than 85%, with an average pores diameter of 100 μm.