Filtration apparatus comprising: - at least one filter medium for purifying a gas from suspended solid particles, said filter medium comprising at least one contaminated side and at least one purified side; - fan means configured to generate a gas flow through said filter medium; - first sensor means for determining at least one pressure difference (ΔP) between said purified side and said contaminated side, and/or second sensor means for determining at least one parameter (Hz) indicative of a power used by said fan means to generate the gas flow through said filter medium; - third sensor means for determining at least one gas flow generated through said filter medium by said fan means; characterized by comprising: - at least one memory containing at least one database containing a plurality of gas flow values through said filter medium and, associated with each gas flow value, at least one first reference value (ΔP') of said pressure difference and/or at least one second reference value (Hz') of said parameter; - electronic control means connected to said database and configured to perform the following operations: * receiving measured values from said first and/or second and third sensor means (8; 9; 10); * associating the gas flow value determined by said third sensor means to a first reference value (ΔP') and/or to a second reference value (Hz') according to said database; * making at least one first comparison between the value of the pressure difference (ΔP) determined by said first sensor means and said first reference value (ΔP') and/or at least one second comparison between the value of the parameter (Hz) determined by said second sensor means and said second reference value (Hz'), * emitting a signal indicating the state of said filter medium on the basis of said first comparison and/or said second comparison. - at least one filter medium for purifying a gas from suspended solid particles, said filter medium comprising at least one contaminated side and at least one purified side; - fan means configured to generate a gas flow through said filter medium; - first sensor means for determining at least one pressure difference (ΔP) between said purified side and said contaminated side, and/or second sensor means for determining at least one parameter (Hz) indicative of a power used by said fan means to generate the gas flow through said filter medium; - third sensor means for determining at least one gas flow generated through said filter medium by said fan means; - at least one memory containing at least one database containing a plurality of gas flow values through said filter medium and, associated with each gas flow value, at least one first reference value (ΔP') of said pressure difference and/or at least one second reference value (Hz') of said parameter; - electronic control means connected to said database and configured to perform the following operations: * receiving measured values from said first and/or second and third sensor means (8; 9; 10); * associating the gas flow value determined by said third sensor means to a first reference value (ΔP') and/or to a second reference value (Hz') according to said database; * making at least one first comparison between the value of the pressure difference (ΔP) determined by said first sensor means and said first reference value (ΔP') and/or at least one second comparison between the value of the parameter (Hz) determined by said second sensor means and said second reference value (Hz'), * emitting a signal indicating the state of said filter medium on the basis of said first comparison and/or said second comparison. * receiving measured values from said first and/or second and third sensor means (8; 9; 10); * associating the gas flow value determined by said third sensor means to a first reference value (ΔP') and/or to a second reference value (Hz') according to said database; * making at least one first comparison between the value of the pressure difference (ΔP) determined by said first sensor means and said first reference value (ΔP') and/or at least one second comparison between the value of the parameter (Hz) determined by said second sensor means and said second reference value (Hz'), * emitting a signal indicating the state of said filter medium on the basis of said first comparison and/or said second comparison.