Pollutant decomposition device, including at least one outer transparent sheet and at least one inner transparent sheet being arranged such that a gap is formed between them and such that the gap is in communication with a surrounding gaseous composition on one side of the device such that the gaseous composition can pass through the gap. The device further including a photocatalyst arranged in the gap for depolluting the gaseous composition that pass through the gap. To obtain optimum decomposition efficiency the outer transparent sheet has a high degree of ultraviolet transmittance compared with the inner transparent sheet.