Hydrogen sulfide (H₂S) is a malodorous, very poisonous, and flammable gas. It can be detected as SO₂ using fluorescence after conversion using a hot catalyst. This technique is well established and as such also recommended by authorities such as the EPA. Our paper describes how at a relative humidity as low as 5 %, significant amounts of H₂S pass the instrument undetected. At ambient levels of relative humidity, up to 1/3 of all H₂S passes the instrument unnoticed.