We have a plant policy that requires that the monitors are worn "in the breathing zone." I can't seem to find an exact definition. But, I have heard it said that this means within 12 inches of our mouth and nose. With that definition, the brim of your hard-hat would qualify. However, our safety engineers warn not to clip them to your hat just because this strains a clip that is designed to grab to cloth. Eventually the clip will fail and the monitor will be lost. I am not aware of an OSHA mandate. As far as I know, the use of personal H2S monitors in an oil refinery is still optional at the discretion of the refinery safety programs. Someone please correct me if I am wrong.
Johnny Pellin