Sensitizer

Sensitization has two stages. First, a person is exposed and the immune system learns to recognize the chemical, usually with no symptoms at all. Later exposures then provoke a reaction. That is why a worker can handle a product for months without trouble and then suddenly develop a rash or breathing problems.

The GHS separates two classes. Skin sensitizers cause allergic contact dermatitis, with itching, redness or blisters, often appearing a day or two after contact. Respiratory sensitizers cause asthma-like reactions such as wheezing, chest tightness and cough. Respiratory sensitization is the more serious of the two, and the label carries the health hazard pictogram. Skin sensitizers carry the exclamation mark pictogram instead.

Sensitization differs from irritation. An Irritant harms tissue directly in nearly everyone given enough contact, while a sensitizer acts through the immune system and only affects people who have become sensitized. Because of this, a dose that is safe for most co-workers can be dangerous for one person.

Once sensitization develops it is usually lasting. Affected workers may need to avoid the substance entirely, so prevention of the first exposures matters most.