An allergen is a chemical or biogenic substance that can cause skin or respiratory allergic reactions. Some common allergens are colophony (rosin), formaldehyde, metals (for example, chromium, nickel), organic dyes, epoxy hardeners, turpentine, isocyanates, reactive dyes, tropical wood dusts and biogenic allergens (for example, fungi, animal proteins, terpenes, dust and storage mites, enzymes).
Allergens may be found in many industrial processes, like fermentation, drug production, baking (for example, flour, grain), paper production, wood processing (for example, sawmills), biotechnology (for example, enzymes, vaccine production, tissue culture) and spice production (for example, garlic powder).
Repeated exposure to a substance designated with an S in Schedule 1 of the ROHS may cause sensitization, that is, a reaction in the form of an allergic (immunological) response of the respiratory tract, mucous membranes, conjunctivas or skin.
http://www.ilocis.org/documents/chpt30e.htm (Figure 30.1)
http://legisquebec.gouv.qc.ca/en/ShowDoc/cr/S-2.1,%20r.%2013