A large number of chemical substances may be used in a single cosmetic product. Most of them comprise one of the following: emulsions, colours, perfumes, preservatives, or special ingredients. Inflammable solvents consisting principally of alcohols and esters are used in several emulsifying agents. Many of them are volatile and evolve inflammable concentrations of vapour at ordinary temperatures. All colouring agents are capable of producing sensitizing reactions. In particular 'para-type' anthraquinone (used in hair dyes) and azo dyes (face and nail preparations) produce not only sensitization reactions but also cross sensitivity to each other. Perfumes often produce contact dermatitis and melanosis and the reaction may be acute and severe. Contact dermatitis may be caused by ionene, balsam of Peru, cloves, oil of bergamot, benzyl alcohol and pine terpenes. Pigmentation may be caused by any perfumed cosmetic and may be localized or diffuse. An ideal preservative should be colourless, odourless, stable and non-toxic, but this is rarely the case. For example, benzoic acid and propionic acid are mild irritants, salicylic acid is a strong irritant and monochloroacetic acid produces severe local reaction of the skin, eye or respiratory tract. Benzaldehyde has an irritant effect on the skin and formaldehyde causes dermatitis, cough, lacrimation and injury to the bronchi.