to every type of fiber. Dyes are classified according to many parameters, such as chemical structure, affinity, application method, desired colour fastness i.e. resistance to washing, rubbing, and light. The properties may vary with different dyes. The selection of dye depends on the objective in dyeing and affinity (to which material is to be dyed). Fastness of color largely depends upon the molecular size of the dyes and the sloubility. Larger molecular size serves better washing fastness results. Indigo dyes have a poor wash and rubbing fastness on denim (cotton), so they are used to produce washed-down effects on fabrics. In contrast, vat or reactive dyes are applied to cotton to achieve excellent washing fastness. The next important criterion for selecting dyes is the assessment of hazards to human health and the environment. There are many dyes especially disperse dyes that may cause allergic reactions to some individuals, and the negative impact on the environment. There are national and international standards and regulations which need to comply. Direct application The term "direct dye application" stems from some dyestuff having to be either fermented as in the case of some natural dye or chemically reduced as in the case of synthetic vat and sulfur dyes before being applied. This renders the dye soluble so that it can be absorbed by the fiber since the insoluble dye has very little substantivity to the fiber. Direct dyes, a class of dyes largely for dyeing cotton, are water-soluble and can be applied directly to the fiber from an aqueous solution. Most other classes of synthetic dye, other than vat and surface dyes, are also applied in this way. The term may also be applied to dyeing without the use of mordants to fix the dye once it is applied. Mordants were often required to alter the hue and intensity of natural dyes and improve color fastness. Chromium salts were until recently extensively used in dyeing wool with synthetic mordant dyes. These were used for economical high color fastness dark shades such as black and navy. Environmental concerns have now restricted their use, and they have been replaced with reactive and metal complex dyes that do not require mordant. Yarn dyeing There are many forms of yarn dyeing. Common forms are the package form and the hanks form. Cotton yarns are mostly dyed at package form, and acrylic or wool yarn are dyed at hank