How safe are modern dyes, compared to ancient ones?

Some modern dyes are dangerous, while others are quite safe. Most but not all ancient dyes require heavy metal mordants, which are more toxic than dyes.

Natural dyes generally require the use of a mordant such as alum, copper, iron, tin, or chrome. Alum is the least hazardous of these, often seen descirbed as "food grade" alum, but the fatal dose of alum for an adult is 30 gram (one ounce), while that for a child is correspondingly much less. The fatal dose of alum for a baby might be as little as one gram. The other mordants, including copper and iron, can be fatal to adults in doses as low as one teaspoon or less, and the chrome mordant (potassium dichromate) is a significant human carcinogen.

One of the few natural dyes that does not require a mordant is indigo, a natural vat dye, which can be prepared from some fifty different species of plants that grow around the world. It is not safe for a child to use because caustic chemicals such as lye are necessary to dissolve it in the dyebath in order to apply it to fabric. However, these chemicals are safe once their pH has been neutralized. The ancient Tyrian purple, prepared from vast quantities of shellfish, is chemically very closely related to indigo.

A very small number of natural dyes do not require any toxic chemicals to apply. One of these is turmeric, the spice, which is used by monks in Tibet to dye their robes. Because turmeric fades quickly in the light, they must redye their robes every year. Walnut hulls are another good natural dye that can be used without any mordant.

Most modern synthetic dyes do not require the use of mordants, so they do not present the risk of deaths by poisoning. However, some are based on the carcinogenic chemicals benzidine or o-dianisidine. Benzidine-based direct dyes were widely used in all-purpose dyes for decades, the kinds of fabric dyes which are sold in grocery stores nearly everywhere, up through the 1970s, and some o-dianisidine based dyes are still available elsewhere. These dyes can cause bladder cancer or other cancers if workers absorb large enough amounts of them, and the development of the cancers can be delayed by twenty years or more after the exposure. Dyes based on benzidine and o-dianisidine are much less easy to buy in local stores now.

The fiber reactive dyes found in tie-dye kits are quite safe. None of them are based on benzidine or o-dianisidine. Since they do not require mordants, they are much safer for children to use than most natural dyes. The washing soda (also known as soda ash) that is used to set fiber reactive dyes is only a mild irritant, which is found in most laundry detergents.

Unlike the mordants used with natural dyes, the dye in currently-used tie-die kits have caused no fatalities. The only health problems seen with fiber reactive dyes are allergies and asthma that can result from repeatedly breathing the dye powder. This is less likely to happen at home, but it is possible if you do a lot of dyeing, so you should be careful to always wear a dust mask when dissolving dye powders, in order to reduce your risk. It is a significant problem in industrial situations in which workers are regularly exposed to dusts in the air. Once a worker has developed an allergy to a specific type of fiber reactive dye, he or she must not work with that dye ever again, though it is generally okay for them to wear clothing colored with that dye.

It is important to avoid unnecessary exposures to even safe household chemicals, in case some problem is discovered in the future which is not currently known. When using dyes, always wear gloves, and wear breathing protection as needed. Never allow children unsupervised access to dyes or other household chemicals. When using toxic chemicals such as household chlorine bleach, be careful to avoid getting any on your skin, and use good ventilation.

Also see:
About Natural Dyes
What are the safest dyes to use? What kinds of dyes are too unsafe to use?
Is it safe to eat or breathe fiber reactive dyes?
Aren't natural dyes safer than synthetic dyes?
Is all-purpose dye safe to use? Is it safer than fiber reactive dye?

What's the difference between mordants and other chemical assistants used in dyeing?

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[This answer was first posted, by me, on Yahoo answers, on June 3, 2008.]