Dear Paula,

I have reviewed many safety-related comments by you and the Forum members. Please pardon me if I missed one that would have answered this. I have read and re-read the indigo section in J. Lyles' The Art and Craft of Natural Dyeing. I only use plant-derived indigo (not synthetic.) I understand that each type of indigo vat has different properties in terms of color depth, appropriateness for resist techniques, etc. I am asking here just about safety. I use a fume canister mask and safety glasses to mix the vats. I (usually) use good gloves while dyeing.
 
I have four related questions:  
First, are there any of these four vat types you would rule out for long-term, virtually daily, professional use on the basis of being too unsafe? 
• Lye-hydrosulfite (e.g., Earthues' recipe),
• Zinc-lime (Lyle's recipe),
• Lyles' Fermentation Vat No. 3: Artificial Sig Vat,
• Pre-reduced Indigo Crystals with soda ash and thiox (either Dharma Trading &/or Earthues recipes) 

Hi Vicki,

Given that you are a careful person who wears gloves as needed, and a properly-fitting respirator and eye protection as needed, I would think you'd be able to use any of the recipes without long-term problems. There are some real safety differences among them, but any can be used safely, with care. 

To see an example of what can happen without any consideration for safety, see the 1970 interview with tie-dyer Maureen Mubeem, quoted in a post I made in 2008 on the Dye Forum. She wrote, "You should also have some kind of rubber cap, like a shower cap or something, though I just put a scarf over my head. The fumes dry my hair and make it brittle. The chemicals seem to react with the oils in your hair and reduce the oil content. You can feel it happening. Another thing I noticed is that after I have used the dyes I get a very heavy chest congestion." Mubeem was using synthetic vat dyes, with the same chemicals that are used for the lye-hydrosulfite recipe for indigo, since indigo is a vat dye. She was not using a respirator. The idea of that chest congestion worries me, because I fear that it might be possible that long-term exposures that produce that symptom could, after some years, lead to permanent lung damage. I think that, if you are free of any short-term symptoms that result from your chemical use, you will be fine in the long term. These chemicals do not produce long-term damage without first causing short-term discomfort.

Let's look at the individual chemicals:

Lye
Lye is sodium hydroxide, also known as caustic soda. Here is a link to a good MSDS for lye. The danger of sodium hydroxide is that an accidental spill can cause burns to the skin, or even permanent blindness if it is splashed into the eyes. I caution against its use by people who are not trained in the safe handling of chemicals, and yet in the lab I worked with sodium hydroxide for many years without the slightest hint of problems. If you already know how to use sodium hydroxide safely, I wouldn't worry about it much. Once the stuff is diluted, it's not particularly bad. It's very dangerous to pour water on top of lye, because the reaction produces heat, which can make the lye boil and spatter, so always add lye to water, rather than water to lye. The same rule applies to any strong acid or base.

The ProChem thiox recipe calls for one cup, or 215 grams, of lye for a 20 gallon vat, or 5 teaspoons or 23 grams for a four-gallon vat. 215 grams of lye is 5.35 moles; diluted into 80 liters, that makes a a solution of 0.0672 molar. A one molar solution of NaOH has a pH of 14; 0.1 molar, a pH of 13. So, the pH of your vat will be between 12 and 13, high enough to require caution, but low enough to be easy for a careful person to use safely.

Hydrosulfite and Thiox
Sodium dithionite, which for some reason we in the dyeing field refer to by an older name, sodium hydrosulfite, is a fire hazard if you store large quantities of it. A little bit of moisture in sodium dithionite, moisture from the air penetrating the package, will cause it to spontaneously combust.  It is safe to store small amounts of sodium dithionite, but for large quantities or long-term storage, it's best to substitute thiourea dioxide, which is approximately five times stronger, but lacks the fire hazard properties of dithionite altogether.

Here are links to a good MSDS page for sodium hydrosulfite and a good MSDS for thiourea dioxide. (Don't confuse thiourea dioxide with thiourea, which is a very different and far more hazardous chemical.)

Thiourea dioxide is similar to sodium dithionite, in that both are reducing chemicals which liberate sulfur dioxide when used to remove the color from fabric dye. The side effect found in both of these, which is not found for the other chemicals, is that people with asthma may be extremely sensitive to the sulfur gas that is produced under some circumstances.

Zinc
Here's a link to a good MSDS for zinc metal powder. The biggest risk from zinc comes from exposure to the fumes when it's heated to high temperatures, which is not an issue to you at all. Of course, it's unwise to breath zinc powder. It is unwise to breathe any chemical powder, but it's not as hazardous as the fumes from heated zinc. I would imagine that ordinary care to avoid breathing the powder or getting it on your skin or in your eyes should be sufficient.

Some years ago on the DyersLIST mailing list, Doug Wilson posted a suggestion against using the zinc-lime bath because of problems in disposing of zinc compounds. Checking the 'Disposal Considerations' section of the MSDS, it appears that this could be a real issue for you. "Whatever cannot be saved for recovery or recycling should be handled as hazardous waste and sent to a RCRA approved waste facility." This is not an issue for the other chemicals you mentioned, as long as you neutralize the pH before dumping them down the drain.

Karren Brito says that handling powdered zinc is hazardous, and suggests that a thiox-based vat is safer, in addition to being less temperamental.

Lime
Lime, or calcium oxide, CaO, is sold for use in pickling; I have a jar of pickling lime in my pantry, solely for its interest as a chemical. Like lye, calcium oxide is corrosive; it has a very high pH and can cause severe burns. If you are careful to follow the usual rules of only adding it to a volume of water, rather than pouring water into a container with dry lime, and follow the usual precautions of wearing gloves and eye protection, I doubt you'll have ill effects, but I would not want to recommend that a novice who is cavalier about safety use it. Here's an MSDS for calcium oxide.

Sig
Even the sig fermentation dye bath contains one potentially hazardous chemical, ammonia. Household ammonia is 5 to 10% ammonium hydroxide. It requires the usual precautions about not getting on your hands or in your face, plus of course the stern warning to never mix with hypochlorite bleach. Here's an MSDS for Ammonia, but some of its warnings look like they would apply only to stronger solutions of ammonia, although the MSDS is specifically for 10% or weaker.

Urea can also be added to the Sig vat. Urea in moderate quantities is harmless stuff, often used as an ingredient in skin moisturizers at concentrations up to 25%, though it can be irritating if used wrong, and urea dust should not be inhaled. Here is an MSDS for urea.

Also see my earlier blog entry, "Safety of the caustic soda and hydro-sulphite used for tie-dyeing in Nigeria".

2. I really like the pre-reduced indigo crystals, but I wonder if workers in other countries are being exposed to risks from the reducing chemicals. Do you know the status of worker safety in those factories?

I don't know anything about this. In a properly-equipped factory with adequate ventilation, it should easily be possible to pre-reduce indigo with no harm at all to the workers, but I don't know what the actual factory conditions may be for these workers.

Note that not all of the pre-reduced indigo is derived from natural sources. You should carefully re-read the claims made by your supplier. Synthetic indigo works as well as natural indigo, but it sounds like it's important to you to use only the plant-derived indigo, so be sure to check this out.

3. If zinc-lime is not too unsafe for daily use, could you recommend a mail-order supplier for these chemicals? And are there any special safety precautions for zinc powder?

Maiwa Handprints in Vancouver sells zinc powder for indigo vats. 

The Village Spinning & Weaving Shop, in California, also sells zinc dust, but they warn, "Zinc in this form is more reactive and hazardous than usual. It is potentially flammable, toxic if ingested. Store carefully, away from acids and oxidizing agents." (Here's a link to a PDF file of their catalog.)

 4. Do you know of any research papers on occupational hazards, or lack thereof, for indigo dyers? E.g., Do long-term users of zinc (or lye, thiox, soda ash) have characteristic health issues or shorter lives? Perhaps the professional Japanese indigo dyers? I could not locate relevant research.

This reminds of a story on the DyersLIST mailing list, or some forum like that, some years ago, saying that many of the Japanese workers who used a particular synthetic black dye, in the early days of synthetic dye use, died of bladder cancer. I was never able to track down the details or the veracity of that story, but it does seem possible, given that carcinogenic direct dyes used to be used in dyeing (and still are, in some places), and latex gloves and respirators were not a part of traditional dyeing practices.

However, I've never heard anything like this about any of the methods of indigo dyeing. Indigo in itself, as you know, is non-toxic. I think that the most likely problem resulting from inadequate safety precautions in indigo dyeing would be a respiratory syndrome caused by overexposure to sulfur-containing fumes from thiox or hydrosulfite. If you ever find yourself with an unexplained cough that seems to be in reaction to one of these chemicals, consider whether it might be an asthmatic type reaction, indicating a need for better respiratory protection. Mild asthma often causes coughing, rather than noticeable wheezing. Be careful that your combination of ventilation and cartridge respirator use prevent any sort of noticeable lung reactions, wear gloves so that your skin does not become irritated, and use eye protection so that you never get a splash of lye or lime in your eyes, and I think you will be fine.

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