I live in Southern California and sometimes hike in areas that can be overgrown with a plant called purple poodle-dog bush. As I found out the first time I went crashing through a stand of the stuff, it's a horrific skin irritant, much worse than poison oak. It has microscopic hairs that get stuck in your skin and act like syringes. The hairs have chemicals on them that are skin irritants. Wikipedia says:

This contact dermatitis is due to prenylated phenolics exuded by hairs (glandular trichomes) of the plant. The principal irritants are derivatives of farnesyl hydroquinone and 3-farnesyl-P-hydroxybenzoic acid.

I'm not a chemist, and all the nomenclature is Greek to me. My question is this. When I hike in an area where it will be hard to avoid the bush, I try to wear long sleeves and change my clothes immediately afterwards. Will throwing my clothes in the washing machine remove these chemicals and/or break them down into harmless ones? What if my shoes aren't machine washable? Will the chemicals break down naturally within a certain period of time? Would other methods such as exposure to sunlight help? Are these chemicals soluble in water, so that simple rinsing would help? How about making sure they aren't lurking in the upholstery of my car?

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    $\begingroup$ I don't have time to answer fully, but I imagine the compounds have a non-zero but rather low solubility in water. The actual amount of irritant substances present may be low enough that it could dissolve even in a regular wash, however. Another thing is that relativity caustic media will cause some degree of deprotonation, forming a salt which is likely far more soluble. The hydroxybenzoic acid derivative is particularly susceptible to this, though even the hydroquinone is slightly acidic. $\endgroup$ – Nicolau Saker Neto Jul 2 '14 at 1:24
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    $\begingroup$ The ultimate problem is likely the hairs which contain the irritants; they might not be easily removed mechanically or chemically, and may impede the access of solvent to the irritants if they are stored in cavities on the hairs. $\endgroup$ – Nicolau Saker Neto Jul 2 '14 at 1:26
  • $\begingroup$ If the solubility is low but the amount of the substance is small, would it make sense to leave the clothes soaking in water for a long time so that the substance would gradually leach out and go into solution? What would be an example of a caustic medium that I could use to soak my clothes? $\endgroup$ – Ben Crowell Jul 2 '14 at 20:11
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    $\begingroup$ Soaking could possibly slowly dissolve everything, but if you were to do that then I would recommend adding a basic compound to the water to possibly increase the speed and efficiency of the process. A safe, cheap and easily washed off base that you could use would be sodium carbonate, $\ce{Na2CO3}$. A solution containing 2 g/100 mL of water will reach somewhere around pH 11 at ambient conditions. Sodium bicarbonate $\ce{NaHCO3}$ can be used too, though it's less basic. $\endgroup$ – Nicolau Saker Neto Jul 2 '14 at 22:38
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    $\begingroup$ I cannot completely guarantee that your clothes will resist the alkaline solution entirely unharmed, though it should be quite safe. At least one commercial non-chlorinated bleaching agent for clothes actually contains sodium carbonate as part of its formulation (though the bleaching compound is actually the hydrogen peroxide). Furthermore, as I mentioned earlier, the hairs themselves may act as a physical barrier and hamper dissolution. In that case, it may be best to strip the hairs off directly using some adhesive (sticky tape?) $\endgroup$ – Nicolau Saker Neto Jul 2 '14 at 22:45

As a starting point, I shall assume that the irritant compounds are freely exposed to the environment (which is not necessarily the case, as I mention further on). As such, one of the conceivably simplest ways to remove them is to wash them off, i.e. disperse them in a solvent and discard the liquid mixture. For several reasons (abundance, extensive solvation ability, non-toxicity, etc.) water is a good choice.

First of all, how well would pure water work at dispersing the prenylated phenolics? Probably not too great. The representative parent compounds of the irritants, hydroquinone and p-hydroxybenzoic acid have modest solubilities in water (59 g and 5 g per 1 L, respectively), and the addition of farnesyl chains in the compounds would quite likely cause a severe reduction in water solubility, unless the farnesyl chains are highly substituted with hydrophilic groups.

Of course, there is no substance that is entirely insoluble in any solvent, though it is very common for certain solutes to require impractical amounts of solvent to disperse completely. It could well be that a washing machine's load of water (of order 100 L) is actually capable of dissolving the farnesylated toxins, especially because natural products are generally present in rather small quantities. However, there are some techniques which can possibly help the process. I list a few in the following:

  1. Use hot water - The majority of substances become more soluble in hot solvents because dissolution very often increases the entropy of a system ($\Delta S>0$), and hence from the Gibbs' free energy relation $\Delta G=\Delta H - T \Delta S$, an increase in temperature leads to a larger decrease in free energy, making the process more favourable. Of course, this only really works well if the water is hot throughout the soaking/washing process; adding hot water and letting it cool off will just cause part of the solute to precipitate out of solution again.

  2. Use a surfactant (detergent) - Since the offending compounds potentially have large hydrophobic chains, they can conceivably removed by the same process we use to wash away oils and fats. An amphipatic molecule contains a hydrophobic part (such as the hydrocarbon chain of sodium dodecylbenzenesulphonate) responsible for interacting with the oils and a hydrophilic part (in SDBS, the sulphonate group), which is well solvated by water. Thus, the surfactant bridges the interaction between oil and water, allowing effective dispersion.

  3. Use a basic solution - In this specific case, the undesirable compounds have some acidic character. In particular, the hydrogen atom in the -COOH (carboxyl) group of the p-hydroxybenzoic acid derivatives are about as acidic as acetic acid. The -OH (hydroxyl) groups in both p-hydroxybenzoic acid and hydroquinone are also slightly acidic, though weaker by some 5-6 orders of magnitude. Thus, mixing in a basic solution may allow an acid-base reaction to take place, forming salts of the organic compounds with significantly increased solubility in water. Many washing powders/liquids are already slightly alkaline, but it is also easy to prepare basic solutions separately. A cheap and very safe base would be sodium carbonate, $\ce{Na2CO3}$, though a few tens of grams may be required to prepare a decent volume of basic medium for soaking. Sodium bicarbonate, $\ce{NaHCO3}$, also works though it is a weaker base and would require a higher amount to reach a given pH value. Caustic soda, $\ce{NaOH}$, is yet another cheap option, but care must be taken to not add too much, as it is a strong base (in contrast to the previous two). I don't know how resistant different fabrics are but, very approximately, any more than a couple of grams of lye per litre of water risks partially hydrolysing the fabric and weakening its structural integrity. Use at your own risk!

  4. Use a bleaching agent (chlorinated or non-chlorinated) - Bleaches work by partially oxidizing organic matter, breaking organic molecules into smaller chunks and making each piece more hydrophilic (due to added oxygenated functionalities). Even if the compounds aren't removed entirely, the reaction products may be much less harmful.

  5. Miscellaneous - You suggest exposure to sunlight. Light is certainly capable of decomposing organic substances, especially UV light, but there's no guarantee that the exposure required to eliminate the irritants won't also break down fabric and bleach dyes. Waiting is also probably not very efficient, as there is no guarantee the irritant compounds are unstable enough to undergo spontaneous decomposition in ambient conditions at a significant rate.

Unfortunately, all this discussion is of limited use if the initial assumption (freely accessible irritants) is incorrect. It could be that the compounds are stored in cavities in the plant hair's surface or interior, and the hairs themselves are insoluble and impermeable to water, effectively locking the toxins away. In this case, the compounds will likely be removed only slowly, as the hairs are progressively worn out by abrasion and other processes. Thus a physical removal method may be preferable. I have heard that adhesives (sticky tape, waxing strips, etc.) can be successfully used to peel out painful hairs. This is also a useful option for surfaces which cannot be washed easily (such as car upholstery).

In practice, I am not sure how this will all work out. It's been a few days since I wrote my comments, so perhaps you've had time to test something out? If so, I would be curious to hear some feedback!

  • $\begingroup$ Sorry about not answering before. I felt an answer needed more polish, but I was having trouble making time for it. Even now I don't think what I wrote is as good as it should be. Also, I suspect a fair amount of the above is not particularly useful to you, but I like my answers to be as thorough as I can make them. $\endgroup$ – Nicolau Saker Neto Jul 8 '14 at 1:57
  • $\begingroup$ Thanks for the very helpful and complete answer. I soaked the clothes overnight in a sodium carbonate solution, then washed them again in the washing machine. Unfortunately it's difficult to tell whether it worked or not. My immune system is currently lit up like a Christmas tree, so I've put the clothes aside and will not try wearing them until my skin is back to normal. If they cause the reaction to flare back up at that point, then I guess we'll know the method didn't work! $\endgroup$ – Ben Crowell Jul 8 '14 at 22:36
  • $\begingroup$ Yikes, I didn't know it was that bad. Do be extra careful then! I wouldn't want to unwittingly inconvenience you a second time. $\endgroup$ – Nicolau Saker Neto Jul 9 '14 at 0:08
  • $\begingroup$ I realize it's been a year, but I thought I'd make a small addition: it may be that dry cleaning could be effective at removing the offending compounds in this case. Dry cleaning often uses non-polar chlorinated solvents like tetrachloroethylene as a washing liquid, which would excel at dissolving the fatty prenylated phenolics, again under the assumption that the solvent can reach the compounds. $\endgroup$ – Nicolau Saker Neto Mar 21 '15 at 14:38

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