Posted by: Green Knight | December 30, 2010

Strawberry Fields Forever

A friend of mine listened to a story on NPR the other night, about California approving the substitution of methyl iodide for methyl bromide as a pesticide for use in strawberry production, and that it’s generated some controversy.  I’ve been asked to look into it, and it was an interesting journey. Normally, when someone asks me about a chemical, I check the periodic table on the wall next to my bed.

Generally, lighter elements tend to be more reactive or otherwise flamboyant than their relatives lower down on the same column. Chlorine, for all its faults, is less nasty than fluorine…hydrofluoric acid will eat through glass. And bromine is one rung lower than chlorine, and iodine one position lower than that. Then there’s the fact that iodine is a trace nutrient, a shortage of which causes goiter. So, my initial thought was that methyl iodide would be a safer chemical than methyl bromide, but it turned out to be only sort of true, or at least not that simple.

Some of you may be familiar with an old-fashioned use of the term “bromide,” which refers to sodium or potassium bromide compounds used as mild tranquilizers for a palliative effect. OK, but those are inorganic salts. There have been some concerns raised about brominated flame retardants, synthetic compounds used in all sorts of products; these are organic chemicals, meaning based on an aliphatic or aromatic carbon structure, not meaning “from my neighbor Farmer Moonbeam’s compost heap.” Metals and other elements that are in an organic chemical structure are often much more available to do biological damage than when in an inorganic state; see my previous blogs on selenium, for example. Break an old thermometer and swallow the pure mercury, and almost all of it will pass right through you. Ingest methyl mercury, however, like the unwitting residents of Minamata, Japan, who just ate the fish they caught, and you’ve got some serious trouble.

Methylated ANYTHING is worse. That glass of cabernet I just quaffed had ethyl alcohol (ethanol) in it, which is of relatively low toxicity (everything’s toxic, it just depends on the dose, as Paracelsus taught us). But methyl alcohol, or methanol, aka “wood alcohol,” is the stuff that makes you go blind. Only difference is one extra carbon and two extra hydrogens in the ethanol molecule.

So I was prepared to find methyl iodide a bit less nasty than its cousin, methyl bromide, especially since my home state has Prop 65 for carcinogens, and generally CA environmental regs are tougher than anyone else’s just as a matter of course. I myself worked for Greenpeace in my hometown of SF from 1980-83. I like that we’re ahead of the game so everyone else has an example to look to.

CHEMICAL COMPARISON TIME

So, how did I do it? Simple; I looked at the various standards from the EPA, OSHA, DOT (Dept. of Transportation), ACGIH (American Conference of Governmental Industrial Hygienists), NIOSH (National Institute for Occupational Safety & Health), NFPA (National Fire Protection Association), and a few others. Some regulatory programs are vigorous and have numbers that are changing and becoming more protective all the time. Others, like the Safe Drinking Water Act, haven’t been updated since they were created, in this case 1974, and neither of our target chemicals are covered by it.

So here we go. For convenience, instead of typing the chemical names every time, I’ll just use MBr for methyl bromide and MI for methyl iodide.

Both are heavier than air, MBr about 3 times and MI about 4x as much.  This is good in that they don’t dissipate and expose large areas, but not so good if you’re working in a low-lying spot, where they would collect.  The MBr’s vapor pressure is 4 times higher than that of MI, which is why it’s a gas at standard temperature and pressure, while MI is a liquid, thus easier and safer to transport and store.  Neither is flammable at less than 200 degrees Fahrenheit.  MI can be slightly reactive at elevated temperatures and pressures (it rates a 1 on a scale of 0-4); not an issue here.  They are both reasonably and about equally soluble in water.  So much for physical characteristics.

In terms of toxicity, MI is about 2.5 to 3 times as toxic as MBr, which surprised me.  More on that later.  Depending on the agency, they are both considered carcinogens, although MI gets more votes than MBr.  The Clean Water Act lists MBr as a priority pollutant, but not MI.  Under Superfund spill reporting requirements, however, a notification for MBr is only required at 1000 pounds (the range is 1 to 5000), whereas it’s 100 lbs. for an MI release.  The DOT Packing Group for both is group I (out of III), where packaging has to conform to the highest standards.  MBr is considered to only exhibit acute (short-term) health effects, while MI produces both acute and chronic effects; the NFPA health rating for each is 3 on the 0-4 scale.  Body systems affected (or “target organs”) are the respiratory system, the kidneys, and the central nervous system, with MI also affecting the liver.  Both are used to kill insects, worms, and rats, and fungi, and there are many types of soil molds that can damage plant roots.  MBr has been used not only for strawberries but also for almonds, rice,etc.

Prior to land disposal, MBr waste must be treated down to 0.11 milligrams per liter, while the standard for MI is only 0.19 mg/L.  As can be seen from that, and the rest of the above, MI is worse than MBr in some ways, and not as bad in others.  So, what does it all mean?  While I favor biological pest control, composting, and using as little pesticides, herbicides, and fertilizers as possible, such techniques are still not that practical on big factory farms.  I think, on the whole, that the substitution makes sense, since MI may be more toxic but is less likely to actually expose those handling and applying it, if proper methods are followed.

A commenter has rightly pointed out  some errors in what I originally covered last here, that is, about relative persistence in the environment of MI vs. MBr.  Is the former more, or less, resistant to degradation than the latter?  The answer turns out to be “YES” (depends on the medium).  I did some more research when I wasn’t so tired, and I’m writing a followup.  I’ll post the comment followed by my corrected treatment of the subject, as well as a few other points, in the next day or so.  Happy New Year!

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Responses

  1. This review is a smokescreen. While he’s made a broad review of the available “data”, he skipped over its impact and relevance. The key is that MI is much less volatile than MB. Because of this it hangs around a lot longer and it is more difficult to aerate or remove after use and thus exposure levels are much greater than MB. When this more difficult aeration issue is combined with MI’s higher reactivity (the carbon-iodine bond in MI is much more susceptible to attack than the carbon-bromine bond in MB) then the result is a much more dangerous chemical. The standards used for MB have grown up, hit and miss, over decades and applying those standards to MI is both foolish and grossly irresponsible.

  2. The most intelligent commentary I have read lately on this subject. Sounds like you are a chemist or even better a thinking person.

  3. thanks, James. Karl’s e-mail bounced back, so i guess he’s just being a sniper. i’m not seeking enemies, just mutual understanding. we’ll see what happens, eh?

  4. A very readable and informative post, Green Knight. All I can say about KarlT is that he could critique, and even add the information you did, WITHOUT insulting the post (“This review is a smokescreen.”) I would have to ask him: a smokescreen for what? You sound like YOU have some axe to grind. Come clean.


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