@Jim: thanks! Not a chemist per se, more like a macroscientist…earth science degrees kind of drag all the other disciplines in, one way or another.
@Karl: This blog is for the general reader, not the chemist, but if simple comparisons of “data” aren’t enough, and if I didn’t cover the impact and relevance enough for you, I’m happy to get more technical. I should also point out to readers that though MBr was used on other crops, strawberries are of more concern because of their talent at absorbing pesticide residues. It’s also important to note that even nearly all “organic” California strawberries have traditionally been grown in soil that was first already fumigated with methyl bromide anyway. See more on this at: http://www.motherearthnews.com/organic-gardening/methyl-bromide-toxicity-what-is-on-your-strawberries.aspx.
First of all, I wasn’t trying to say that methyl iodide is okay, or harmless; I was just comparing it to what was being used before. I wasn’t applying MBr standards to MI; I was comparing the two to look at similarities and differences. My goal in this blog is to look at environmental issues rationally and in a way that compares risks and puts them into perspective…some things people are concerned about pale in comparison to issues they’re not even aware of, like mixing bleach with ammonia (I’ve had students who did that in past years – before they ever took my Hazwoper class – much to their regret).
I remember 30 years ago when I worked for Greenpeace in San Francisco, and we were all gonna die from malathion exposure because of the medfly crisis. That pesticide is one of the most innocuous ever produced, and is a lot less nasty than its relative diazinon, which was still available for residential use until 6 years ago on New Year’s Eve. There’s an anniversary for ya.
Let’s look at toxicity for a moment. I generally look at OSHA workplace exposures, the 8-hour Permissible Exposure Limit, and the instantaneous Immediately Dangerous to Life or Health (IDLH) numbers. By this comparison, MI is worse, having a PEL of 5 parts per million and an IDLH of 100 ppm, vs. 20 and 250 for MBr; however, OSHA has had more stringent worker protection standards thrown out in the courts before, and such is the case with MBr, which previously had a PEL also of 5 ppm. For toxicology, the lethal dose for 50% of the test population (LD50) in rats is 214 milligrams per kilogram body weight for MBr, and 76 mg/kg for MI. [Test results from different species vary considerably and are not reliably transferrable to humans or to other species…dioxin kills the hell out of hamsters but doesn’t do much to rats, for example.]
Let’s compare both to carbon monoxide for a moment. CO is deadly, and every winter I hear about whole families going to the hospital, or the morgue. It has a NIOSH Recommended Exposure Limit of 35 ppm, an OSHA PEL of 50 ppm, and an IDLH of 1200 ppm. The Compressed Gas Association has a limit of 10 ppm in breathing-air SCBA cylinders for firefighters and cleanup personnel. The IDLH is much higher than for our methylated halogens, but the 8-hour exposure numbers are comparable. CO is all around us: if we don’t clean the stove, the furnace, or the chimneys, if we use outdoor appliances such as grills indoors, we’re exposed; it’s also prevalent in car exhaust, tobacco smoke, and factory emissions.
To show how toxicology data can fluctuate and not always be comparable, look at the IDLH levels for MI, MBr, and CO in the last two paragraphs, and then read the following from the Wikipedia listing for methyl bromide: “Brief exposure to high concentrations and prolonged inhalation of lower concentrations are problematic. Exposure levels leading to death vary from 1,600 to 60,000 ppm, depending on the duration of exposure (as a comparison exposure levels of 70 to 400ppm of carbon monoxide cover the same spectrum of illness/death).” [from Hazards in the Chemical Laboratory, Muir, G.D., ed.,The Royal Institute of Chemistry, London, 1971.] In that study (admittedly 40 years ago), the findings were that CO is more toxic than methyl bromide. William Ruckelshaus, first head of the EPA and then running it again under Reagan, apocryphally said “give me the numbers and I can make them say whatever you want ‘em to.” I met Bill at a conference once, and would have asked him about that, but didn’t want to antagonize him on first meeting, and he seemed like a nice guy, even though I didn’t like his boss at all.
Methyl bromide and methyl iodide are both naturally produced by marine algae and kelp, serving as the main mechanism of cycling iodine, a vital nutrient, back to the land surface, and incidentally responsible for the characteristic smell of the sea (Lovelock, James, The Ages of Gaia, A Biography of Our Living Earth, W.W. Norton, 1995). Bromine is so far not recognized as a trace nutrient.
Is methyl iodide reactive? As I said, it gets a “1” rating on the NFPA’s 0-4 scale, meaning yes but not very. It is reactive with strong oxidizers, but so is just about everything else; a piece of wood, for example. Bringing up this non-issue is a red herring, or “smokescreen” if you like, because those potential chemical combinations occur in the lab, not out in the field. And it’s reactive with nucleophiles, including – mildly – with water, which we WANT. It’s hydrolysis, one of the three degradation processes, which also include biotic and photolytic activity, that break it down to innocuous or less hazardous byproducts.
Conversely, is methyl iodide more persistent in the environment than methyl bromide? Yes, in soil and water, but not in air. MBr’s higher volatility is a major reason why the 1987 Montreal Protocol mandated that it be phased out, because it does far more damage to ozone than does chlorine, and has a much longer atmospheric residence time than does MI. Something of a tradeoff, but that’s how it usually works (see Nielson & Allard, 2003).
And speaking of its environmental fate, in order of publication, you may wish to consult:
Risk Management for Hazardous Chemicals, Volume 7, Jeffrey W. Vincoli, CRC Press, 1996,
Organic bromine and iodine compounds, Alasdair H. Neilson, A.-S. Allard, Birkhäuser, 2003,
and, of course,
“Methyl iodide (Iodomethane), Risk Characterization Document for Inhalation Exposure, Vol. III, Environmental Fate,” prepared by the Environmental Monitoring Branch, Department of Pesticide Regulation, California Environmental Protection Agency, in February 2010.
The US EPA’s fact sheet on the review process is also worth a look: http://www.epa.gov/pesticides/factsheets/iodomethane_fs.htm. The review took four years, and was one of the most thorough ever performed by the agency under its FIFRA program (that’s the Federal Insecticide, Fungicide, and Rodenticide Act, for those put off by acronyms.)
Here’s what Vincoli’s book has to say about methyl iodide: it’s not persistent in water, with a half-life of less than 2 days. It doesn’t bioaccumulate up the food chain; the amount in tested fish was the same as in the water they swam in. It’s highly soluble in water, and most of it that gets into a body of water, 99.5% of it, ends up in the air, and is then photolyzed (broken down by ultraviolet light). It exhibits only slight acute & chronic toxicity to aquatic life. Insufficient data on plants, birds, and terrestrial animals, he says, but that was 14 years ago. [NOTE: The CalEPA did some toxicology tests on quail in its recent study to determine lethal oral dose and air concentration; people forget that to find out how toxic a chemical is to animals, you have to expose them to it.]
Here’s some information from the CalEPA study: breakdown in water (hydrolysis) by non-biological means is slow with a half-life of 113 days. [“Half-life” simply means the time it takes for half of the material to break down into something less nasty.] Photolysis in water is faster, at a 13.1 day half-life. MI exhibits low sorption (bonding) to soil, and with its high vapor pressure, it’s therefore mobile in soil/water systems. It’s quickly metabolized by soil microorganisms under aerobic conditions with an aerobic soil metabolism half-life of 2 hours. Under anaerobic conditions (low oxygen), the degradation rate is slower, with a soil metabolism half-life of 41.8 hours.
“Iodomethane is more persistent than methyl bromide in soil and water. Gan and Yates (1996) reported half-lives ranging from 13 to 43 days in unsterilized soil, and neutral hydrolysis half-lives in the range of 50 – 113 days have been reported (Mabey and Mill, 1978; Schwarzenbach et al. 1993, DPR, 2002b).” Despite this, impact to groundwater was expected to be minimal due to volatilization and degradation in the upper soil layers. MI may be less volatile than MBr, but it still evaporates from moist soil & shallow groundwater readily, just somewhat slower than methyl bromide.
Degradation in soil and groundwater results in iodide ions, which aren’t cause for alarm any more than chloride ions are…soil salts are many and common. Another byproduct is methanol, but “Methanol is a degradation product of iodomethane hydrolysis, and methanol is known to be relatively susceptible to biodegradation (USEPA, 1994).”
Now let’s talk residues. Remember that MBr and MI aren’t sprayed on the strawberries, or even on the plants before the berries grow. They’re applied to the soil using either of two infiltration methods, to sterilize it by eliminating weed seeds, nematode worms, insects, and harmful fungi. Then the plants are planted. As we’ve seen, soil breakdown rates have been studied, so if the planting is done after waiting an appropriate time, there shouldn’t be much left to be uptaken by the plants. The problem is that washing the outer surface of the fruit, in this case, won’t do any good, because it gets in from the inside, so that issue needs to be addressed in regs dealing with application and planting schedules.
A relative of methyl bromide called ethylene dibromide used to be used to fumigate grain silos for weevils, so there was direct contact with the foodstuffs, and while MI and MBr are “suspect human carcinogens,” EDB is one of the few known human carcinogens, along with asbestos, benzene, Aflatoxin B1, and a few others (more on aflatoxin in my next post). EDB residues were being found on yer cornflakes in the box, so it was banned from most uses in the late 1980s. One alternative for killing the weevils was food irradiation, and since most people know next to nothing about ionizing radiation, they were afraid their food would be rendered radioactive…not possible unless using neutron rad, which you only get at power plants and weapons facilities. A little knowledge, right?
So, let’s sum up what we know about the old (methyl bromide) and the new (methyl iodide). MBr was used as an insecticide, rodenticide, herbicide (for weed seeds) and fungicide…Nielson & Allard call it “an important fungicide,” but Wikipedia’s listing says that it “has poor fungicidal properties.” It was used on strawberries, almonds, rice, tomatoes, and ornamental shrubs, and to fumigate ham/pork products (!). [The latter reminds me that they used to use methylene chloride, a known human carcinogen and hepatotoxin (liver damage), to decaffeinate coffee.] It’s also used on wood products, whole logs, and railroad ties to eradicate termites and other insects, and to control Bermuda grass on golf courses…I don’t know of too many environmentalists who are golfers, but there ya go. It was also used in fire extinguishers for electrical fires until less toxic halons became available, because it’s electrically non-conductive and “leaves no residue,” according to the Wikipedia listing (which is interesting, eh?).
They both have the [skin] designation, so applicators should be wearing fully-encapsulating suits if exposed directly, although the soil-delivery systems are automatic and use plastic tarps to avoid evaporation into the immediate atmosphere. Since MI is less volatile, even though more toxic, it presents less of an exposure potential to workers.
MI degrades rapidly in the atmosphere, which is why it’s a valid substitute for MBr in terms of ozone-layer protection. They’re both heavier than air, so how do they get 8-25 miles up to mess with the ozone? Answer: they get caught up by winds and blown around, like other heavier-than-air things like soot, dust, and volcanic ash; the various CFCs like Freon also weigh roughly the same as our methylated halogens.
As shown above, MI is not considered all that toxic to aquatic life, and exposures to terrestrial life are limited by the density and the use of ground covers during application to the strawberry patch. In surface and groundwater they degrade relatively quickly, compared to, well, see the next paragraph.
MI is more persistent than MBr in soil and water, but not by all that much, and it should be noted that all pesticides are reviewed for environmental persistence, as are new chemicals that aren’t pesticides. Our soil fumigants in question persist for days or weeks, while the bad actors we used in the “before time,” like DDT and chlordane, or PCBs and dioxins, last for decades, and are still contaminating sites around the world 50 or more years later.
In closing (and putting this together has not been a walk in the park; I should be getting paid for this, lol), it appears that the substitution is an improvement. A slight one, perhaps, but better than continuing to use the other stuff as long as they do it right. Again, I’m not a fan of pesticides, and use alternate methods around my own place, but the “Mother Earth News” article at the top of this post shows what the growers are up against. How much do you like strawberries?
For anyone who wants to explore these matters further, I recommend any of the informative textbooks by my former toxicology professor, Stanley E. Manahan, based in Columbia, Missouri, especially Environmental Chemistry (9th edition, 2009).