OZONE, A NATURAL OXYGEN (O3; 3 oxygen atoms) molecule that can also be synthesized, is among many other things an alternative to fumigants such as chlorine, methyl bromide, phosphine, and sulfuryl fluoride. Modern day humans have an almost schizophrenic, Jekyll-and-Hyde relationship with ozone, alternately and simultaneously seeking to eliminate and preserve ozone in the environment.
At stratospheric heights 12-15 miles (20-25 km) above the Earth’s surface, in what is called the ozone layer, 90% of the planetary ozone swirls and drifts about in molecular clouds, protectively absorbing mutation-inducing ultraviolet solar radiation wavelengths (also linked to immune disorders, cataracts, skin cancer, crop damage). For that reason an international treaty, the Montreal Protocol, was enacted in the mid-1980s to protect the ozone layer against known and unknown (to be discovered in the future) molecules destructive to ozone. That’s the good Dr Jekyll aspect of ozone (with apologies to Strange Case of Dr Jekyll and Mr Hyde author Robert Louis Stevenson).
Closer to Earth’s surface, where the 10% of planetary ozone not in the upper atmosphere resides, metropolitan areas such as Los Angeles and Phoenix suffer from ozone pollution and come under regulatory fire from the U.S. Environmental Protection Agency (EPA) because what is good 12 miles high is harmful at ground level. In “America’s Most Polluted Cities,” Forbes.com’s (04.28.10) Tim Kiladze calls it “harmful ozone, a ground level gas that contributes to urban smog and inflames the lungs, causing shortness of breath, wheezing and throat irritation.” The American Lung Association State of the Air 2010 report web site formulates EPA ozone and particle pollution data for American cities and counties into searchable form.
So, will ozone gas as an alternative to ozone-destroying fumigants like methyl bromide create more ground level ozone pollution or help replenish and preserve the protective stratospheric ozone layer?
“Chemicals can have their seasons, just like fashions. What one age admires as fine, another will reject as folly, and a good example of this is ozone,” writes John Emsley in his 1998 book, Molecules at an Exhibition. “A century ago ozone was also something to worry about, and for exactly the opposite reason: it was thought there was not enough of it around. Ozone was deemed to be natural, wholesome and invigorating, and the very locations where its levels were highest proved this: up in the mountains and along the coasts…Such was the esteem in which the Victorians held ozone that they had generators pumping it into churches, hospitals, theatres and even their underground railways.”
At the 2009 Entomological Society of America (ESA) annual meeting in Indianapolis, ground-level ozone was back in fashion as an environmentally-friendly fumigant molecule (unstable; decomposes quickly). Methyl bromide (another natural molecule) faces demise as a fumigant under the Montreal Protocol for destroying ozone. And stored grain insect pests are becoming resistant to phosphine. Carbon dioxide, a waste gas exhaled by humans and a feedstock of sorts for green plants, has some fumigant potential but is too slow-acting and unfashionable as a warming greenhouse gas.
Though it is easy to pump ozone through hoses into grain bins, ozone does not penetrate the dense grain well and fumigations can last 3-10 days. Purdue University’s Marissa McDonough noted that the grain is more quickly disinfested of red flour beetles (Tribolium castaneum), maize weevils (Sitophilus zeamais) and other pests by physically moving the grain in layers exposed to ozone sprays in passing.
Potency, ease of use, and low cost make for a good fumigant, said Kansas State University’s Mahbub Hasan, who noted that phosphine, sulfuryl fluoride and ozone are all potential alternatives to methyl bromide fumigation for dry-cured hams attacked by red-legged ham beetles (Necrobia rufipes), ham mites (Tyrophagus putrescentiae), cheese skipper flies (Piophila casei) and carpet beetles (Dermestidae). Carbon dioxide was too slow, said Hasan, taking six days to knock out ham mites. An integrated (IPM) approach combining biological controls, cooling, pheromone traps, ozone and botanical fumigants may ultimately be devised for protecting stored foodstuffs.
However, ozone remains out-of-favor in the plant world. “Tomato plants may be more susceptible to wounding by caterpillar pests in ozone than in ambient air,” Western Illinois University’s Sue Hum-Musser told the ESA meeting in Indianapolis. “Several genes play important roles in plant defenses against the combined stress of ozone and insect herbivory. Increasing ozone levels cause damaging effects on the plant community, and insect pests cost billions of dollars to agriculture.”
Fashion is a fickle thing, and future ozone fashions will likely remain a twenty-first century Jekyll and Hyde muddle.