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The Most Toxic Pesticide You've Never Heard Of

Bad air quality on a Kern County farm | Photo: staticaantics, some rights reserved
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Farm workers sort potatoes. Photo: Ridofranz
Workers on farms where this soil fumigant is used run a substantially higher risk of cancer. Photo: Ridofranz

If you’re worried about the weed-killer glyphosate, a.k.a. Roundup™, you’re not alone. The herbicide is getting increasing critical exposure in the news — and on social media — as we learn more about its potential effects on the environment and human health. Roundup use is growing exponentially, so that concern is sensible. But there are other commonly used pesticides that don’t get nearly as much public attention, despite the fact that they’re significantly more dangerous to people and the planet. In this short series, we'll discuss five common pesticides whose ill effects on human health and the environment are demonstrably worse than Roundup's. 

If you value your health, you might want to hold your breath when you visit Shafter, California.

The small town northeast of Bakersfield is slowly moving away from its agricultural roots, with growing numbers of distribution warehouses, call centers, retail, and even a municipal fiber-optics internet backbone — the first in the Central Valley. But Shafter’s heart is still agricultural. The pleasant Kern County town is surrounded by almond and pistachio orchards, vineyards, and fields of cotton, alfalfa, carrots, and potatoes.

It’s also surrounded by a miasma of agricultural chemicals.

Air sampling in town has detected high ambient levels of the insecticide chlorpyrifos, the fungicide chlorothalonil, and toxic breakdown products of the soil fumigant metam sodium. And it’s also revealed measurable levels of a chemical that’s gotten a major corporation hauled into court: the soil fumigant 1,3-dichloropropene.

1,3-dichloropropene is used as a fumigant to kill soil pathogens before planting crops such as carrots, potatoes, sweet potatoes and other field vegetables, cotton, and orchard and vineyard crops. It’s a simple molecule, with just three carbon atoms, three hydrogen atoms, and two chlorine atoms. (Curious about the “1,3” part of the name? That denotes which carbon atoms the chlorines are bonded to. There are also “1,2” and “2,3” versions of dichloropropene, all with different properties.) That structure makes dichloropropene one of the simplest of a class of chemicals called organochlorines, which include some of the most toxic pesticides available.  

A molecular model of 1,3-dichloropropene | Image: Jynto and Ben Mills, some rights reserved
A molecular model of 1,3-dichloropropene. Green atoms are chlorine, black carbon, and gray hydrogen. | Image: Jynto and Ben Mills, some rights reserved

Banned as unsafe by the European Union, 1,3-dichloropropene is nevertheless one of the most commonly used pesticides in the United States, pound for pound. It’s used to kill nematodes, pathogenic fungi and bacteria, insects, and weed seeds in fields before planting. It’s been found in groundwater in many farm states, and — as in Shafter — in the air near and downwind of farms where it’s used.

Dichloropropene has been known since 1986 to cause cancers of various kinds in lab animals, and is classified as “likely to be carcinogenic to humans” by the EPA. It also causes respiratory ailments, serious kidney damage, reproductive problems, and eye and skin irritation. A 1989 study of 15 California farm workers exposed to dichloropropene found that nine of them had kidney damage.

Usually sold under the trade name “TeloneTM” by manufacturer Dow AgroSciences, dichloropropene is sometimes mixed with the fumigant chloropicrin to boost its effectiveness. Chloropicrin is an even more potent carcinogen, and some formulations of the mixture are banned from use in California as a result.

In 2011, California’s Department of Pesticide Review (DPR) started taking weekly air samples in Shafter and two other California farm towns: Salinas and Ripon. Those air samples are tested for the presence of 32 different pesticides, as well as five troublesome chemicals pesticides create when they break down.

Those tests revealed that Shafter’s air has a dichloropropene problem. As the DPR reported in 2014,

The data from the Shafter site showed that for the 4 years of monitoring, 1,3‐dichloropropene was detected at an average concentration of 1,135 ng/m3 [nanogram per cubic meter of air]. If this level continued for 70 years, it would exceed DPR’s cancer risk regulatory goal by 1.7 times.

That “if this level continued for 70 years” thing may seem an odd phrasing, but it’s DPR’s way of translating a common lifetime exposure standard into something approaching English. What they're trying to say is that living out your life in Shafter brings with it a measurably greater risk of developing cancer from the dichloropropene in the air.

Terminology: herbicides kill plants, insecticides kill insects, fungicides kill fungi. Fumigants kill almost everything. And “pesticide” is a blanket term that includes all of the above.

How much greater? In 1986, the EPA estimated that residents living downwind of fields treated with dichloropropene would develop as many as five extra cancers per 10,000 people. Field workers on treated farms may develop one or two cancers per thousand people. Workers in enclosed sheds run the worst risk of all, at six cancers per thousand people.

Dichloropropene’s small molecular structure makes it easy for the pesticide to volatilize — to dissipate into the air when it’s applied to a farm field. But once it dissipates dichloropropene can stick around in the air downwind, especially if it enters enclosed spaces. In 1990, a junior high school in Merced was found to a whole lot of dichloropropene in its indoor air: 900 times as much as the state considered safe at the time.

California temporarily suspended use of the pesticide as a result of the school contamination and other similar incidents. After pressure from Dow and its customers, who were anxious to replace the ozone-destroying fumigant methyl bromide (still in use in California strawberry fields), the state relaxed the temporary ban in 1995. Since then, state scientists and Dow AgroSciences have fought over restrictions on 1,3-dichloropropene use in the state, with the DPR often apparently siding with Dow against the recommendations of its own staff.

To the DPR’s credit, for some time, California has been the only state with limits on the amount of dichloropropene that can be used in a specific area. The rules set an upper limit on the amount of dichloropropene that can be used in a particular “township.” (Surveyors have divided much of the United States up into townships, each a square of land six miles on a side.) This “township cap” was set at 90,250 pounds per township in 2002, though townships could bank their unused poundage for use in later years, up to a maximum of 180,250 pounds per township per year.

Environmental activists, other agencies, investigative journalists, and the DPR’s own scientists held that this cap didn’t go far enough to reduce the threat to public health from 1,3-dichloropropene exposure. Activists called the “banking” rule a loophole that allowed unsafe use of the pesticide in communities already hard-hit by pesticide exposure. Scientists pointed out that nothing kept the pesticide inside the airspace over a township, meaning that people downwind of several townships using more than their share of 1,3-Dichloropropene could receive far more exposure than anticipated.

In October 2016, the DPR announced a new township cap intended to solve the banking problem. As DPR Director Brian Leahy put it, the new cap was stricter, compared to the two-tiered banking arrangement, and more straightforward. “I believe that overhauling the way we manage the pesticide, to be based upon a fixed amount, will be health-protective and simpler to manage,” said Leahy.

The problem: The new cap was actually higher than the previous one, raised from 90,250 pounds per township to 136,000 pounds: a 50 percent increase. It’s just that the banking loophole was eliminated.

Dichloropropene use in the United States, 2014 | Map: USGS
Washington used more dichloropropene than any other state, followed by California, Georgia, Florida, North Carolina and Idaho.  | Map: USGS

Environmental and public health activists were livid when the new cap was announced. “Once again, DPR has chosen to listen to Dow Chemical instead of agency scientists,” said Mark Weller, co-director of Californians for Pesticide Reform. “It’s beyond me how DPR can spin this as an improvement. And it’s simply egregious that DPR has again ignored the voices of the frontline communities most at risk, who have repeatedly demanded better protections from this cancer-causing chemical.”

Though Dow got what it wanted from the DPR, its troubles in California aren’t over. A month before the new cap was announced in October, the Oakland-based Center for Environmental Health sued Dow AgroSciences for failing to warn Shafter residents that its chemical was a known carcinogen, as required under California’s Safe Drinking Water and Toxic Enforcement Act of 1986, a.k.a. Prop 65. The group said it had filed suit because state and local authorities had failed to live up to their legal obligation to do so. Dow had provided warning signs for farms, said the Center, but had done nothing to warn residents who don't venture onto the farms.

Potatoes | Photo: 16:9clue, some rights reserved
Consumers of potatoes grown with 1,3-dichloropropene are, in effect, requiring these workers to expose themselves to this toxic chemical. | Photo: 16:9clue, some rights reserved

In its public response to the lawsuit, Dow pointed out that California has the most stringent regulations covering 1,3-dichloropropene of any state in the country. They’re not wrong. No other state has even insufficient local caps on dichloropropene’s use. Among U.S. counties, Shafter’s Kern County comes in just sixth in the amount of dichloropropene used, at least in federal states for 2014, the most recent available that include California. 

Which five counties used more dichloropropene in 2014 than Kern County’s 558,833 pounds? They’re all in Washington. Grant County near Walla Walla leads the pack, with just under 1.5 million pounds of the stuff used in 2014. A sign at the county line informs travelers on Interstate 90 that Grant County is “the nation's leading potato producing county." Grant County grew more than 1.3 million tons of potatoes in 2014, which means it took just over a pound of 1,3-Dichloropropene to grow each ton of potatoes. Put that way, it might not seem like a big deal.

Then again, the county has just 89,120 residents, which means local farmers used more than 16 pounds of the pesticide for every man, woman, and child within the county line, many of them farmworkers prone to higher exposures than their neighbors.

In 1992, Carolyn Cox put it this way in the Journal of Pesticide Reform:

Consumers of the potatoes, carrots, strawberries, pineapples, and other crops that are grown after fumigation with 1,3- dichloropropene are in effect requiring these workers to expose themselves to this toxic chemical.

Food for thought.

The other pesticides we're covering in this series are the weedkillers atrazine and 2,4-D, the insecticide chlorpyrifos, and the soil fumigant metam sodium.

Banner: Bad air quality on a Kern County farm | Photo: staticaantics, some rights reserved

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