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The pesticide that sickened a Delaware family in the Virgin Islands is banned for indoor fumigation but U.S. growers will still legally use more than 375 metric tons of the chemical on fields this year through special waivers, federal regulators said.

Methyl bromide is blamed in the accidental poisonings of a mom, dad and their two teen sons at a Caribbean resort. The boys remained critically ill Monday at a Philadelphia hospital. Criminal investigators are examining how and why the bug killer got sprayed in a room beneath the family's rented villa two days before they arrived in mid-March.

U.S. law forbids exterminators from using methyl bromide but the Environmental Protection Agency grants a "critical use exemption" to certain farmers — primarily strawberry growers — letting them inject the chemical directly into their soil, the EPA said.

And some organic advocates are worried about the pesticide perhaps reaching grocery-store fruit.

"You have nurseries producing strawberry transplants — the nurseries are the main users of methyl bromide in the U.S. today. The plants in the fields are all started in nurseries. That ground at the nursery is all fumigated," said Jonathan Winslow, field services manager at Farm Fuel, Inc., a farmer-started, organic distribution and research company on the central California coast.

"The agency (EPA) is still working to determine how much methyl bromide to allow for critical uses in 2016 and 2017"

"So that strawberry transplant can get pulled out of the ground at the nursery and moved to an organic field and be produced under an 'organic' certification," Winslow said. "The use of methyl bromide has diminished, yes. But I am concerned about it. That's why I work for an organic company."

The pesticide is so nasty that, in 1987, the United States and 26 other nations signed a treaty called the Montreal Protocol, vowing to phase out methyl bromide mainly because it depletes the ozone layer. Today, nearly 200 countries have signed that agreement.

But the use of the neurotoxin goes on in farming.

"In the United States, strawberries and tomatoes are the crops which use the most methyl bromide," the EPA says on its website. "Other crops which use this pesticide as a soil fumigant include peppers, grapes, and nut and vine crops."

The EPA offers a "critical use exemption" for methyl bromide to users "who have no technically and economically feasible alternatives and where the lack of methyl bromide will result in a significant market disruption," the EPA said in a statement emailed to NBC News.

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"There is no specified end date for the critical use exemption, but the amount of methyl bromide allowed for critical uses has declined from 7,659 metric tons in 2005 to 376 metric tons in 2015. The agency is still working to determine how much methyl bromide to allow for critical uses in 2016 and 2017," the EPA said.

In California, where most U.S. strawberries are grown, farmers have relied for decades onmethyl bromide to control diseases and pests.

According to the California Department of Pesticide Regulation, growers in that state used 3.8 million pounds of methyl bromide on 30,000 treated acres in 2012 (the most recent year for which figures are available.) By comparison, California growers used 6.5 million pounds on 53,000 treated acres in 2002.

"Exposure to this chemical will affect not only the target pests it is used against, but non-target organisms as well"

"The use of methyl bromide in California agriculture has dropped significantly over the years as the fumigant is being phased out," said Charlotte Fadipe, spokeswoman for the California Department of Pesticide Regulation.

There appears to be a disconnect, however, between California and federal officials as to when the pesticide may be banned outright.

In California, regulators expect the EPA's critical use exemptions to "go away" in 2017, Fadipe said.

Growers who now get that federal waiver use tractors to drag machines across their plots. Those machines inject the pesticide 12 to 24 inches below the surface of the soil, the EPA said.

That fumigation is done before a crop is planted and it "will effectively sterilize the soil, killing the vast majority of soil organisms," the EPA adds.

Immediately after the pesticide is injected, a field is temporarily covered with a plastic tarp, trapping the chemical in the ground.

"Exposure to this chemical will affect not only the target pests it is used against, but non-target organisms as well," the EPA says on its website. "Because methyl bromide dissipates so rapidly to the atmosphere, it is most dangerous at the actual fumigation site itself."

The people who handle the odorless pesticide are usually considered most at risk.

Human exposure to high concentrations of methyl bromide can cause the central nervous system and the respiratory system to fail, the EPA says. Symptoms of poisoning can include slurred speech, blurred vision, and temporary blindness. More severe symptoms may include bleeding in the brain, heart, and spleen.

In grocery-store fruit, the amounts of potential exposures that concern some organic advocates are far, far smaller and would not lead to such symptoms.

But those advocates still hope to see a full ban enforced in two years.

"Methyl bromide is still being used in a very small scale in agriculture in the ground, and it's done under these critical use exemptions. Originally, that was supposed to have ceased by January 2015," said organic advocate Winslow.

"They're talking about maybe having these critical use exemptions available at an even more diminished amount next year — to finally cease in California in 2017," Winslow added. "We'll see if that actually happens."