Last October, dozens of DEA agents and police officers, some clad in hazmat suits, raided multiple marijuana grow-ops in Denver. The use of the protective clothing in the operation appeared excessive — after all, the agents' targets were indoor pot farms, not meth labs. But the drug cops may have had a legitimate reason to be wary: City inspectors would later catch growers slathering their plants with pesticides.

The DEA doesn't necessarily care if growers use toxic chemicals on their weed — the federal government considers cannabis illegal whether it's organic or not. An agency spokesman told VICE News that agents "wear personal protective equipment and gear as dictated by the circumstances of the particular grow," and referred questions about the raid to federal prosecutors in Denver, who said "no charges have ever been filed based on the use of pesticides" in a marijuana grow.

It appears the raids were unrelated to extensive pesticide use later documented by Denver's Department of Environmental Health (DoEH), which led to thousands of plants being quarantined, the destruction of crops, and an ongoing investigation. According to records obtained by the industry watchdog Cannabis Consumers Coalition and shared with VICE News, Denver health inspectors found plants treated with a variety of chemicals.

The boom caused by state-level cannabis legalization — Colorado, Washington, Oregon, Alaska, and Washington, DC, allow recreational use of the drug, while 23 states permit some form of medical marijuana — has prompted farmers to embark on larger-scale grow operations to meet the higher demand. A mature pot plant can be worth as much as $5,000, and pests can destroy thousands of them in a matter of days. With stakes that high, some growers are doing whatever it takes to protect their investments.

Mitch Shenassa, author of The Cannabis Aficionado's Handbook, told VICE News it doesn't take an expert to spot pesticide-tainted pot. "If it turns black and sort of pops, then it probably has pesticides in it," he said. "It's sort of like the difference between drinking antifreeze and nice French wine."

Even in states where growing pot is legal, using commercial pesticides without EPA approval is still prohibited. But the fact that weed remains strictly illegal under federal law has left a gaping loophole: As a federal agency, the EPA can't approve any chemicals for use on an outlawed crop. Even legal growers had no guidelines until recently, and several states are still catching up. With a black market still supplying most of America's recreational pot smokers, the vast majority of marijuana farmers are free to put whatever they want on a product that most consumers plan to inhale into their lungs.

The impact of pesticides on marijuana crops is still relatively unknown. The products that growers use aren't necessarily illegal, and the EPA has approved many of them for use on food crops. But their safety profile covers only ingestion — not inhalation. Jay Feldman, director of Beyond Pesticides, a DC-based nonprofit that monitors pesticide and environmental policy, told VICE News the lack of regulation poses a genuine public health concern.

"When you prescribe marijuana for medical use, especially for patients who have cancer, or immune-deficiency conditions, or seizures, the EPA has extra responsibility to evaluate these chemicals," Feldman said.

In March, Colorado became the first state to issue informal pesticide guidelines for pot farmers, working with the state's Department of Agriculture to "assist growers" in selecting what to use on their weed, suggesting products that have been cleared for use on similar plants, such as tobacco. Washington, Nevada, and Illinois have since followed suit with similar lists.

Sixteen of the legal marijuana states require some form of lab analysis before a cannabis product can hit the shelves. The testing, however, can be unreliable. Industry experts told VICE News that there's no standardized process for lab testing pot, little oversight of what actually happens in the facilities, and an incentive for businesses to mislabel pot in order to help their clients sell to dispensaries.

"Unfortunately, none of the testing labs are accredited in Oregon right now," said Sam Chapman, founder of New Economy Consulting, which specializes in the marijuana industry. "You can take one product to one lab and it will test at 27 percent [THC], and in another lab it will be 12 percent. There's no standard operating procedure or consistency between labs, which almost makes them pointless. You can give me a gram of marijuana and I could write on a piece of paper in crayon that I tested it at 97 percent THC, and technically that would be valid."

Another industry expert pointed out that even the most professionally run labs are not required to test for contaminants other than pesticides — like the pests themselves. At the recent "Weed the People" party celebrating legalization taking effect in Oregon, Patrick Irwin, a representative of the CO2 Company, which specializes in cannabis extractions, showed VICE News a large glass jar of buds from a crop infested with spider mites. It had been submitted to the "growers garden," where 1-gram samples were handed out to the public.

In severe cases, Irwin said the mites can leave spider-like white webbing around the leaves and buds. In this instance, the leaves were pale and dotted with tiny, almost imperceptible, white spots. The bugs sometimes die — often after being sprayed with pesticides — leaving their carcasses and bits of feces on the weed. Savvy dispensary owners would likely spot and refuse to distribute such a product, but legally there's nothing wrong with offering it to the public.

"Your weed could test totally clean for pesticides, but it could be full of rotten bugs because they don't test for that," Irwin said. "It blows my mind."

Jeffrey Raber, who holds a PhD in chemistry and runs a cannabis analytics lab in California called WercShop, conducted a study in 2013 for the Journal of Toxicology, which examined how much pesticide residue is transferred to a cannabis consumer through inhalation. In his experiment, Raber used three pesticides available for commercial use on food produce. He found that pesticides are easily transformed into a vapor and inhaled with marijuana smoke.

It's possible that a seemingly innocuous pesticide could transform to a more toxic or volatile substance when heated, but Raber, whose lab was raided by Pasadena police in April, told VICE News that's still uncharted territory from a research perspective. Still, he said it's clear that smoking pesticide-laced pot is unhealthy.

"If you smoke something," he said, "you're basically injecting it directly into your bloodstream."

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According to a report compiled by marijuana industry groups, the market for legal marijuana in the US grew 80 percent last year, bringing in a total of $2.4 billion. That figure is projected to swell to $11 billion by 2019 as the so-called green rush continues. Cannabis is going corporate fast, but, as the recent raids in Denver show, there are few ground rules, and some of the big industry players have already run afoul of new pesticide restrictions.

Larisa Bolivar, director of the Cannabis Consumers Coalition, was determined to find out which pesticides the Denver inspectors identified during their raid and whether disciplinary action was taken. Bolivar filed a request under the Colorado Open Records Act for the companies' grow logs, inspection reports, and other documents, which she shared with VICE News.

LivWell, one of Colorado's largest grow operations, also maintains a chain of medical marijuana dispensaries across Colorado, with nine locations in five cities selling what their website describes as "high quality medicine." According to Bolivar's records, the company was treating its plants with Eagle 20, a fungicide that was not among those recently recently approved by the state for use on cannabis plants.

After the raid, the Denver Department of Environmental Health placed a "hold" on contaminated plants. Growers could harvest and maintain their crop, but not sell it for consumer use. LivWell had a total of 60,000 plants temporarily taken off the market, worth up to $300 million.

Either the Colorado Department of Agriculture or "an independent accredited laboratory" was required to carry out a pesticide analysis to prove that the level of chemical residue on the buds was "insignificant" and safe enough for human consumption.

A LivWell spokeswoman told VICE News that after running their contaminated plants through the lab tests, business "went back to normal." She declined to share the results of the chemical testing with VICE News, and would not comment further on the specifics of the raid.

LivWell's logs appear to show regular applications of Eagle 20, which is used by growers to treat powdery mildew, a white mold that spreads rapidly and can be toxic to humans. Eagle 20 is registered by the EPA for use on stone fruits like peaches, as well as on ornamental roses and lawns — but not on tobacco. It's also classified a carcinogenic "bad actor" by the Pesticide Action Network, which claims it may cause long-term damage to the human reproductive system.

LivWell weren't the only ones in Colorado using Eagle 20. Others, such as Organic Greens and Herbal Alternative, advertise themselves as "organic," but their grow logs seemingly tell a different story.

"They were operating unethically across the board," Bolivar told VICE News. An Herbal Alternative employee initially denied using pesticides when contacted by VICE News, then clarified and said that the company no longer uses them. Organic Greens, whose parent company is Natural Remedies, did not respond to repeated requests for comment.

Bolivar said other big industry players in Colorado such as MMJ America and The Green Solution were using Avid, a pesticide used commercially to kill mites on Christmas trees and other ornamental plants. Like Eagle 20, the Pesticide Action Network says Avid can potentially have harmful effects on the human reproductive system. Bolivar noted that MMJ America later invited the Cannabis Consumer Coalition to tour their facilities after the raid as a show of good faith.

Documents obtained by Bolivar show that many of the targeted companies were also using Mallet, Akari, and Abamectin, insecticides that, according to the warning labels on the products themselves, can cause respiratory problems, birth defects, and other problems depending on the level of exposure.

John Scott, the Colorado Department of Agriculture director, told VICE News that he is unable to disclose whether disciplinary action will be taken against the growers who used the banned pesticides because an investigation is still underway. Dan Rowland, a spokesman for the city of Denver, told VICE News the most the city can legally do is put a grower's plants on "hold," and that some business owners opted to destroy their plants rather than deal with the hassle.

"It is a complex jurisdictional issue and a unique situation," Rowland said. "If our environmental agency was concerned that the chicken a restaurant was using was unsafe, we'd ask that they throw it out. But it's a new industry and a new area with very young science, so we didn't condemn any of their product. If it was a different product, we might have. It's been an educational process."

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There are many growers who go out of their way to avoid using pesticides at all. Several farmers told VICE News they are keenly aware that harsh chemicals could seriously harm medical patients — or recreational users — and instead rely predominantly on preventative measures to nip pest infestations in the bud.

Cristian Koch, the founder and master grower at HiFi Farms in Portland, Oregon, explained the company's technique of strengthening a plant's immune system by introducing carnivorous mites, such as microscopic worms or ladybirds, which prefer to eat other mites rather than cannabis foliage.

"The plants can do half the work for you," he told VICE News. Koch said HiFi also uses imported neem oil from India; a concoction of wheatgrass, rosemary, and thyme; and a compost mixture that cultivates bacteria in the soil to break down the eggs of spider mites and other pests.

Dustin, the head grower at Otis Gardens in Oregon, agreed to show VICE News around a state-of-the-art warehouse grow-op of about 100 large plants on the condition his last name not be used. The crop is entirely organic, and he stressed that prevention is the key to avoiding pesticides entirely. Anybody who sets foot in his grow rooms has to step on a mat with disinfectant, and hand sanitizer dispensers are set up outside each door.

"Everything is grown in a sealed environment," Dustin said. In one grow room, he sprinkled carnivorous mites from a tall white plastic cylinder "like parmesan" onto the potted soil beneath several mammoth, six-foot tall plants.

"Most of the research you find published [on pesticides], even if it's safe for food crops, it hasn't been tested on cannabis flowers," he said. "We're very selective about the pest control methods we use."

If pests get past their first lines of defense, Dustin's preferred strategy is to heat up the grow rooms — which are normally kept at a steady, air-conditioned 78 degrees — to about 115 degrees for an hour.

"It kills most pests, and most beneficial insects are resistant to it," he said. "It stresses out the plants, you might lose a day or two of growth, but if it kills off a significant portion of the pests, the plants will thank you for that."

Both HiFi Farms and Otis Gardens are licensed to grow for medical purposes, but even some growers who work illicitly choose to eschew pesticides. A grower based in New York who asked to be identified as Elly has 99 plants, which are grown almost exclusively for a 15-year-old girl with severe cerebral palsy who is fed a mixture of cannabis and coconut oil through a tube every day.

"I just really wouldn't want to use anything else," Elly said of his organic approach.

Of course, weed that is grown and sold exclusively on the black market has no restrictions whatsoever. In some cases, such as at outdoor farms in northern California's infamous Emerald Triangle, police have found jugs of pesticides and other evidence that suggests the use of chemicals in these clandestine grows is widespread. With no oversight, these chemicals often seep into local streams and waterways.

Industry advocates hope that as more states legalize weed, the black market will dissipate and the federal government will allow agencies like the EPA to develop guidelines to help prevent the use of harmful pesticides on pot. States that have already legalized the drug are also constantly updating their rules and regulations, trying to create a regulatory framework for what is still a very new industry.

Colorado officials require all marijuana products sold in the state to be properly labeled, listing the pesticides used during production. They also require medical marijuana to be tested in a lab for pesticide residue before hitting the shelves. But even with standardized lab testing, the question of which pesticides are safe to use on marijuana crops remains unanswered.

Asked about pesticide oversight, EPA spokeswoman Cathy Milbourn sent VICE News a letter the agency had previously sent to Mitchell Yergert, head of the plant division at the Colorado Department of Agriculture. Yergert wrote to the EPA earlier this year seeking advice on how to address the issue. In response, the agency suggested that Colorado pursue a "Special Local Needs" (SLN) registration, a status granted when states can prove they have a unique agricultural condition that makes them exempt from federal guidelines. The EPA wants Colorado to identify pesticides approved for crops similar to marijuana, such as tobacco, tea, barley, and sage.

For Scott, the fact the EPA is willing to work with his agency is a good sign. The Colorado Department of Agriculture chief said they are still in the process of compiling a database of pesticide products they believe might be safe for marijuana cultivation.

But ultimately, Feldman, the director of the pesticide and environmental policy monitoring group, said it's unlikely that the EPA will grant Colorado's request for SLN status, since it would entail the federal government treating marijuana like a legal substance. "They're caught in a bind," he said.

Though Denver is way ahead of the curve compared to other states and cities that have legalized weed, Rowland, the Denver spokesman, said the city is still trying to decide what's safe for consumers while balancing the interests of business owners.

"Obviously the easiest thing to do would be to set the limits at zero," he said. "But we're trying to find a middle ground and navigate this emerging agriculture and industry. We determine the tolerances by looking at tolerances that are set on other products, and we take the lowest one and apply it to marijuana."

He added that the city has faced a steep learning curve, but suggested that increased awareness is already leading to progress.

"I didn't know anything about pesticides on marijuana six months ago," Rowland said. "It's in everyone's best interest to keep consumers safe, and it's cool that we're having this conversation."