Much of the marijuana sold in Colorado pot shops is polluted with pesticides, heavy metals and fungi, and often contains little cannabidiol, the “good” ingredient in cannabis, a study found. The surprising results of the investigation into the composition of the state’s legal weed, a product that is highly regulated but seldom tested for contaminants, were unveiled Monday at the national meeting of the American Chemical Society in Denver.

"It's pretty startling just how dirty a lot of this stuff is," Andy LaFrate, founder of Charas Scientific, a Denver lab licensed to test cannabis, told Smithsonian Magazine. While dispensaries are now required to have their products routinely tested for potency, state regulators have been slow to enforce rules regarding the testing of marijuana for impurities. In Washington state, which legalized marijuana for recreational use the same year as Colorado, officials were quick to begin analyzing cannabis products for things like E. coli, salmonella and yeast mold, which led to 13 percent of pot products sold there last year being rejected for failing such safety tests, Smithsonian said.

The study found today’s legal weed is much more potent than the pot of 30 years ago. In many cases, samples tested today contained two to three times as much tetrahydrocannabinol, or THC, the high-inducing ingredient in cannabis, as marijuana in the 1980s.  

In decades past, weed typically contained less than 10 percent THC, NBC News reported. Modern pot contains on average 18 percent to 30 percent THC, a claim supported by previous marijuana potency tests. Such powerful pot is the result of decades of selective breeding and improved cultivation techniques that allow growers to favor certain compounds in the plant, including pot’s psychedelic ingredient.

LaFrate’s team tested hundreds of strains of marijuana from dozens of Colorado producers using a method of separating a product’s individual compounds called liquid chromatography. Much of the weed they analyzed came from recreational pot stores, the first of which opened in January 2014. They discovered marijuana samples frequently contained fungi, bacteria and heavy metals, which the plant can absorb from the soil in which it is grown. Marijuana edibles -- food products infused with cannabis -- were found to contain solvents like butane, a potentially dangerous substance linked to heart muscle sensitization.

Among researchers’ other discoveries was many medical marijuana strains contained little cannabidiol, the compound in pot that is used to treat a range of ailments from anxiety to Alzheimer’s. They found the average amount of cannabidiol in Colorado’s medical marijuana was just 0.1 percent.

"It's disturbing to me because there are people out there who think they're giving their kids Charlotte's Web,” LaFrate told NBC News, referring to a form of concentrated cannabis used to treat epilepsy in children. “And you could be giving them no CBD -- or even worse, you could be giving them a THC-rich product which might actually increase seizures.”