Marijuana at a medical marijuana dispensary in Los Angeles. Twenty-three states and the District of Columbia have legalized medical marijuana. David McNew/Getty Images hide caption

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Using chemicals to control bugs or mold is common among commercial cannabis growers. But with no federal oversight, experts are concerned growers may be using dangerous pesticides. Ted S. Warren/AP hide caption

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Polls show changing American opinion on marijuana, and it's having an effect on politics. Leon Neal/AFP/Getty Images hide caption

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Leaders of the Alaska Cannabis Club share a joint at their medical marijuana dispensary in Anchorage. On Tuesday, Alaska became the third state in the nation to legalize recreational marijuana use. Mark Thiessen/AP hide caption

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Deuel County Sheriff Adam Hayward shows off a container of confiscated marijuana in Chappell, Neb., in July. Nikki Kahn/The Washington Post/Getty Images hide caption

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A marijuana bud displayed in Denver. Don't legalize pot, the pediatricians say, but don't lock teenagers up for using it, either. Seth McConnell/The Denver Post/Getty Images hide caption

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