Some may tout marijuana as a safe, recreational drug with valuable medicinal properties, but toke up your joint and it likely contains mold, pesticides, even dead insects, according to researchers at the University of New Haven in Connecticut.
Much of it could be seen with the naked eye on the surface of plants, said Heather Miller Coyle, a forensic botanist and associate professor at the university, who has built a DNA database of different types of marijuana that has helped federal law enforcement learn where illegal pot growers and dealers get their product.
Now she has turned her attention to public health and is urging states where marijuana is legal to pass strict certification laws.
Marijuana is a controlled substance that is illegal under federal law. But in states where it is used legally for medicinal purposes, there is limited testing, and the testing that is done is for medical potency, not purity.
"Every other medicine out there is controlled and monitored for quality and not administered in a smokable format," Coyle told ABCNews.com.
"There's a lot of concern about the way these forms of medical marijuana are grown," she said. "A lot of the time, they are grown in a noncertified fashion, especially in California.
"They are grown in open fields or illegally on federal park lands," she said. "Pesticides are dumped on them to prevent damage and increase yield. Some are grown between crops of different species, and chickens are running around and fertilizers are being used.
"They are grown in the wild by people and are not certified by anyone," said Coyle. "What would the effect be on a person who is immune compromised or seriously ill?"
Coyle said potential health hazards could include allergic reactions, especially in the inhalation of molds or spores.
"In a normally healthy person, that might not matter, but for someone who is immune compromised, it could cause inadvertent death," she said.
Chemicals might also be toxic if smoked. Newer herbal blends made with synthetic THC, the psychoactive ingredient in marijuana, are also worrisome, said Coyle.
For the past five years, Coyle's research has been funded by a $100,000 grant from the U.S. Office of National Drug Control Policy's High Intensity Drug Trafficking Areas program. She and her students have developed a bar-coding system to identify the different varieties of marijuana and to study its genetic foundation. The system helps to detect contaminants and could be widely used to test medical marijuana.
"The health hazards have not been fully investigated, and it's not being grown in controlled greenhouse environments with state regulations and safety visits," she said. "Medical marijuana is of great concern because of claims that it has medicinal value."
National advocates for legalization such as the Washington, D.C.-based Marijuana Policy Project support efforts to certify product safety.
"Our mission is to end marijuana prohibition and to replace it with a system that regulates marijuana like alcohol," spokesman Mason Tvert told ABCNews.com. "In some sense, you could say our goal is to ensure that marijuana is being tested."