The Centers for Disease Control says most of the patients sickened in an ongoing wave of lung illnesses reported using vaping products containing THC, the active ingredient in marijuana.
No single substance or device has been definitively tied to the illnesses.
According to a report looking at patients who used THC products in Illinois and Wisconsin, most got the products from family and friends, off the street, or through illicit dealers.
Meanwhile, tests on a sampling of 18 THC cartridges commissioned by NBC News found troubling results.
CannaSafe, one of the nation's leading cannabis testing facilities, found no dangerous contaminants in three samples purchased from legal dispensaries, but pesticides and high levels of Vitamin E were found in most of the black market samples.
Vitamin E is known to cause lung damage when inhaled, and the pesticides contain a substance that can transform into hydrogen cyanide when burned.