According to CBS News, research being presented to the American Chemical Society has found that the potency of marijuana ha increased significantly in the past 20 to 30 years. Currently, the average potency of THC in marijuana is around 20 percent, but some samples were found to have up to 30 percent THC. This compares to an average of four percent THC in the 1980′s and 15 percent in 2012. This could mean an increased risk for addiction or adverse and unpredictable reactions. Additionally, cross-breeding may be causing changes as well, as many samples were found to contain little to no cannabidiol, the compound which gives pot its medicinal properties. There have also been tests for contamination since any plant is susceptible to microbial growth, and some marijuana plants have been found to be harboring mold, E. coli and salmonella.