Evan Mascagni '11, a Research Assistant at the California Anti-SLAPP Project (CASP), published an article at PolicyMic on pesticide contamination of cannabis. 

Mascagni writes, "While it is possible to at the least wash off non-organic fruits and vegetables to remove some pesticide residue before consumption, there is no way to wash the pesticides off a cannabis flower or bud. As a result, when marijuana is inhaled or ingested for the effects of its natural chemical compound, tetrahydrocannabinol (THC), additional unknown chemical residues enter one’s system as well."

Read the full article at PolicyMic.