Ahh so wiki has spoken, but I do think it's important to put things in context:
In vivo and in vitro studies are showing yerba mate to exhibit significant cancer-fighting activity. Researchers at the University of Illinois (2005) found yerba mate to be "rich in phenolic constituents" and to "inhibit oral cancer cell proliferation" while it promoted proliferation of oral cancer cell lines at certain concentrations.[26][clarification needed] This activity was due in part to inhibition of topoisomerase II activity in yeast.[26]
Conversely, yerba mate consumption has been associated with increased incidence of bladder, esophageal, oral, squamous cell of the head and neck, and lung cancer.[27][28][29][30][31][32] However, a case-control study[33] showed no increased incidence of bladder cancer in mate drinkers.
A study by the International Agency for Research on Cancer showed a limited correlation between oral cancer and the drinking of large quantities of hot mate. Smaller quantities (less than 1 liter daily) and warm rather than hot mate consumption were found to increase risk only slightly; alcohol and tobacco consumption had a synergistic effect on increasing oral, throat, and esophageal cancer. The increased risk, rather than stemming from the maté itself, could be credited to the high temperatures in which the mate is consumed in its most traditional way, the 'chimarrão'. The cellular damage caused by thermal stress could lead the esophagus and gastric epithelium to be metaplasic, adapting to the chronic injury. Then, mutations would lead to cellular dysplasia and to cancer.[34] Given the influence of the temperature of water, as well as the lack of complete adjustment for age, alcohol consumption and smoking, the study concludes that mate is "not classifiable as to its carcinogenicity to humans".[35]
Researchers in Mississippi found that both cold and hot water extractions of yerba mate contained high levels (8.03 to 53.3 ng/g dry leaves) of carcinogenic polycyclic aromatic hydrocarbons (PAHs) (i.e. Benzo[a]pyrene).[36] However, these potential carcinogenic compounds originate from drying process of the maté leaves, which involves smoke from the burning of wood, rather than from the mate itself
I, for one, have been drinking only the air dried mate and like I said earlier I only drink it cold. I also don't smoke cigarettes. So I am not worried.
How much more carcinogenic could it be than say, corporate deodorants, or many of the strange "food" ingredients in a lot of the modern supermarket products?