Atmospheric scientists at the Desert Research Institute (DRI) have turned their attention toward the growing e-cigarette industry and found that toxic aldehydes, such as formaldehyde, are formed during the chemical breakdown of the flavored e-liquid during the rapid heating process (pyrolysis) that occurs inside e-cigarettes or electronic nicotine delivery systems (ENDS).
Since the dose makes the poison, is this a concern? Not really, but in the modern world of International Agency for Research on Cancer (IARC) hazard assessments, risk has become irrelevant and the presence of any compound, even in trace levels, is declared carcinogenic if the levels are within seven orders of magnitude, so 10,000,000 to 1.