“Fire breathing is a stunning but potentially injurious stunt. The fire-breathers direct a mouthful of fuel forcefully or creates a fine mist by spitting through pursed lips which is ignited over a flame resulting in a stunning visual show of plume, pillar, ball, volcano, or a cloud of fire [Figure 2].”
The potential injuries are described by Dr. Sanjay Saraf, from the Department of Plastic Surgery, NMC Specialty Hospital, Dubai, UAE, in a 2012 article for Indian Dermatology Online J. 2012 Jan-Apr; 3(1): 73–74.
“There are several immediate and long-term health hazards associated with fire-breathing act. Burns are the most obvious fire-breathing danger; however, there could be facial hair loss, ingestional toxicity, cutaneous irritation, peptic ulcers, fire-eater’s pneumonia (hydrocarbon pneumonitis/chemical pneumonia),[3,4] inhalational injuries, and Acute Respiratory Distress Syndrome.“
See: ‘Fire-breathing burn’
BONUS: Fire-breathers looking for a safe(r) non-toxic alternative (using corn starch) can check this video >