On Fire!

Not to be confused with Burning.

On Fire! is a debuff which prevents regeneration and causes the afflicted NPC or player to lose 4 health per second. When an enemy is Oiled(Desktop, Console and Mobile versions), it will lose an additional 25 health per second, totaling to a loss of 29 health per second.

The debuff can be canceled by jumping into water, or by pouring a Water Bucket over the player's head. The Obsidian Skin Potion also grants immunity to this debuff for the duration of its effect. (Desktop, Console and Mobile versions) Molten armor provides immunity to the debuff while worn.

Causes

From environment

From	Duration	Chance	Total damage
Lava	7 / 14 / 17.5 seconds	100%	28 / 56 / 70

From player

	Duration	Chance	Total damage
Wand of Sparking(Desktop, Console and Mobile versions)	1–4 seconds	50%	4–16
Volcano	3 seconds	50%	12
Molotov Cocktail(Desktop, Console, Old-gen console and Mobile versions)	3–6 seconds	100%	12–24
Imp Staff(Desktop, Console, Old-gen console and Mobile versions)	3–6 seconds	100%	12–24
Molten Pickaxe	3 seconds	10%	12
Molten Hamaxe	3 seconds	20%	12
Flaming Arrow	3 seconds	33.3%	12
Flare	5–10 seconds	100%	20–40
Blue Flare	5–10 seconds	100%	20–40
Flower of Fire	5 seconds	50%	20
Flamarang	3 seconds	50% (Desktop, Console and Mobile versions) 20% (Old-gen console and 3DS versions)	12
Flamelash	4–8 seconds	50%	16–32
Flaming Mace(Desktop, Console and Mobile versions)	2–3 seconds	16.67%	8–12
Sunfury	3 seconds	25%	12
Flamethrower (Old-gen console and 3DS versions)	20 seconds	100%	80
Elf Melter (Old-gen console and 3DS versions)	20 seconds	100%	80
Cascade(Desktop, Console and Mobile versions)	1–4 seconds	33.33%	4–16
Inferno Fork (Old-gen console and 3DS versions)	8–15 seconds	100%	32–60
Fire Gauntlet (Old-gen console and 3DS versions)	2–6 seconds	100%	8–24
Magma Stone (Old-gen console and 3DS versions)	2–6 seconds	100%	8–24
Flask of Fire	3–6 seconds	100%	12–24
Flameburst Tower(Desktop, Console and Mobile versions)(rod)	3–5 seconds	33.33%	12–20

From enemy

From	Duration	Chance	Total damage
Meteor Head	7 / 14 / 17.5 seconds	1/3 (33.33%)	28 / 56 / 70
Skeleton Archer(Arrow attack)	7 / 14 / 17.5 seconds	1/3 (33.33%)	28 / 56 / 70
Pirate Crossbower(Arrow attack)	7 / 14 / 17.5 seconds	1/3 (33.33%)	28 / 56 / 70
Elf Archer(Arrow attack)	7 / 14 / 17.5 seconds	1/3 (33.33%)	28 / 56 / 70
Hell Armored Bones	10 / 20 / 25 seconds	1/2 (50%)	40 / 80 / 100
Diabolist(Fireball attack)	8–15 / 16–30 / 20-37.5 seconds	1/1 (100%)	32–60 / 64–120 / 80-150
Fire Imp(Burning Sphere attack)	7 / 14 / 17.5 seconds	1/3 (33.33%)	28 / 56 / 70
Betsy(Desktop, Console and Mobile versions)(Breath attack)	7-10 / 14-20 / 17.5-25 seconds	1/1 (100%)	28-40 / 56-80 / 70-100
Golem(Fireball attack)	5–7 / 10–14 / 15-21 seconds	1/2 (50%)	20–28 / 40–56 / 60-84
Lunatic Cultist(Desktop, Console and Mobile versions)(Fireball attack)	0.5–2.5 / 1–5 / 1.25-6.25 seconds	1/1 (100%)	2–10 / 4–20 / 5-25

Other

From		Duration	Chance	Total damage
Red Potion(Desktop, Console and Mobile versions)(except For the worthy and Get fixed boi worlds)		1 / 2 / 3 hours	1/1 (100%)	14400 / 28800 / 43200

Immune NPCs

Enemies immune to On Fire!

Ancient Vision(Desktop, Console and Mobile versions)
Betsy(Desktop, Console and Mobile versions)
Bone Serpent
Bubble Shield(Desktop, Console and Mobile versions)
Clumsy Balloon Slime(Desktop, Console and Mobile versions)
Corite(Desktop, Console and Mobile versions)
Corrupt Mimic(Desktop, Console and Mobile versions)
Crimson Axe
Crimson Mimic(Desktop, Console and Mobile versions)
Cursed Hammer
Deadly Sphere(Desktop, Console and Mobile versions)
Demon
Desert Spirit(Desktop, Console and Mobile versions)
Dungeon Guardian
Dungeon Spirit
Dutchman Cannon(Desktop, Console and Mobile versions)
Enchanted Sword
Fire Imp
Flying Dutchman(Desktop, Console and Mobile versions)
Ghost
Golem
Golem Fist
Golem Head
Granite Elemental(Desktop, Console and Mobile versions)
Granite Golem(Desktop, Console and Mobile versions)
Hallowed Mimic(Desktop, Console and Mobile versions)
Headless Horseman
Hell Armored Bones
Hellbat
Hellhound
Ice Mimic
Jungle Mimic(Desktop, Console and Mobile versions)
Lava Bat
Lava Slime
Lunatic Cultist(Desktop, Console and Mobile versions)
Martian Saucer(Desktop, Console and Mobile versions)
Martian Saucer Cannon(Desktop, Console and Mobile versions)
Martian Saucer Turret(Desktop, Console and Mobile versions)
Martian Walker(Desktop, Console and Mobile versions)
Meteor Head
Mimic
Mysterious Portal(Desktop, Console and Mobile versions)
Mysterious Tablet(Desktop, Console and Mobile versions)
Nebula Pillar(Desktop, Console and Mobile versions)
Phantasm Dragon(Desktop, Console and Mobile versions)
Pirate's Curse(Desktop, Console and Mobile versions)
Poltergeist
Possessed Armor
Present Mimic
Probe
Pumpking
Reaper
Red Devil
Rock Golem(Desktop, Console and Mobile versions)
Solar Pillar(Desktop, Console and Mobile versions)
Stardust Pillar(Desktop, Console and Mobile versions)
The Destroyer
Voodoo Demon
Vortex Pillar(Desktop, Console and Mobile versions)
Wall of Flesh
Wraith

Notes

In terms of sources available to the player, the On Fire! debuff is the easiest to inflict to opponents.

Trivia

The debuff icon appears to depict the Guide Voodoo Doll.
It is the only debuff which has an exclamation mark in its name. Happy!, Panic!, and Ballista Panic! have exclamation marks in their names as well, but they are buffs rather than debuffs.