Effect: Gain Armor and Overheal for each poisoned enemy. +0.005 Armor and +0.005 Overheal per poisoned enemy per copy. Caps at 0.4 raw Armor (80 enemies) and 25% Overheal (50 enemies) per copy. Dynamic - fluctuates with current poison count.
Unlock: Kill 5,000 enemies using Amog
Gas Mask is an Epic item that grants Armor and Overheal based on the number of enemies currently poisoned on screen. The bonus is dynamic - it updates in real time as enemies become poisoned or die. When your poison coverage is high, Gas Mask provides meaningful damage reduction and a temporary HP buffer. When no enemies are poisoned, it does nothing.
The exact values per poisoned enemy are 0.005 Armor and 0.005 Overheal per copy, as confirmed by community testing in the Steam discussions. One copy caps at 0.4 raw Armor contribution (equivalent to roughly 80 poisoned enemies to hit the cap) and 25% Overheal (requiring about 50 poisoned enemies). These numbers are subject to Armor's hyperbolic scaling formula, meaning 0.4 raw Armor doesn't translate to 40% effective damage reduction - it converts to approximately 35% due to diminishing returns.
Multiple older guides and even some in-game text describe Gas Mask as giving "Lifesteal." The correct effect is Overheal - a temporary HP buffer above your Max HP, displayed as a yellow bar. The wiki confirms this, and community testing corroborates it. Gas Mask does not restore HP or heal you through damage dealt. Additionally, the bonus does not appear on the stat screen - the Armor and Overheal values fluctuate invisibly based on live enemy poison count, which is why many players assume the item isn't working.
| Poisoned Enemies | Raw Armor (1 copy) | Overheal (1 copy) | Notes |
|---|---|---|---|
| 10 | 0.05 | 5% | Small swarm, modest buff |
| 25 | 0.125 | 12.5% | Medium coverage |
| 50 | 0.25 | 25% (cap) | Overheal capped at 1 copy |
| 80 | 0.4 (cap) | 25% (cap) | Both stats capped at 1 copy |
| 80+ | 0.4 (cap) | 25% (cap) | No further benefit from 1 copy |
Kill 5,000 enemies using Amog across any number of runs. The kill counter is cumulative. Amog is a late-unlock character himself (requires poisoning 50,000 enemies with Moldy Cheese), so Gas Mask is one of the later items to unlock in the game. Once you have Amog available, run him with Moldy Cheese, high attack speed, and Cursed Tome for maximum enemy density to accumulate the 5,000 kills quickly. After meeting the condition, purchase Gas Mask from the Unlocks shop with Silver.
Amog - The wiki directly connects Gas Mask to Amog and it unlocks through him. Amog's Plague passive spreads poison stacks to all nearby enemies when any poisoned enemy dies, creating large groups of simultaneously poisoned enemies - exactly what Gas Mask needs to reach its Armor and Overheal caps. The megabonk.org Amog guide lists Gas Mask as a core item alongside Moldy Cheese and Overpowered Lamp. In a dense Amog run with chain Plague explosions, Gas Mask's poison count can spike to very high values simultaneously, briefly maximizing both the Armor and Overheal buffers.
Sir Oofie - Sir Oofie's Reinforced passive gives +1% Armor per level, making him naturally oriented toward high Armor values. Gas Mask adds a further dynamic Armor component on top of his existing investment, and Armor follows hyperbolic diminishing returns - so Gas Mask's 0.4 raw Armor per copy contributes meaningfully alongside Sir Oofie's passive rather than stacking wastefully. Sir Oofie's stationary playstyle means enemies cluster around him, making it straightforward to keep many nearby enemies poisoned with Moldy Cheese or Cursed Doll for sustained Gas Mask uptime.
Robinette - Robinette's high kill rate with Cursed Tome for enemy density creates dense swarms where Moldy Cheese stacks can keep a large number of enemies simultaneously poisoned. Her builds already include Moldy Cheese as a near-universal pickup, so Gas Mask slots in naturally without changing the build direction. The survivability from Gas Mask's Armor and Overheal helps Robinette sustain through the heavy damage pressure that high-difficulty Cursed Tome runs create.
Gas Mask gives +0.005 Armor and +0.005 Overheal per poisoned enemy per copy, dynamically updating as enemy poison status changes. One copy caps at 0.4 raw Armor (approximately 80 poisoned enemies) and 25% Overheal (50 enemies). The effect is not shown on the stat screen. Gas Mask gives Overheal, not Lifesteal - older guides and some in-game text use "Lifesteal" but this is incorrect.
Overheal. The official wiki page confirms the effect is Armor and Overheal. Some older sources and community guides incorrectly say Lifesteal because that is what earlier in-game descriptions said before it was updated. Overheal creates a temporary yellow HP buffer above your Max HP. Gas Mask does not restore HP through damage dealt - that would be Lifesteal, which comes from the Bloody Tome or Chonkplate.
Gas Mask's Armor and Overheal contributions are dynamic - they update in real time based on how many enemies are currently poisoned, and this fluctuating bonus is not reflected in the stat display. The benefit is real and works in combat, but you will not see your Armor or Overheal numbers change in the UI as you poison more enemies. This is the most common source of confusion about whether the item is working.
Kill 5,000 enemies using Amog. Progress accumulates across all runs. Amog himself must first be unlocked by poisoning 50,000 enemies with Moldy Cheese. Once Amog is available, run him with Moldy Cheese, high attack speed, and Cursed Tome for efficient kill accumulation. After meeting the condition, purchase Gas Mask from the Unlocks shop with Silver.
