Damage types

All damage done by one entity to another has at least one type associated with it. For instance, If a player falls from a high place, they'll be hurt with the damage type. When the player spots that damage type, the game may do something like shake the user's screen.

0 - GENERIC
Regular damage. Has typical effects. (Like HUD indicators pointing towards the damage source, pain sounds.)

1 - CRUSH
Physics collisions/getting squished.
 * HEV says "major/minor fracture detected". Damage amount not affected by difficulty. Makes hunters flinch. Makes manhacks explode on death.
 * Chell emits pain sounds.
 * Screen flashes red.
 * Does not hurt marines.
 * Screen flashes darker red.
 * Plays quieter bullet impact sounds.

2 - BULLET

 * Plays bullet impact sound. HEV says "blood loss detected".
 * Plays bullet impact sound. Turrets interpret this as friendly fire from other turrets regardless of the source.
 * Plays bullet impact sound.
 * Plays bullet impact sounds when hurting infected. May seem odd without a gunfire sound & visual accompanying it. Makes witches emit extra high-pitched screams.

4 - SLASH

 * Creates blood particle on player. HEV says "major/minor lacerations detected". Prevents using for half a second in episodic.

8 - BURN

 * Screen flashes red. Burning sound effect plays (HL2 only). Ignites zombies (if hurting at a high enough DPS relative to their health) and headcrabs. Kills rollermines. Slowly chars citizens if combined with DMG_DIRECT. Causes flammable breakable props (such as oil barrels) to ignite.
 * Does not ignite turrets.
 * Does not ignite players.
 * Shows flame and blur effects. Ignites infected.
 * Ignites marines and swarm.

16 - VEHICLE

 * Shows broken player.mdl if it kills the player! If killing a hunter, decreases the  global setting.
 * Shows Chell's model in a t-pose when killed!

32 - FALL

 * Plays splat sound if the player dies from it. Damage amount not affected by difficulty. HEV says "major/minor fracture detected".
 * Does not play bullet impact sound (others will).
 * Character does not emit pain sounds (will from other sources).
 * Character emits more dramatic pain sounds. Plays crunch sound if they die from it.
 * Does not play any impact sounds, ever.

64 - BLAST
Explosions.
 * Temporarily deafens player. Causes flammable breakable props (such as oil barrels) to ignite. Makes antlions, rollermines, and manhacks explode on death. Makes Alyx flinch (episodic). Makes hunters flinch. May ignite headcrabs.
 * Temporarily deafens the player if they take significant damage.
 * Character emits more dramatic pain sounds. Temporarily deafens the player if they take significant damage.
 * Gibs players if the damage done is at least 10 more than their current health.
 * Temporarily deafens the player if they take significant damage. Kills infected instantly, regardless of damage. Knocks witches away from the damage source, but may cause them to be stuck in their stumbling animation until another begins playing (for a reason such as being disturbed).
 * Uses a special pronged directional HUD indicator. Drones may get flung toward the player camera.
 * Knocks turrets around.
 * Does not play impact sounds, decreases armor.

128 - CLUB
Melee bashing weapons.
 * Makes manhacks spark, and makes them explode on death. Instantly kills barnacles and headcrabs. HEV may say "minor fracture detected".
 * Character emits more dramatic pain sounds.
 * Character emits dramatic death sounds when they die from it.
 * Instantly kills jockeys.

256 - SHOCK
Electrical damage. Creates sparks.
 * Missing a dust texture!
 * Shocks swarm in the same manner as an electric attack.

512 - SONIC
Damage caused by a rapid change in air pressure.
 * Plays special pain sound. HEV may say "internal bleeding detected".
 * Always gibs swarm.
 * Plays special pain sound.

1024 - ENERGYBEAM/RADIUS_MAX

 * Nullifies distance-based damage falloff.
 * Hurts infected noticeably less per hit.
 * Damages s.

16384 - DROWN
Health will slowly regenerate to what it was before being hurt, but not when done through.
 * Plays drowning sound, screen flashes blue. Damage amount not affected by difficulty.
 * Plays drowning sound, screen flashes blue.
 * Screen flashes blue/red.

32768 - PARALYZE/FORCE_INCAPACITATE

 * HEV says "warning, blood toxin levels detected".
 * Jostles the player's view like how grenades do. Higher damage means the view jostles more. Doesn't actually cause damage.
 * Instantly knocks survivors down, even if it only takes off 1 HP. Instantly kills tanks.

65536 - NERVEGAS

 * HEV says "warning, biohazard detected".

131072 - POISON/NOCLOSEDISTANCEMOD/BLURPOISON

 * HEV says "warning, blood toxin levels detected". Screen flashes yellow. Health will slowly regenerate to what it was before being hurt.
 * If using DMG_USEDISTANCEMOD, cancels the damage increase when the target is close, meaning the damage will only decrease (when farther away).
 * Does not show a red arrow pointing to the damage source (others will).
 * Applies the blur and trail effects received from buzzers.

262144 - RADIATION/HALF_FALLOFF

 * Screen flashes white. HEV says "warning, hazardous radiation levels detected". If using a, a Geiger counter sound plays from the trigger's origin if it's in the player's PVS and they're close enough.
 * Halves the damage reduction that explosives suffer due to distance from their target. Used on weapons like rockets to ensure that they always do a non-trivial amount of damage. If using a, (inconsistently) emits a Geiger counter sound like radiation from HL2 and gibs players identically to blast.
 * Does not knock marines back.

524288 - DROWNRECOVER
Used internally to make the player regain health when they stop drowning. Works like generic when using.

1048576 - ACID/CRITICAL/CHOKE

 * Screen flashes white. HEV says "warning, hazardous chemical detected".
 * Crits. Always gibs players if combined with blast. Character emits dramatic death sounds if killed by the damage.

2097152 - SLOWBURN/USEDISTANCEMOD

 * Makes the weapon's damage increase or decrease depending of the distance between the victim and the inflictor. Normal damage is at 512 units.

4194304 - REMOVENORAGDOLL

 * Instantly removes the victim's remains upon killing them. Does not work for scanners. The NPC will still drop whatever weapon they held.
 * Instantly removes the victim's remains upon killing them. Marines will drop whatever weapons they had.

8388608 - PHYSGUN

 * Hit by gravity gun punt. Instantly kills birds. Flips antlions over. Always hurts headcrabs by 1/3rd of their max health. Only hurts manhacks by one point.
 * Makes boomers explode instantly.

16777216 - PLASMA/IGNITE/MELEE

 * Damage from a never-used weapon (the cremator flamethrower). Darkens screen and plays burning sound.
 * Acts like generic. Might have once ignited players, but does not.
 * Creates a huge blood spurt on infected and cuts them up.

33554432 - AIRBOAT/USE_HITLOCATIONS/STUMBLE/INFEST

 * Damage from the airboat gun. Needed to destroy some objects. Kills manhacks instantly.
 * Makes the damage capable of doing a headshot. Does not apply to.
 * Right-click shoving. Makes most infected stumble away from the damage source. Stops jockeys (but not hunters) mid-jump.
 * Damage done from parasites. On death, the marine's body explodes and emits green blood.
 * Attempts to play the danger zone red area's hit sounds instead of the regular ones. The console reports that the sounds are not precached, so nothing plays.

67108864 - DISSOLVE

 * Dissolves victims and their weapons. Dissolves secondary ammo dropped by elite soldiers. Prevents scanners from doing a divebomb.
 * Dissolves victims.

134217728 - BLAST_SURFACE

 * Will not damage players who are underwater.
 * Like blast, may fling drones toward the camera.

268435456 - DIRECT
Indicates that the damage is happening directly to the victim. Used in HL2 to distinguish from flames on a victim's body vs flames that aren't.
 * Does not knock marines back or play the "hit" sound (only pain sounds).

536870912 - BUCKSHOT/DISMEMBER
Shotgun damage.
 * Knocks back common infected, but not boomers.
 * Damages s (at 1/5 the rate of energybeam, however).

Half-Life 2 severity notes
In a single hit of damage is considered "trivial" if the player still has at least 76 health left after it, or if it did less than 5 damage. Damage is "major" if the amount done was over 25, and damage is "critical" if the player has less than 30 health after being hurt.

Trivial damage will not trigger HEV voicelines unless the damage is an "over time" type, IE paralyze, nervegas, poison, radiation, drownrecover, acid, and slowburn. Trivial damage will also never trigger the "automatic medical systems engaged, morphine administered" line or any of the low health warnings, regardless of the damage type.

Major damage will make the HEV note a minor fracture for club and internal bleeding for sonic, note major damage for fall, crush, and slash, and will trigger the "automatic medical systems engaged, morphine administered" line if the player's health was above 75 before getting hurt.

Critical damage will cause the low health warnings to play, such as "seek medical attention" as long as the damage was not also considered trivial.