There are plenty of effects that appear in Escape from P.R.I.P.Y.A.T. Most of them are negative and Stalker should be prepared for any of those.
Caused by: HE Grenades [in future - some artifacts], Bloodsucker (NPC & Player).
Effect: Rapidly saps health (about two points per second), bleeding deals more damage if player has <50% HP.
How to notice: Health drops at steady rate.
How to counter: Use bandage.
Advice: Avoid grenades - their blast range can damage through weak walls. Bloodsuckers may be hard to repel as they are hard to notice but can be spotted easily by disortions.
Caused by: Stamina dropping below 30%.
Effect: Slows down player.
How to notice: Walk speed becomes much lower.
How to counter: Drink energy drink or eat food.
Advice: Stop sprinting when stamina is below 50% so there will be ~20% emergency stamina to run.
Caused by: Needler (NPC & Player).
Effect: Slowly saps health and stamina per interval (random), poison deals more damage if player hass <50% HP.
How to notice: Both health and stamina will drop significally in intervals.
How to counter: Take Antidote or Alpha Mutagen (has random chance to heal poison!)
Advice: As Needlers are relatively weak, they still can ambush you. Be alert in the Underground and have your fellow Stalker watch your back. Always have spare money or Alpha Mutagen when venturing down the Underground as Antidote is very rare.
Caused by: Pripyat borders, toxic waste, Gamma wave.
Effect: Lowers stamina and health if player has been too long in radiated area.
How to notice: Geiger meter starts to crackle whenever player is close to a radiated area, when player is radiated already the vision will become sharper and grayed out.
How to counter: Equip respirator, take anti-rad pills or drink vodka.
Advice: Simply avoid radiated areas, when you'll have to go through them equip a respirator so you'll be able to cross them without being too radiated. If Gamma wave occurs you should quickly get under a roof.
Caused by: Alcohol
Effect: Screen becomes bit blury and starts to swing around.
How to notice: It's obvious - player can't even walk straight.
How to counter: Just wait until intoxication wears off or take some hangover pills.
Advice: Getting wasted is not the smartest idea - 5 regular vodkas or 2 Cossacks are enough to become unable to shoot at anything.