It's an event that happens periodically, and is similar to the blowouts from STALKER. If you're outdoors, you get knocked out for 40 seconds, and lose 6500 blood. Indoors, you simply get knocked out for 40 seconds. You can find experimental drugs, to prevent any ill effects from the event, allowing you to loot player bodies while they are unconcious and such. Watch out for Bloodsuckers.