micky.c

const int buttonPin = 2;     // the number of the pushbutton pin
const int staubi =  13;      // the number of the LED pin
const int nebel = 12;     // the number of the pushbutton pin
const int vent1 = 6;
const int vent2 = 7;

volatile int buttonState = 0;         // variable for reading the pushbutton status

void setup() {
  pinMode(staubi, OUTPUT);
  pinMode(buttonPin, INPUT);
  pinMode(vent1, OUTPUT);
  pinMode(vent2, OUTPUT);
  pinMode(nebel, OUTPUT);
  attachInterrupt(0, pin_ISR, CHANGE);
}

void pin_ISR() {
  buttonState = digitalRead(buttonPin);
  digitalWrite(staubi, buttonState);
  digitalWrite(nebel, buttonState);
  }

void loop() {
  digitalWrite(vent1, HIGH);   // Vent1 an
  delay(15000);              // 15s einatmen
  digitalWrite(vent1, LOW);    // Vent1 aus
  delay(3000);              // Pause 3s
  digitalWrite(vent2, HIGH);   // Vent2 an
  delay(15000);              // 15s ausatmen
  digitalWrite(vent2, LOW);    // Vent2 aus
  delay(3000);              // Pause 3s
}

Micky, I too am a learner like you. I am NOT an expert. Somewhere I remember reading that all the parameters that are in ISR should be labelled as volatile. I have not run your code, but can you please try labelling "staubi" and "nebel" as volatile and give it a try ?

My application too is similar to yours, I need the ISR to run as long as the switch is "ON", and should exit, the moment the switch is thrown into "OFF". Still breaking my head.... :)

You need to pull pin 13 up (or down) with a resistor and let the pushbutton switch pull the input down (or up) when it is pressed.