How to slowly decrease health script?

I have been looking for a script to slowly decrease the health of a player. (e.g when in a fire environment)

I currently have a basic script that will display the health bar, can anyone help me create a script to gradually reduce the players health?

var health100 : Texture2D;
var health95 : Texture2D;
var health90 : Texture2D;
var health85 : Texture2D;
var health80 : Texture2D;
var health75 : Texture2D;
var health70 : Texture2D;
var health65 : Texture2D;
var health60 : Texture2D;
var health55 : Texture2D;
var health50 : Texture2D;
var health45 : Texture2D;
var health40 : Texture2D;
var health35 : Texture2D;
var health30 : Texture2D;
var health25 : Texture2D;
var health20 : Texture2D;
var health15 : Texture2D;
var health10 : Texture2D;
var health5 : Texture2D;
var health0 : Texture2D;

var LIVES = 21;

function Update () 
{
     switch(LIVES)
     {
         case 21:
             guiTexture.texture = health100;
             break;

         case 20:
             guiTexture.texture = health95;
             break;

         case 19:
             guiTexture.texture = health90;
             break; 

         case 18:
             guiTexture.texture = health85;
             break;

         case 17:
             guiTexture.texture = health80;
             break;

         case 16:
             guiTexture.texture = health75;
             break;

         case 15:
             guiTexture.texture = health70;
             break;

         case 14:
             guiTexture.texture = health65;
             break; 

         case 13:
             guiTexture.texture = health60;
             break;

         case 12:
             guiTexture.texture = health55;
             break;  

         case 11:
             guiTexture.texture = health50;
             break;

         case 10:
             guiTexture.texture = health45;
             break;

         case 9:
             guiTexture.texture = health40;
             break;

         case 8:
             guiTexture.texture = health35;
             break; 

         case 7:
             guiTexture.texture = health30;
             break;

         case 6:
             guiTexture.texture = health25;
             break;

         case 5:
             guiTexture.texture = health20;
             break;

         case 4:
             guiTexture.texture = health15;
             break;  

         case 3:
             guiTexture.texture = health10;
             break; 

         case 2:
             guiTexture.texture = health5;
             break;

         case 1:
             //gameover script here
             break;  
     }
}

EDIT***

After looking around some more I have come across a thread that seems relevant to what I want. But the problem Im having is combining the first script with the new one?

Slow Damage Over Time Script

Just subtract a life when player is in the fire... Check again in 3 seconds, and keep subtracting... Pretty easy.

Save the time, compare it with the refreshed time, but add 5 to the saved time, then that'll make it every 5 seconds...

Look around, easy.

I think what Ben wants is a health bar system replace of changing 21 pictures manually to represent current health.

Refer to below example of how to make a health bar:

http://answers.unity3d.com/questions/4456

How would I use this code with the code above. Ive tried simply attaching the script to the player and tagging a sphere collider “Fire”, but the code still does not work…any suggestions?

thanks for your help

function OnTriggerStay(collisionInfo : Collider){
  if(collisionInfo.gameObject.tag == "Fire"){
  Health-=FireDamage*Time.deltaTime *1;
  enable = false;
  yield WaitForSeconds(1);
  enabled = true;
  }
 }
}