GrenadeScript.cs Troubles

I have a script and when I "throw" the grenade I want all enemies in a certain radius to lose health. how would I do this?

Here is my script:

 using UnityEngine;
 using System.Collections;
 
 public class Grenade : MonoBehaviour 
 {
     public bool isRadius1;
     public bool isRadius2;
     public Collider radius1;
     public Collider radius2;
     public int damage;
     public KeyCode throwKeycode;
     public Rigidbody grenadeObject;
     public Transform spawnPosition;
     public float throwSpeed = 0f;
     public float timer = 0f;
 
     void Update ()
     {
         if(Input.GetKeyDown(throwKeycode))
         {
             StartCoroutine("ThrowGrenade");
         }
     }
 
     IEnumerator ThrowGrenade ()
     {
         Rigidbody grenade = Instantiate (grenadeObject, spawnPosition.position, spawnPosition.rotation)as Rigidbody;
         grenade.AddForce (transform.position * throwSpeed);
         yield return new WaitForSeconds(timer);
         DamageEnemies (radius1);
     }
 
     void DamageEnemies (Collider col)
     {
         
     }
 }

Thanks!