string[] oreTypes = new string[] { "blah", "two", "three" };
float[] oreSec = new float[] { 1.0f, 1.0f, 0.5f };

private void textBox1_Leave(sender object, EventArgs e)
{
        //This gets called when the user leaves the text box with the security status in it
	comboBox1.Items.Clear();
	for(int x = 0; x < oreTypes.Length; x++)
	{
		//If the security the user entered is less than the system's security (i.e the system is safer) add it
		if (float.Parse(textBox1.Text) <= oreSec[x])
		{
			comboBox1.Items.Add(oreTypes[x]);
		}
	}
}