Bob,
I will have to dig my books out as I am retired from firefighting, but all of my hazmat classes they teach that for most flammable products the IDLH is about 10% of LEL.
Here is a MSDS for
gasoline. It does not have an IDLH level, but the STEL (short term exposure limit) is 500 ppm and the LEL is 14,000 ppm. In this case OSHA would require respiratory PPE at 500 ppm if you are going to be exposed for over 15 minutes. The STEL would be a concentration that would be significantly lower than the IDLH concentration.
One of the chemicals used at a plant I often work at is acrylonitrile, with an IDLH of 85 ppm and LEL of 31,000 ppm. Another used at that plant is styrene with an IDLH of 700 ppm and LEL of 11,000.
I do see that there are a lot of the ones on the NIOSH list that the IDLH and the LEL are the same. It appears that when they changed the IDLH levels that they have made them equal to the LEL for the cases where they were previously above LEL. It looks like that what we were taught in the hazmat classes was not entirely correct.
As far as your last question, I can only say this.
If you really have a Class I, Division 1 location, I can not see permitting any one to enter the area without PPE suitable for the product as the IDLH for the products is equal to, or in a number of cases substantially less than the, LEL. I know that protection of people and selection of PPE for people is not the purpose of the area classifications, but based on the criteria used to classify the areas, the very fact that an area has been classified as a division 1 area should indicate that extreme caution is required to even enter the area.