What is the LEL (Lower Explosive Limit) and why is it important in air monitoring?

1. The minimum oxygen concentration needed for combustion; it determines if SCBA is required
2. The minimum concentration of a combustible gas or vapor in air that can ignite; readings above 10% LEL typically trigger evacuation
3. The maximum concentration of a gas before it becomes too rich to burn
4. The lowest radiation reading that constitutes a hazard