What does the upper explosive limit (UEL) indicate?

The upper explosive limit (UEL) indicates the maximum concentration of combustible vapor in the air required for an explosion to occur. When the concentration of a flammable substance in the air is above this limit, there is not enough oxygen available for combustion, meaning that no ignition can occur even if an ignition source is present. Understanding the UEL is crucial for safety in handling hazardous materials, as knowing this limit helps in assessing risks and planning for effective ventilation and containment measures.

In contrast, other options pertain to different aspects of combustion or chemical reactions. The maximum temperature for ignition concerns the conditions needed for a substance to ignite, rather than the concentration of vapor. The point at which a chemical reaction becomes uncontrollable relates to reaction kinetics and not to explosive limits. The amount of air necessary to sustain combustion focuses on the essential conditions for a fire to maintain itself, which doesn’t specifically address the limits of vapor concentration in the context of explosions.