At what pressure must vapor pressure not exceed for a liquid to be considered flammable?

A liquid is considered flammable based on its vapor pressure, which is the pressure exerted by the vapor when it is in equilibrium with its liquid form at a given temperature. Generally, a liquid is classified as flammable if its vapor pressure at 100°F does not exceed a certain limit. The threshold can vary depending on regulations, but typically, a vapor pressure limit of around 40 psia is recognized in many safety standards.

When the vapor pressure of a liquid exceeds this level, it indicates that the liquid can produce sufficient vapor to form an ignitable mixture with air, increasing the risk of fire or explosion. Thus, if the vapor pressure of a liquid is above 40 psia, it is likely to be classified as flammable. This understanding is crucial for safety in handling and storing liquids at potentially hazardous conditions.

In this context, the other pressure levels presented do not align with the general threshold for flammability, thereby making 40 psia the correct answer.