What specific environmental condition requires adherence to the 'two-hour rule' being reduced to only one hour for perishables transport?

If the air temperature exceeds $90^{ ext{F}}$

The 'two-hour rule' is a guideline dictating the maximum safe time perishable items can remain outside of controlled cold storage, such as during transport from the point of purchase to home. This rule establishes a baseline limit of two hours for perishables to sit at room temperature. However, this safety margin is drastically reduced when the external environmental conditions are hotter, accelerating microbial growth exponentially. Specifically, if the ambient air temperature surpasses $90^{ ext{F}}$, the maximum safe holding time is cut in half, allowing only one hour before the food must be returned to refrigeration or freezing temperatures to minimize pathogen proliferation.