Why does acetaldehyde accumulation cause acute symptoms such as flushing and nausea when ALDH is deficient, even though BAC may remain unchanged?

ALDH deficiency increases hepatic blood flow which both raises BAC and produces flushing by thermoregulatory disruption, so the symptoms are due to higher BAC values.

Because acetaldehyde directly increases the solubility of ethanol in blood which artificially lowers measured BAC while increasing peripheral toxicity and causing flushing.

Acetaldehyde is a toxic intermediate produced in the liver that causes vasodilation and irritative symptoms; ALDH deficiency slows its clearance so it accumulates and produces these effects independent of unchanged ethanol blood percentages.

When ALDH is deficient, ethanol is rapidly converted to acetate which triggers histamine release from mast cells producing flushing and nausea even though acetaldehyde levels are low.