Why do some spirits cause severe hangovers while others do not?

Why do some spirits cause severe hangovers while others do not?
The hangover and 'blackout' effects linked to whisky and brandy largely trace back to fusel oil content, specifically isobutanol and isoamyl alcohol. Chinese national standards for these spirits set higher alcohol limits at up to 6.00 g/L for brandy, while vodka is capped at a mere 5 mg/L, reflecting a thousand-fold difference. Chinese baijiu, by contrast, rarely exceeds 130 mg/L even in the lowest-grade edible alcohol used for blending, because its flavor profile relies on acids and esters rather than higher alcohols. Fusel oils metabolize slower than ethanol, cross the blood-brain barrier, and linger in the brain, causing next-day headaches and neurological irritation. This explains why premium vodkas, distilled repeatedly to remove impurities, produce cleaner experiences than higher-alcohol aged spirits.