volatile acidity, commonly referred to as 'VA,' is a family of compounds produced by acetic bacteria that create ‘off’ smells like nail polish remover, wet paint, and apple cider vinegar. Acetic bacteria need oxygen to proliferate, so VA is often associated with oxidative winemaking styles, low SO2 additions, and other oxidative characteristics.
in moderate quantities, volatile acidity can add a lively pop to wines that might otherwise come off as heavy--for instance, it's not uncommon to find elevated levels in classic examples of powerful red wines from Priorat or central Italy. like all things in wine, it comes down to balance: structured wines like these can hold and even benefit from levels of volatile acidity that would overwhelm less muscular cuvees.
generally speaking, volatile acidity becomes apparent at concentrations of around 50mg/l in unstructured wine, and up to 75mg/l in more structured examples. wines with upwards of 100mg/l may start feeling like vinegary citrus juice.