More unprotonated nicotine in tobacco smoke increases smoke "strength", "impact", or "kick". However, the irritation and harshness of smoke at higher pH makes it harder for smokers to inhale it. Cigarette design ensures that the smoke has enough unprotonated nicotine to rapidly transfer nicotine into the brain but not so much of it as to be too harsh for the smoker. Variability in the acid-base nature of tobacco leaf is considerable. Flue-cured ("bright") tobacco typically produces acidic smoke, whereas air-cured ("burley") tobacco typically produces basic smoke. The natural acids in tobacco smoke (e.g., formic acid, acetic acid, and propionic acid) can protonate nicotine, while the natural bases (e.g., ammonia) tend to neutralize the acids and keep more nicotine in the unprotonated form. Acidic tobacco additives such as levulinic acid are used as "smoothing" agents to reduce smoke harshness.