Lactic Acid is an alpha hydroxyl acid that exfoliates the skin. The Ordinary Lactic Acid 5% + HA 2% offers very mild exfoliation and is supported with a purified Tasmanian pepperberry known to reduce signs of inflammation and sensitivity that is often associated with exfoliation. This formula contains a studied Tasmanian Pepperberry derivative to help reduce irritation associated with acid use. This derivative is of plant origin and varies in colour seasonally and this colour variation may be apparent in the formula from time to time. When to not use The Ordinary Lactic Acid 5% + HA 2%: Lactic Acid should not be used on sensitive, peeling or compromised skin. Please refer to additional sun protection note and other warnings in provided Directions. This pH of this formula is approximately 3.8. Lactic Acid has a pKa of 3.8 and pKa is the most important aspect to consider in formulating with acids. pKa implies acid availability. When pKa is close to pH, there is an ideal balance between salt and acidity, maximizing effectiveness of the acid and reducing irritation. Higher pH numbers in such a case would increase salt which counter-intuitively would make the formula even more irritating than if the formula was more acidic.