Regardless of its advantages and benefits, vitamin C can be unstable and easily oxidized. This is caused by factors such as exposure to light, air, high temperature, and humidity. When vitamin C is oxidized, its usage is no longer effective for skin health.

The translated text while preserving HTML tags: "The oxidation of skincare changes is almost similar to makeup products. Dream friends can pay attention to the signs of oxidized products such as changes in color, texture, and aroma. If there are changes in these three aspects, the active ingredients in skincare may no longer be effective."

The easiest way to prevent skincare from oxidizing is to store it in a cool place like a wardrobe, air-conditioned room, or refrigerator. Also, pay attention to the expiration date. "It is advisable to use vitamin C products within a period of three months," said Houshmand.

Most serums have symbols indicating the period of time the product can be used after opening. This also serves as an indication of when the serum expires. In addition, choose products that are packaged in dark, opaque glass bottles and sealed to prevent exposure to air and light.