Cosmetics with natural ingredients such as highly dosed vitamins and essential fatty acids (omega-3 and omega-6 fatty acids) usually have a minimum shelf life during which they are fully effective.
The minimum shelf life refers to a room temperature of 20-25°C unless otherwise stated.
A rule of thumb says that lowering the storage temperature of food or cosmetics by 10°C can double the remaining shelf life. To give you an example: buying a cream with a minimum shelf life of 2 months counted from the day of purchase means that said cream can still be used within 2 months after the use-by date if stored in the fridge at a temperature of about 10°C.
Hence, the shelf life extends the lower the temperature. Caution however is recommended for aqueous products since the storage temperature here should not be lower than 0°C.
Vice versa, the shelf life reduces if the room temperature is increased by 10°C, in other words the temperature is at 30°C. In this case, the shelf life then will halve to about one month. In analogy, 40°C means that the actual shelf life already will be expired after about half a month.
Thus, storing a cream with sensitive active agents in the car at summer temperatures involves that the cream already will be spoilt after a short time.
Dr. Hans Lautenschläger