What does size mean anyway? There are two sizes that are relevant to skin care. One is the molecular weight and the other one is particle size. Molecular weight refers to the weight of one molecule expressed in daltons. On the other hand, particle size refers to the width of a particle expressed in some unit of length (usually micrometers or nanometers). For example, the molecular weight of water is 18 daltons, and the particle size of one molecule of water is 0.27 nanometers.
In skin care, both sizing parameters are used. For active ingredients such as alpha hydroxy acids and hyaluronic acids, molecular weight is a much more relevant number. When talking about powders, especially sunscreen filters like zinc oxide and titanium dioxide, it makes much more sense to talk about their particle size. Of course, size is only one parameter, so one must consider other examples of when size matters in skincare.
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