Spreadability is a huge consideration when choosing an emollient and we’ll be talking about it a lot in this series. The spreading value is measured by “dropping a fixed amount of oil to the skin and measuring its area size after a fixed period of time” (Reference: Emollients, chapter 16)
In general, it means dropping emollients on skin – usually on the forearm – then letting them sit for 10 minutes so they can spread naturally. The oil is blotted, and the area is measured in square millimetres. (The measurement for this is mm2/10 minutes.)
There are different ways to do this, so I’m going with Croda’s method for this series, which is to apply a 4 milligram dose of oil to the forearm at 23˚C at 60% relative humidity, which is the most common measurement I’ve found.
A low spreading emollient will cover less than 300 mm2 of our skin in 10 minutes; a medium spreading emollient, 300 to 1000 mm2 in 10 minutes; and a high spreading emollient, above 1000 mm2 in 10 minutes. We can also find super-fast and ultrafast spreading emollients.