There are a handful of persistent skincare myths that never cease to frustrate me.

Like "chemicals are inherently bad". A chemical is just a compound (mixture of two element). This means water is a chemical - but uranium is not. And I know what I'd rather put on my face. It also bothers me every time I see someone claiming rosehip oil is rich in vitamin C. As a water-soluble acid, vitamin C does not naturally occur in rosehip oil. Now, I'm adding another annoying myth to the register. The idea that oils 'hydrate' your skin.

I first came across this claim after a content creator I followed on Instagram launched her own beauty brand. She'd write regularly about how 'hydrating' her face oils were - and how her skin felt immediately more hydrated after one use. I chalked up the error to her being new to beauty, and carried on.

But since then, it's something I notice all the time. Products flow across my desk regularly and it's not uncommon for me to find body oils, face oils, shower oils - all sorts of oils - claiming to 'hydrate'.

But the fact of the matter is, oils do not and cannot hydrate your skin. In fact, when used incorrectly, they can actually dehydrate your skin.

Why oils don't 'hydrate'

Of course, anyone who passed year 10 chemistry would know that the word 'hydrate' pertains to water. It's the same with skin. Hydrated skin means that the skin is holding a healthy level of water. Dehydrated skin on the other hand means that the water levels of skin are lower than they should be. When your skin is dehydrated it can appear sunken, sullen and dull. But when your skin has a healthy level of water it will appear bright, bouncy and full.

Oil of course, is not water. As such, adding oil to your skin will not increase its water levels aka 'hydration'. But, the confusion does have a source. Because what oil does do is help to combat transepidermal water loss.

What is transepidermal water loss?

Transepidermal water loss refers to the process where water passively evaporates out of the skin. In the Australian summer, where the humidity is higher and water vapour pressure gradient is down, the evaporation of water from our skin is lessened. But in lower humidity settings like winter where water vapour pressure gradient is up, the transepidermal water loss will be more significant - that is, more water will evaporate.

So where do oils come in? Well, many oil skincare products contain lipids. Lipids occur naturally in the skin. They're fats that preserve the skin barrier function. In very simplistic terms, lipids are mortar that fill the gaps between the bricks (or corneocytes) that make up our skin. This creates a seal or barrier that prevents water escaping. Of course, our skin barrier function can become disrupted by a number of environmental factors - hot showers for one. And gaps can appear in between the bricks of our skin allowing for accelerated transepidermal water loss. This is what leads to that scaly, flaky, dry skin situation that many people suffer with in dry weather.

With this in mind, applying certain oils, ones that are rich in lipids (I like Jojoba Co. Ultimate Youth Potion for example) can help preserve the skin barrier function - very simply, they plug the holes in your skin. This can support skin to lose less water than it ordinarily would. But it certainly doesn't add water to the skin.

How to hydrate your skin

Rather than hydrating, it's been suggested that oils can actually prevent hydration. If you were to apply an oil before applying a hyaluronic acid - a compound that adds water or hydration to the skin - the hyaluronic acid and water it attracts may not be able to pass through the oil layer and get to work in the lower layers of your skin.

This is why getting your skincare order correct is important. Always apply oils or lipid-rich products last in your routine - at the very least apply your serums first. Also, using hyaluronic acid products can help with adding water to your skin since HA attracts 1000 times its weight in water. But, the best way to hydrate your skin is not through a product, but from within. If you ensure you're drinking enough water, your skin should be hydrated and stay hydrated.

Of course, lots of environmental and intrinsic factors can affect skin hydration. For example, if you suffer from eczema your skin may be more prone to water loss or if you live in especially low humidity conditions. But, as a general rule, if you have a healthy skin barrier, drinking a healthy amount of water will is the best way to keep yourself and your skin hydrated.