And… well, you know what that means:

curling up with some hot cocoa,

watching friends on Netflix and

staring at your dry, cracked skin.

Wait, what?

[Reactions intro]

When temperatures drop and heaters kick on,

indoor air gets drier.

That sends many of us scrambling for

a moisturizer to ease the unsightly

and irritating effects of dry skin.

But what do these products actually do?

As the name suggests, their job is

to make your skin more moist.

Skin dries out by a normal process with a

fancy scientific name: transepidermal water loss.

Or, if you like funny-sounding abbreviations, tewl.

Blood vessels only supply moisture to

the middle layer of skin, the dermis.

From there, water moves outward through

to the epidermis before it evaporates.

A moisturizer’s job is to trap or replenish

the moisture in the epidermal layer.

Moisturizers come in three varieties:

occlusives, emollients and humectants.

Most products you buy have a

combination of some or all of these.

Occlusives are the old school moisturizers,

and they work in the simplest way possible.

They form a barrier over the skin that water

can’t penetrate, stopping evaporation

and keeping your skin moist.

The best in the biz is petroleum jelly, sold as vaseline.

It cuts tewl by 98 percent.

The long carbon chains of the molecules

that make up occlusives repulse water.

The only problem?

You don’t really want to walk around covered in vaseline.

We hope.

More popular these days are the emollients.

Instead of coating the skin, these are

designed to penetrate, making skin

feel softer and more flexible.

They are made from similar chemicals as

occlusives: molecules with long fatty

carbon chains like stearates and castor oil.

But they work differently.

The outermost layer of your skin has a

‘brick-and-mortar’ structure where the bricks are

dead cells, called corneocytes, and the mortar

is made of fatty layers of lipids.

Corneocytes are linked by proteins that form a

strong barrier between your body and the bacteria,

microbes and toxins in the outside world.

The ‘brick-and-mortar’ stacks are thicker

in places like your palms, but thinner

on softer skin like your face.

When the moisture level in the air goes down,

the protein links break apart and fractures

develop between groups of corneocytes.

Emollients get beneath the skin’s surface

and fill in these gaps, keeping tewl

under control and helping your skin feel smooth.

The third kind of moisturizers are humectants.

Broadly, these molecules help attract

and retain moisture in the epidermis.

Humectants help get the younger, moist cells

towards the outer layer of skin, as well

as reduce the flakiness of dry skin.

Humectants also stimulate the body’s

natural production of ceramides,

waxy molecules that reduce tewl.

So don’t leave your skin out to dry this winter.

Grab some lotion and stay hydrated.

Your corneocytes will thank you.

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