As the first line of defense to the onslaught of the outside world, our skin fends off cosmic UV rays, reaction-hungry free radicals, and the imminent impact of concrete pavement on an icy day. But laboring on the front lines of anatomy can take its toll—wrinkles, acne, and sunburn are just a handful of the common battle scars of daily life. Caring for the body’s largest organ is crucial to overall health—and can allow us to take control over the signs of wear and tear we’d rather avoid.

Yet, determining the best skincare routine can be a daunting task. Estimated at $141 billion, the skincare industry produces countless serums, moisturizers, cleansers, toners, and more that promise to fix everything from eczema to hyperpigmentation.

Deciphering strange ingredients on the backs of bottles can be overwhelming—and consulting with a dermatologist is the best way to be sure you’re doing what’s right for the skin you’re in—but understanding the structure of skin, how these products work, and the chemistry behind them can help you navigate the available options on your own.

Skin is comprised of three major layers: the epidermis, dermis, and sub cutis. The layer hugging our insides is known as the sub cutis—fat that insulates our body and absorbs shock to prevent injury. Directly above sits the dermis, which consists primarily of the protein collagen. Collagen provides our skin with structure to give it strength and flexibility. Finally, the epidermis is the outermost wrapping, and the one skincare products are most concerned with.

Of course, a single layer of skin is made up of millions of cells. The top 15 to 20 cell layers of the epidermis are dead, and referred to as the stratum corneum. These cells are surrounded by fatty lipids that repel water (just as other fats like grease do). Without them our bodies would swell—and burst—with each shower or dip in the ocean. This waterproof barrier also keeps out all water-based skincare products, making it difficult to add moisture back into dry skin.

Since water from inside our body is constantly seeking to escape—a process dermatologists call trans epidermal water loss (TEWL)—the cells in the stratum corneum attempt to trap it in. To do this, each cell contains a mixture of amino acids and salts called natural moisturizing factor (NMF) which is produced by proteins in the living cells below. NMF acts like a sponge, soaking up water to ensure we don’t shrivel up into nasty human raisins.

Tiny holes called pores dot the entire surface of our skin. Each pore is the opening for a follicle made up of a hair and an oil gland. To keep skin lubricated, the gland periodically releases sebum (oil) that travels up the hair and out onto the surface. Sometimes these pores become clogged by dead skin cells, bacteria, or an overproduction of oil resulting in the painful, red pimples characteristic of acne.

Meanwhile, desmosomes link skin cells of the stratum corneum together. Enzymes within the surrounding lipids break down these protein bridges when water content in the skin is high, paving the way for younger cells from lower layers to move up. Dermatologists call this cell turnover. When water content is low (thank the icy winter winds, dehydration, or simply the sands of time) these enzymes struggle to break apart desmosomes, resulting in flaky, scaly skin. This is why moisturizing is crucial: it ensures mechanisms in the skin can operate at full capacity.

Water barrier function is the ability of the skin to self-moisturize, and it varies from person to person. Therefore, the degree and frequency to which any individual moisturizes also fluctuates, but most dermatologists agree a solid moisturizer is the backbone of a good skincare routine. Increasing the skin’s water content not only results in softer, more flexible skin, but it also reduces the appearance of fine lines and minor scarring. However, no product can add moisture into the skin (hello fatty lipid layer!)

Ingredients in moisturizers can be divided into three categories: humectants, emollients, and occlusives. While most moisturizers are a combination of all three, the ratio is adjusted to tweak consistency and intensity.

Humectants are like sponges, and include the naturally-occuring NMF. They attract water from below the epidermis and from the atmosphere, drawing it into the stratum corneum. For people with very oily skin, humectants alone may provide plenty of protection.

Scientists can create NMF in the lab, so it sometimes pops up in products. But glycerin and hyaluronic acid are more common humectant ingredients.

Glycerin, also known as the “moisture magnet,” is a water soluble alcohol. Because it holds several times its weight in H2O, it can occasionally draw in so much of the substance on a humid day that it begins to feel sticky. But because it’s very cheap, it’s abundant in everything from cleansers to eye creams.

Another common humectant, hyaluronic acid, is a carbohydrate found in copious amounts throughout the body from skin cells to connective tissues. Some companies market it as an anti-wrinkle ingredient that can promote collagen growth, but it’s actually too large to penetrate deep into the skin, according to Frey. However, because it is such an efficient humectant, it can temporarily reduce the appearance of fine lines and wrinkles by plumping them up with moisture.

Lighter, oil-based substances such as jojoba oil or squalene are classified as emollients. Emollients are typically unsaturated fatty acids that are prone to oxidizing. Oxidation is when components in the oil break down due to heat, light, and oxygen exposure, turning the product into a rancid, smelly, and unsightly mess (read: bacteria heaven). Therefore, they occasionally cause inflammation, clogged pores, and a spattering of acne. While they prevent some water loss, they are not the most effective moisturizers—something often masked by their ability to make skin feel smooth.

“Basically they ooze between cracks in the skin cells, making the skin feel soft,” says Frey. “Ironically, it’s an important ingredient because it’s the one customers associate with the smooth feeling of moisturizer.”

The third category, occlusives, are the heavyweight champions of the moisturizer world. Thick, fatty, and waxy, these substances prevent dehydration by forming a water-resistant barrier on the surface of the skin. These ingredients might feel greasy, sticky, and gross—the category includes beeswax and petroleum jelly—but studies consistently show that they don’t actually clog pores. And they’re super effective: Petroleum jelly can prevent 98 percent of water loss.

But a moisturizer usually won’t just include any of those three components. While they get a bad reputation for being unnatural, preservatives are also essential: “You stick your hands into theses jars and introduce bacteria, and all of these lotions and creams are actually really nice mediums to grow mold,” says Victoria Fu, a cosmetic scientist with a degree in chemical engineering. “You need preservatives to prevent the growth of bacteria.” Common preservatives that needn’t sound any alarm bells include phenoxyethanol and disodium EDTA.

While they may sound intimidating, desmolytics are just a fancy name for chemical exfoliates. A healthy stratum corneum sheds cells every two or three weeks according to Fu and her fellow cosmetic chemist, Gloria Lu, who started a science-backed skincare blog. But as we age, skin’s ability to shed old cells, a process called desquamation, slows. Desmolytics speed cell turnover by breaking up the protein bridges—desmosomes—holding cells together, revealing a fresh and smooth layer beneath. In the short term, exfoliating treats flaky skin, but in the long-term it can improve acne and hyperpigmentation, a harmless condition where patches of skin become darker, usually as a result of aging or sun damage.

The two main classes of desmolytics are Alpha Hydroxy Acids (AHAs) and Beta Hydroxy Acids (BHAs). Both are water-soluble weak acids. Glycolic acid, a molecule in sugar cane juice, is the smallest of the common AHAs, so it penetrates deeper in the skin—but can also result in more irritation. Therefore, those with sensitive skin should look for products with lactic acid or mandelic acid, which are larger in structure. BHAs like salicylic acid are another alternative for those with sensitive skin as they are weaker than AHAs.

Printed in bold letters or metallic font across a glass bottle or porcelain tube, actives are ingredients like retinoids, Vitamin C, and Niacinamide that generate a bunch of buzz, but are rarely really explained. While dermatologists disagree on their efficiency, actives are a class of skincare ingredients meant to target specific dermatological woes.

Some of the most common actives are from a class of Vitamin A derivatives known as retinoid, and include trendy ingredients like retinol and retinoic acid. While retinol is widespread in drugstore and over-the-counter products, Fu and Lu say most of the existing scientific research backs retinoic acid, which is only available as a prescription-strength cream. FDA-regulated retinoic acid is the gold standard for skin rejuvenation, Frey says, but long-term effects are still unknown. It is not suitable for pregnant women as it can cause birth defects. Dermatologists and scientists are unsure of the molecules’ exact mode of action, but they seem to target nuclear receptors in skin cells that alter how they grow and differentiate, increasing epidermal thickness and reducing the appearance of wrinkles.

Over-the-counter products in the U.S. contain retinol—a fat-loving compound that likely penetrates the skin, where enzymes then metabolize it into retinoic acid. Frey says in her experience retinol works well for acne, but it’s anti-aging claims are murkier due to poorly designed studies. Because it potentially speeds cell turn over, any retinoid product must be worn alongside sunscreen and is best applied at night.

Vitamin C is another frequently heralded skincare active, but science backs only its chemically active form, L-ascorbic acid. If products list another Vitamin C derivative, it is best to steer clear. Active Vitamin C works by targeting free radicals in the air. Free radicals are unstable atoms formed by UV rays and pollution that cause a wide range of damage to the skin, from hyperpigmentation to wrinkles. Unstable and reactive, Vitamin C is an antioxidant, meaning it latches onto these free radicals so your skin doesn’t have to.

“Your body has its own natural antioxidant defense system to help us battle these day-to-day stressors,” says Lu. “But with topical products the idea is to supplement and strengthen our own defense system.”

But Vitamin C’s reactivity means it breaks down rapidly when exposed to oxygen or sunlight. Therefore, it must be kept in dark bottles or aluminum tubes, and used rapidly. The best packaging to ensure L-ascorbic acid’s efficiency is an airless pump. Due to its ability to irritate the skin, it should not be applied at the same time as retinoid.

A lesser-known favorite of Fu and Lu’s is Vitamin B3, sometimes referred to as Niacinamide. According to the two chemists, Niacinamide is known to regulate sebum—the oil our skin naturally produces—and brighten complexions. The relatively-gentle active is suitable for even those with dry and sensitive skin.

No matter which actives, exfoliates, or moisturizers you decide to invest in, Frey, Lu, and Fu all agree the most important product is a broad-spectrum sunscreen.

“More and more studies show that protecting the skin from ultraviolet rays is by far the best anti-wrinkle product,” says Frey.

Broad-spectrum sunscreens block both Ultraviolet A (UVA) and Ultraviolet B (UVB) rays. UVAs are longer in wavelength and penetrate deep into the skin, causing signs of aging. UVB are the rays that cause sunburn since they are shorter in wavelength and target the surface of the skin. Regardless of their ability to age the skin, both rays cause cancer, which is why Frey strongly recommends wearing sunscreen 365 days a year—even when it’s overcast or cold. PopSci recently awarded a 2019 Best of What’s New Award to SPF company Supergoop for its protective eyeshadow, but the company has an entire fleet of products designed to make sun protection fit seamlessly into any and all of your daily beauty routines.

Regardless of aesthetics, skin health should always come first.

“You are going to hear four to 10,000 times a day, whether it’s subliminal or not, that you are not good enough, because that’s how the skincare industry works,” says Frey. “I instead tell women, live a healthy lifestyle. Eat a good diet, exercise, and wear sunscreen.”