In this installment of Science Sunday, we’re diving into the science behind wet hair and how to best care for it during the drying process. 

Recent studies are challenging our understanding of how hair responds to water and drying routines—and it turns out that protecting hair while it’s wet is just as important as how you dry it. Even if you believe air drying is gentler, understanding hair’s unique structure is key to minimizing damage and maintaining its health.

Why wet hair needs special care

Hair fibers change when they become wet (just like a sponge), and that’s because they can absorb up to 30% of their weight in water.  This absorption affects every layer of the hair and alters its properties—making it more prone to damage. 

The inner cortex, which primarily consists of polypeptide keratin chains that are arranged within cells called macrofibrils and surrounded by the cell membrane complex (CMC), swells from the inside as water is absorbed. At the same time, the outer cuticle layers also swell and expand, causing the cuticles to lift, and the hair's surface becomes rougher.

Hair's strength, elasticity and shape rely on hydrogen bonds—temporary connections between proteins within the cortex that act like tiny molecular Velcro. When water molecules flood in, they break these bonds by attaching to the keratin polypeptide chains in the hair's core instead—a bit like water disrupting a perfectly arranged stack of building blocks.  

The good news is that this process also allows hair to be reshaped. When you style wet hair, you’re essentially using water to break and reform these hydrogen bonds—which is why it can hold new shapes when drying (like straightening or curling). As the hair dries, the reformation of the hydrogen bonds holds it in its new shape.

Why natural isn’t always better

Hair absorbs a lot of water, so drying isn't just about removing surface moisture. Even when it feels dry, it can still hold significant water inside, continuing to make it vulnerable to damage. Understanding the drying process and protecting hair during drying is crucial to maintain its health.

Letting hair air dry may seem like the gentlest option (after all it's natural, right?), but new research suggests this isn't always the case. Air drying hair can take hours, which means it’s in a prolonged weakened state.

The specific concern here is the potential damage to hair’s Cell Membrane Complex (CMC), which is the glue that holds its structure together. The CMC is made of natural oils, proteins, and other cellular components, and extended  water exposure can compromise it.On the other hand, using high-heat tools can also damage wet hair if not used properly. Extreme heat can cause the water inside hair fibers to rapidly turn into steam, which in turn creates tiny bubbles that can burst and damage hair’s structure—leading to rough, brittle strands. Under a microscope, these burst bubbles look like blisters or volcanic eruptions.

Understanding heat’s effect on hair

Surprisingly, the temperature of most hair dryers is much lower than what's needed to break down hair proteins and cause damage. The real culprit behind damage is often wet hair combined with mechanical stress—the physical manipulation of hair.  

This stress comes in many forms. Rough towel-drying can act like sandpaper on your hair's cuticles. Common styling habits like brushing and combing during blow-drying can stretch and fatigue the weakened hair structure. It's like stretching a weakened rubber band—the combination of heat and physical stress can lead to breakage.

To minimize stress on hair, it is important to handle it with adequate care. Detangling when it's protected by a conditioning  product, for example, can significantly reduce stress and the chances of breakage. Minimizing vigorous or rough handling of wet hair when drying, and ensuring hair is properly protected through styling will also go a long way.

Choosing the best drying method

Now that we understand how hair responds to water, stress, and heat, let's discuss techniques to keep it in great condition while drying.

Blot, don’t rub: Use a soft or microfiber towel to gently blot sections of hair rather than rubbing, which can roughen up hair cuticles and cause more damage.

Air drying: If you prefer to air dry, reduce the time your hair stays wet by gently blotting it first. Afterwards, apply leave-in products to protect your hair while it dries. You can also try a hybrid approach: air dry until damp, then finish with low heat to speed up the process.

Heat drying:  For those who prefer using heat, section the hair  to make uniform drying easier, and keep the dryer about 8 to 10 inches away from the strands. Use the lowest heat setting that effectively dries hair, and always point the dryer down the hair shaft to reduce frizz and help smooth the cuticle. Choose brushes made from natural fibers to minimize heat damage, as metal brushes can create hot spots.  This technique helps prevent both heat and mechanical stress.

Keeping hair strong + protected

Whether you choose air drying or heat drying, some disruption to the hair's internal bonds is inevitable during washing and styling. That's why protecting and restoring your hair's protein structure with the K18Repair Service is crucial for maintaining healthy hair.

To protect hair at home, use the K18 Leave-In Molecular Repair Mask. This clinically proven product targets broken polypeptide chains deep within the hair shaft, reconnecting them to restore strength from the inside out. It also contains emollients selected to protect the hair surface from styling stresses. It’s effective on all hair types, regardless of drying method.

For ongoing daily protection, use Damage Shield Conditioner on non-treatment days. When heat styling, a few drops of K18 Molecular Repair Oil after towel drying will help shield hair from heat damage,  while providing additional structural support during drying.