THE NEW SECRET TO YOUR Best Hair Eve

We'll let you in on the term that's popping up all over the beauty world: hair porosity. Understanding your hair's relationship to moisture is key to choosing the right products and techniques for the optimal health (and look) of your strands. Here, everything you need to know about hair porosity, including the types, how to determine yours and care tips from top hair pros.

- CELIA SHATZMAN and KATHERINE TINSLEY

What exactly is hair porosity?

"Porosity refers to how well the hair is able to absorb or hold on to moisture," explains Marisa Garshick, M.D., a dermatologist and an assistant clinical professor of dermatology at CornellNew York Presbyterian Medical Center in New York City. "The cuticle, hair's outermost layer, determines how much moisture passes in and out, which can dictate how porous your hair is." Much of hair's inherent porosity is genetic, but it can also be altered by external factors like chemical treatments and heat damage, says Lacy Redway, a celebrity hairstylist in Los Angeles and New York City and a Dove Hair Partner. "Hair can be categorized as low porosity, medium porosity or high porosity," notes Kamara Brown, a hairstylist in West Hollywood, CA.

Decode YOUR TYPE

IF YOUR HAIR IS: Low Porosity

In this type, hair's cuticles are very tight, making it difficult for moisture to enter or escape, Brown says. Because of this, hair can be either too dry due to lack of hydration or excessively oily because of buildup on strands, says Erinn Courtney, a hairstylist in Los Angeles. If hair takes a long time to blow-dry or is resistant to coloring (e.g., gray hair), these can be signs of low porosity. Straight strands are naturally oilier, while curly or coily hair with low porosity tends to feel brittle and dry.

CARE CUES: