Why glycerin makes your hair frizzy in summer

Same gel, same routine, totally different hair from one week to the next? The weather may be reading your ingredient list before you do.

Glycerin is a humectant

Humectants attract water. Glycerin is the most common one in hair products, along with honey, propylene glycol, sorbitol, and hyaluronic acid. On your hair, they pull moisture toward the strand, which is great when the air has the right amount of water in it.

Dew point, not humidity, is the number to watch

Relative humidity changes with temperature, so it can be misleading. Dew point tells you how much water is actually in the air, and most weather apps show it. Humectants tend to behave best when the dew point is moderate, roughly 40 to 60°F.

• High dew point (muggy, above about 60°F): humectants keep pulling water in, strands swell, and frizz takes over.
• Low dew point (dry, below about 35°F): there's little water in the air, so humectants can pull moisture out of your hair instead.

What to use when the weather turns

In humid weather, look for anti-humidity polymers such as polyquaternium-10 or -11 and polyamide-1. They form a light barrier that helps curls hold their shape. They can build up, so plan a clarifying wash now and then.

In cold, dry weather, film-formers like aloe vera, flaxseed, marshmallow root, and panthenol hold moisture in without relying on the air. Light oils and butters on top help seal it.

You don't have to ban glycerin

Where glycerin sits on the label matters. Near the end of a long list, it's a small amount and may never cause a problem. If it's in the top few ingredients and your hair goes wild in summer, swap that product for a humidity-resistant one during muggy months.