Polypropylene is an excellent choice if you tend to run hot or sweat heavily on the slopes. It's a synthetic fiber that moves moisture away from your skin faster than almost any other fabric, and it dries incredibly quickly. This means less of that clammy, wet feeling against your face during high-intensity skiing or competitive racing.
The trade-off is that polypropylene isn't the softest material against bare skin, and it can hold onto odors over time—so it's not ideal for multi-day trips without washing. It also provides minimal insulation on its own, since its focus is on moisture management rather than warmth.
If you're charging hard, hiking for turns, or just naturally sweat a lot under your helmet, polypropylene will keep you drier and more comfortable than fleece or merino. For casual resort days or extreme cold, you might prefer something softer and warmer.